Nutria (Myocaster coypus) in Louisiana

The nutria or coypu (Myocastor coypus) is a rodent native to South America that has been introduced almost worldwide since the early 1900’s, originally with the intent of fur farming in many cases. The nutria is a large (over 6 kg), semi-aquatic rodent with a voracious appetite and high reproductive potential. Nutria became established in the Louisiana wetlands in the 1930’s. The habitat proved to be ideal and populations exploded, reaching an estimated 20 million animals in less than 20 years. Trapping of nutria for their pelts formed the backbone of the Louisiana trapping industry from the 1960’s until the early 1980’s when prices for furs on the world market and in Louisiana fell drastically. Since then the annual trapping harvest, which was over one million animals per year for many years, has dwindled to 29,544 in the 2000-2001 season. Since the virtual cessation of the annual harvest, nutria numbers have increased dramatically. Reports of damage to wetland habitats emerged in the late 1980’s. Numerous studies of the wetland environments of Louisiana since then have documented the deleterious effects nutria grazing is having on the habitat. While nutria serve as an important prey item for the alligator, effects of nutria activity on other animals are primarily negative. Their most important impact is habitat modification and in many cases, habitat destruction. When impacts of intense nutria herbivory are added to the abiotic forces that are degrading the Louisiana coastal marshes the potential for lasting loss of wetland area is magnified. This report reviews the general biology and natural history of nutria; the chronology of nutria establishment in Louisiana and historic population fluctuations; interaction of nutria with other animals in Louisiana, and impacts of nutria herbivory on the wetland plant communities. CHAPTER 1 General Biology and Natural History of the Nutria Introduction The nutria, or coypu, (Myocastor coypus) is a rodent native to southern Brazil, Bolivia, Paraguay, Uruguay, Argentina, and Chile (Cabrera and Yepes 1940, Cabrera 1961). Five subspecies of Myocastor coypus are recognized in its native range, with M. c. coypus occurring in central Chile, M. c. melanops restricted to Chiloe Island, Chile, M. c. santacruzae found in Patagonia, M. c. bonariensis in northern Argentina, Bolivia, Paraguay, Uruguay, and southern Brazil, and M. c. popelairi in Bolivia (Osgood 1943). Myocastor coypus is the sole member of the family Myocastoridae, which belongs to the Nutria (Myocaster coypus) in Louisiana 4 large group of native South American rodents of the suborder Caviomorpha (this group also includes guinea pigs, chinchillas, and New World porcupines, among several other groups of South American rodents). Woods and Howland (1979) compared the cranial musculature of the nutria with that of its near relatives, and Murphy et al. (2001) placed the nutria in a phylogenetic framework based on analysis of mitochondrial DNA sequences. Since the early 1900s, the nutria has been introduced almost worldwide, and today it is established in the United States, Canada, England, France, Holland, Scandinavia, Germany, the Caucasus, northern and central Asia, Japan, the Middle East, and East Africa (Aliev 1966a, Van den Brink 1968, Corbet 1978, Hall 1981, Bar-Ilan and Marder 1983). Head and body length of adult, non-captive nutria ranges between 472 and 625 mm, and weight averages approximately 6.7 kg in males and 6.3 kg in females (Gosling 1977). Specimens as large as 17 kg (> 37 pounds) have been reported (Grzimek 1975). The upper parts of the nutria range from yellowish brown to dark brown and the underparts are pale yellow (Chabreck and Dupuie 1970). The head is large and roughly triangular in shape, with eyes, ears, and nostrils located high on the head reflecting the aquatic habits of the nutria (Mann 1978). The tail is round in cross-section (unlike the laterally flattened tail of the muskrat (Ondatra zibethicus), scaly, and thinly haired except at the base (Woods 1984, Nowak 1999). The digits of the hind legs are partially webbed, whereas those of the forelegs are not. Females have four pairs of thoracic mammary glands that are located on the side of the body, rather than on the belly (Dobson and DeViney 1967, Gosling 1980). Presumably, this positioning of the mammary glands Nutria (Myocaster coypus) in Louisiana 5 allow the young to nurse with their nose above the water’s surface while the mother is swimming (Newson 1966). Figure 1 Native range of Myocastor coypus in South America. Modified from Woods et al. (1992) Figure 2 Range of Myocastor coypus in North America. Modified from Le Blanc (1994) Nutria (Myocaster coypus) in Louisiana 6 Habitat Use Nutria are semi-aquatic rodents that live along lakes, marshes, and slow-moving streams, and are abundant in freshwater, brackish, and saltwater marshlands. In their native range, nutria seem to prefer freshwater situations, however they are known to occur in both brackish and saltwater areas at several localities, such as the Chonos Archipelago in Chile (Nowak 1999). Along the Gulf Coast of the United States, nutria are most abundant in freshwater situations, and seem to prefer areas with dense stands of Chairmaker’s bulrush (Scirpus olneyi = Schoenoplectus americanus). Throughout their range (both native and introduced), nutria prefer wetlands with emergent (above-water) vegetation and areas with succulent vegetation along the banks. Although most nutria populations occur at low elevations, populations exist at elevations above 1000 m in the Andes of South America (Greer, 1966). A study of nutria density and distribution in the Pampas region of Argentina showed nutria density to be positively correlated with availability of grasslands used for cattle grazing and negatively correlated with local human perturbations (Guichón and Cassini 1999). In contrast to studies conducted throughout the introduced range of the nutria, Guichón and Cassini (1999) found little evidence of crop damage by nutria and concluded that nutria may not be a threat to agriculture in their natural range. Nutria often collect a large mat of vegetation, which they use as a feeding, grooming, and resting platform. Although they occasionally take over muskrat and armadillo burrows for nesting purposes, nutria are avid diggers and often dig their own burrows in banks along waterways and wetlands (Lowery 1974). Burrows are most common along banks with 45-90° slopes (Peloquin 1969) and range in size from short, Nutria (Myocaster coypus) in Louisiana 7 unbranched tunnels 1-6 m in depth, to elaborate burrow systems, often extending 15-46 m or more into the bank (Atwood 1950, deSoriano 1960, Laurie 1946, Peloquin 1969, LeBlanc 1994). Multiple entrances to a single burrow system are common. Their nests are crude mats of local vegetation located on narrow soil shelves (0.3 m wide) or in large chambers (up to 1 m in diameter) within the burrow (Willner 1982, LeBlanc 1994). Well-worn paths, or runways, usually emanate from the borrow opening and extend into the nearby vegetation. Burrow systems provide, not only protection from predators, but also effective thermal buffering—a study in Argentina by deSoriano (1960) showed internal burrow temperatures to range between 8-10°C daily (2° range), while outside temperatures ranged between –4° and 24°C (28° range). Social Behavior The nutria is a gregarious rodent and often lives in groups containing from 2 to 13 or more individuals (Ehrlich 1966, Warkentin 1968, Gosling 1977). These groups usually are composed of related individuals, including one to several adult females, their young, and one adult male. As young males mature, they are driven away from the group by the resident adult male (Warkentin 1968, Gosling 1977). As a result, young males are often solitary. Resident males participate actively in nest defense (Carill-Worsley 1932, Ryszkowski 1966, Ehrlich 1966), and Warkentin (1968) reported that females are behaviorally dominant over males, except while mating. Nutria males are territorial and typically exclude other males from their territories, which are normally larger than those of females (Doncaster and Micol 1989, Gosling and Baker 1989). As a result, males spend more time in the water patrolling for intruders Nutria (Myocaster coypus) in Louisiana 8 than do females. In France, this behavior may contribute to the observed male-biased mortality (Doncaster and Micol 1989) because males may continue to defend their territories even when water temperatures fall dangerously low (Moinard et al. 1992). In Louisiana, activity patterns of nutria appear to be influenced by ambient temperature (Warkentin 1968). When temperatures were below 28°C, diurnal activities were restricted primarily to sleeping and sunning. At temperatures above 28°C, most animals fed, groomed, or slept. No animals were observed sunning when ambient temperatures rose above 34°C. Nutria are known to huddle in small groups during cold nights (Gosling et al. 1980a), which would appear to be an adaptation for energy conservation (Contreras 1984). Moinard et al. (1992) showed that metabolic energy expenditures of nutria huddling in groups of three were reduced approximately 20% over single (non-huddling) individuals. In a laboratory study of non-evaporative heat loss from the tail of the nutria, Krattenmacher and Rübsamen (1987) showed that heat loss from the tail is of major thermoregulatory importance. As with most mammals, the olfactory lobes of the nutria’s brain are well developed and much of the nutria’s social behavior is influenced by the sense of smell. Oily secretions from scent glands located near the mouth and anus are used, not only in grooming, but also in marking of territories (Ehrlich 1958). As with other aquatic and semiaquatic mammals who spend much of their time foraging in murky waters (e.g., Sea Lions, Zalophus, and River Otters, Lutra), the nutria’s vibrissae (whiskers) are richly endowed with sensory neurons at their base, which may enable them to navigate in dark waters using only the sense of touch (Mann 1978). Unlike many other aquatic and semiaquatic mammals who show a reduced dependence on the sense of hearing, the Nutria (Myocaster coypus) in Louisiana 9 nutria seems to have a well developed sense of hearing (Mann 1978), which probably reflects its dual existence in both water and air. Eyesight in the nutria is thought to be poor (Le Blanc 1994). Feeding and Nutritional Ecology Nutria are generally thought to be strict vegetarians, but like many other rodents, they may consume small arthropods and nestling birds that they happen to encounter while foraging. Nutria feed while on land or in the water, using their forelegs to deliver food materials to the mouth. Although voracious eaters, nutria rarely cause habitat damage, except at high densities (Hillbricht and Ryszkowski 1961, Ehrlich and Jedynak 1962, Harris and Webert, 1962, Ellis 1963, Wentz 1971, Litjens 1980). A study of nutria food consumption in Chile documented an average of 1,100 g of vegetation (range 700 to 1,500 g) per individual per day (Christen 1978), which amounts to approximately 25% of individual body mass consumed per day. The diet consists of a wide variety of plant materials, including leaves, stems, roots, and bark (Warkentin 1968, Murua et al. 1981). Unlike muskrats, nutria consume only small quantities of algae (Willner et al. 1979). Nutria may have been introduced to certain regions of the world based on the hope that they would eat undesirable aquatic plants (Woods et al. 1992). However, nutria do not seem to be an effective control agent for introduced species, such as common water hyacinths (Eichhornia crassipes), alligatorweed (Alternanthera philoxeroides), coon's tail (Ceratophyllum sp.), and bladderwort (Utricularia sp.). Instead, nutria seem to prefer native plants and also are known to eat crop plants, such as rice, sugarcane, Nutria (Myocaster coypus) in Louisiana 10 alfalfa, ryegrass, and fruit and nut trees (Schitoskey et al. 1972, Kuhn and Peloquin 1974). In addition to crop plants, nutria are also known to damage trees including conifers, deciduous forest trees, and seedlings of bald cypress, Taxodium distichum (Blair and Langlinais 1960, Kuhn and Peloquin 1974, Myers et al. 1995). A study of nutria diet in their natural range (the Pampas region of Argentina) showed that 40-60% of their diet consisted of aquatic monocots and 30-35% consisted of terrestrial monocots (Borgnia et al. 2000). Nutria consumed dicots only occasionally (015%). In Argentina, spikerush (Eleocharis bonariensis) was the preferred monocot in winter and spring, and duckweed (Lemna sp.) was preferred in summer and fall. A study of nutria diet in Maryland (Willner, et al. 1979) showed that nutria feed heavily on plant roots. Likewise, Ellis (1963) and Gosling (1974) reported that root crops are an important dietary constituent during winter in England. In Louisiana, nutria are known to eat the roots and rhizomes of many native plant species. Because of this behavior, they are considered wasteful feeders (Linscombe et al. 1981), and estimates suggest that nutria may waste more than 90% of the plant material damaged while feeding on the bases of plants (Taylor et al. 1997). Nutria appear to be opportunistic feeders in Louisiana (Atwood 1950, Wentz 1971, Shirley et al. 1981, Tarver et al. 1987). Common food plants in Louisiana include cordgrasses (Spartina alterniflora, S. cynosuroides, and S. patens), duckweeds (Lemna minor and Spirodela polyrrhiza), arrowheads (Sagittaria latifolia and S. platyphylla) and chairmaker’s bulrush (Scirpus olneyi = Schoenoplectus americanus), but nutria also consume many other native and non-native plant species (Lowery 1974, Conner 1989, Wilsey et al. 1991). Ellis (1963, Nutria (Myocaster coypus) in Louisiana 11 1965) reported that nutria feed on a large variety of crops in England, including cowbane (Circuta virosa) and great water dock (Rumex hydrolapathum). There have been numerous studies of the impact of nutria on wetland plant communities in Louisiana. Certain of these studies conclude that nutria and other herbivorous vertebrates reduce the number of plant species living in a study area (e.g., Fuller et al. 1985, Rejmanek et al. 1990, Shaffer et al. 1990, Nyman et al. 1993), whereas other studies suggest that herbivores have little or no effect on plant species diversity (Smith 1988, Taylor and Grace 1995). Regardless of the potential effect of nutria on plant species diversity, all studies agree that nutria can have major impact on total aboveground biomass of important native plant species, such as chairmaker’s bulrush, Scirpus olneyi = Schoenoplectus americanus (Johnson and Foote 1997) and arrowheads, Sagittaria latifolia and S. platyphylla (Llewellyn and Shaffer 1993). The negative impact of nutria on soil building processes, such as below-ground plant productivity and surface litter accumulation, was documented by Ford and Grace (1998). Like many other vegetarian species, nutria are coprophagious (i.e., they reingest fecal pellets to extract additional nutrients). Although defecation occurs throughout the feeding period (with approximately 86% of the feces produced while in the water), coprophagy appears to occur only at the nest (Gosling 1979). A study of the digestive tract of the nutria by Snipes et al. (1988) revealed an unusually large cecum (first portion of the large intestine) that can hold approximately 47-55% of food material in the entire digestive tract. Nutria are proficient divers, and can remain submerged for periods exceeding 10 minutes (Katomski and Ferrante 1974). Studies by Ferrante (1970) indicate that nutria Nutria (Myocaster coypus) in Louisiana 12 show bradycardia (slowing of heart rate) and peripheral vasoconstriction (reduction of blood flow to the skin and appendages), while diving. Apparently, the nutria’s respiratory system is able to tolerate the physiological consequences of diving, which include high levels of carbon dioxide and lactic acid in the blood (Ferrante and Miller 1971). Although nutria have low red blood cell counts relative to other mammals, their red blood cells are unusually large (Scheuring and Bratkowska 1976). Although nutria can be active both day and night, they are primarily nocturnal and most feeding activity occurs at night (Gosling 1979). However, in instances of low food availability, feeding may be observed at all hours of the day (Lowery 1974). Gosling (1979) reported that the period of nocturnal feeding activity in nutria is shorter during colder weather, however Chabreck (1962) reported no relationship between activity and temperature. Nutria metabolism is quite labile and correlates positively with ambient temperature (Segal 1978). Nutria studied in Cuba during the summer showed basal metabolic rates consistent with expectations based on body mass (Kleiber 1961, Segal 1978). However, when air temperatures drop, the metabolism of the nutria also drops and core body temperature in 0°C weather may drop to 33°C. Hull (1973) showed that newborn nutria control their body temperatures over a wide range of ambient temperatures. Primary bile acids of the nutria are similar to those in humans, which has made the nutria a potential model organism for study of gallstone formation in humans (Tint, et al. 1986). Nutria (Myocaster coypus) in Louisiana 13 Breeding Biology, Population Density, and Genetics As with most rodents, the nutria is a prolific breeder. Females are polyestrous (i.e., they show post-partum estrus, and are ready to breed again within a day or two following birth of a litter; Matthias 1941, Skowron-Cendrzak 1956, Gosling 1981a). The gestation period of the nutria ranges from 127 to 139 days (Atwood 1950, SkowronCendrzak 1956, Weir 1974), which is somewhat longer than would be predicted based on body size alone (Blueweiss et al. 1978, Kleiman et al. 1979, Sacher and Staffeldt 1974). Litter size normally is 3 to 6 individuals, but litter size can range anywhere from 1 to 12 individuals (Federspiel 1941, Gosling 1981b). Litter size generally is smaller in winter (Gosling and Baker 1981), and is larger in areas with mild winters and abundant food (Brown 1975). In Maryland, reproductive output for the nutria was estimated at 8.1 young per female per year (Willner et al. 1979). In areas with plentiful food and low predation, adult females may produce three or more litters per year (mean = 2.7) with an average total of 15 young per female per year (Brown 1975). A high percentage of nutria litters (estimated between 26 and 28%) are wholly or partially aborted during gestation (Gluckowski and Maciejowski 1958, Newson 1966). Evans (1970) reported that a large percentage (estimated at 40%) of nutria embryos are resorbed in the mother's uterus and thus do not survive to birth. Newson (1965, 1966) reported that the nutria embryo develops slowly during the first month of gestation. Ovarian hormone cycles of the nutria are not well understood, and the biological consequences of food restriction on catabolism and ovarian activity are only vaguely understood (Sirotkin et al. 2000). The normal (non-pregnant) estrous cycle of the nutria Nutria (Myocaster coypus) in Louisiana 14 varies from 5 to 60 days (Newson 1966, Wilson and Dewees 1962), with one to four days of estrus ("heat") during the cycle. This varia tion, along with the fact that healthy females may show no cycles for several months, suggests that estrus in females is induced by copulation (Asdell 1964). The structure of male and female reproductive systems of the nutria have been studied in detail by Hillemann et al. (1958) and Stanley and Hillemann (1960). Nutria are non-seasonal breeders (Brown 1975, Gosling et al. 1980b, Kim 1980). Studies in Louisiana report high birth rates in December, January, June, and July (Adams 1956). In Oregon, birth rates peak in March, May, and October (Peloquin 1969). The female nutria usually gives birth to her litter in an open nest at the water’s edge or in a nest chamber within her burrow system (Gosling et al. 1988). Nutria young are precocial, and are born fully furred, active, and with eyes open. Mean birth weight is approximately 225 g for both sexes (Newson 1966), although males eventually become 15% heavier than females as adults (Doncaster and Micol 1989). Young nutria are able to swim and eat very soon after birth, and they gain weight rapidly during their first months of life (Peloquin 1969). Dixon et al. (1979) reported that growth rate of young may be retarded by cold weather. Weaning occurs at five to eight weeks of age (Gosling 1980), and sexual maturity may be reached in four to eight months, depending on food availability and habitat conditions. Male young born in early summer may breed within four to six months, whereas those born in early winter may not breed until reaching seven or eight months of age (Pietrzyk-Walknowska 1956, Evans 1970). In females, age of first reproduction ranges from 6 to 14 months (Gosling 1974, Nutria (Myocaster coypus) in Louisiana 15 Konieczna 1956). The sex ratio in adult populations ranges from 0.6 to 1.6 males per female (Le Blanc 1994). The potential life span of the nutria is approximately 6.5 years (Woods et al. 1992), although Le Blanc (1994) stated that captive animals may live as long as 15-20 years. Nutria can be aged based on molar wear and eruption patterns (Aliev 1965b, 1965c, 1965d), body mass (Willner et al. 1980), or mass of the lens of the eye (Gosling et al. 1980b). Willner et al. (1983) proposed a four-parameter model for aging nutria that incorporated body length, body mass, hind foot length, and tooth eruption. Whereas tooth characteristics and body mass show sexual dimorphism and can be influenced by food type and abundance, Gosling et al. (1980b) showed that sex and environmental factors had little effect on lens mass. Nutria can be divided into rough age categories (i.e., juvenile, subadult, and adult) based on weight and pelage characteristics (Brown 1975) or length of the hind foot (Adams 1956). Estimates of fecundity rate, age distribution, and mortality schedules for a population of nutria in Maryland were calculated by Willner et al. (1983). Density of nutria populations will vary with climate, habitat, food availability, predation and hunting pressure, prevalence of diseases and parasites, pollution, density of competitors, and many other environmental variables (Brown 1975, Willner, et al. 1979). Doncaster and Micol (1989) found that nutria densities in their study area (in France) were independent of food availability, but this contradicts the findings of other researchers working in other areas (e.g., Lowery 1974, among others). Nutria populations appear to be very sensitive to climatic fluctuations. Populations can grow dramatically during mild winters and in the presence of heat-producing pollution Nutria (Myocaster coypus) in Louisiana 16 (Doncaster and Micol 1989, Litjens 1980). Cold weather can cause direct mortality of nutria and can also cause dramatic loss of fat stores, which may increase abortion rates, thereby causing reproductive failure (Newson 1966 and Norris 1967). Local nutria populations are susceptible to severe storms and prolonged flooding. Waldo (1957) estimated that perhaps 60 to 65% of the nutria population in the White Lake and Grand Lake marshes (southwestern Louisiana) perished as a result of hurricane Audrey in 1957. Further east, at Marsh Island, perhaps 70% of the nutria were killed or driven away during the same hurricane (Harris and Chabreck 1958). Density estimates of nutria populations range from 0.1 individuals per hectare in a Louisiana study (Valentine et al. 1972) to 138 individuals per hectare in Oregon (LeBlanc 1994). Greer (1966) estimated densities of 25 animals per hectare in Malleco Province, Chile. In Maryland, Willner, et al. (1979) estimated population densities to range between 2.7 to 16 individuals per hectare. Other estimates of population density are listed in Table 1. Table 1. Published estimates of nutria population density. Locality Density Estimate (individuals/hectare) Reference Chile 25 Greer (1966) France 2.42 in May, 9.14 in November Doncaster and Micol (1989) Florida 5.9 (unpolluted pond), 24.7 (polluted pond) Brown (1975) Louisiana 0.1 to 1.29 over 5-year period Valentine et al. (1972) Louisiana 43.7 Kinler et al. (1987) Louisiana 44 (in floating freshwater marshes) LeBlanc (1994) Maryland 2.7 to 16 Willner et al. (1979) Oregon 138 (in freshwater marshes) LeBlanc (1994) There have been only a few genetic studies of nutria populations. An electrophoretic study of a Maryland population by Morgan et al. (1981) found complete absence of genetic variation in liver enzymes and serum and lens proteins. This result is Nutria (Myocaster coypus) in Louisiana 17 not unexpected in an introduced population that likely experienced a genetic bottleneck at the time of introduction. Ramsey et al. (1985) surveyed protein variation in feral, introduced nutria from a variety of locations and found only three of 22 presumptive loci to be polymorphic. Average individual heterozygosity was about 5% in Louisiana populations, but only 0.2% in England and 0% in Washington state. Isolated inland populations and populations periodically reduced by severe weather in Louisiana had less variation, perhaps due to founder effects or genetic drift. Maum (1986) found electrophoretic variation in three coastal Louisiana populations, as well as morphological variation in pelt and cranial characteristics. Nutria populations contain at least two antibody blood groups, types CO1 and CO2 (Szynkiewicz 1968). Szynkiewicz (1971) reported variation in beta-globulin gene frequencies among several Polish populations of nutria, and Brown (1966) reported ontogenetic (age-related) differences in lipoproteins as well as serum concentrations of globulins and albumins. Chromosomal studies of Myocastor coypus report a diploid number of 42 chromosomes and a fundamental number (= number of chromosomal arms) of 76 (Tsigalidou et al. 1966, George and Weir 1974, Kasumova et al. 1976). No chromosomal variation in nutria has been reported to date. Movements and Dispersal Because it is an amphibious mammal that moves easily on both dry land and in water, the nutria has high potential for long-distance dispersal. Swimming by the nutria is particularly energy efficient—while swimming, the nutria’s head and back are slightly Nutria (Myocaster coypus) in Louisiana 18 above the water's surface and propulsion is provided by means of alternate thrusts of the webbed hind feet and graceful side-to-side undulations of the tail (Gosling 1979). Despite this ability to easily traverse both land and water barriers, nutria tend to remain in the vicinity of their natal area for their entire lives (Aliev 1968). However, freezing weather or drought may cause them to migrate in search of more favorable climate or habitat (Aliev 1968). The daily home range of nutria is usually restricted to within 45 meters, or so, of their burrow entrance (Adams, 1956), individuals are often observed as much as 180 meters from their burrow opening (Nowak 1999). A study in the Netherlands documented daily movements of nutria up to 300 m by water and 50 m by land while foraging (Kim 1980), and Linscombe et al. (1981) reported movements of up to 3.2 km in Louisiana. Finally, Aliev (1968) documented a nutria range extension of 120 km over a 2-year period in Eastern Europe. Estimates of home range size for nutria vary considerably with season, reproductive condition, and food availability. In a study of tagged nutria in Louisiana (all males), 50% were recaptured within 91.4 meters of their burrow, 80% within 0.4 kilometers, and 20% between 0.4 and 1.25 kilometers (Robicheaux, 1978). Home range size was estimated at approximately 13 hectares in Louisiana (LeBlanc 1994). Doncaster and Micol (1989) estimated the size of nutria home ranges in France to be approximately 2.47 hectares for females and 5.68 hectares for males. These authors concluded that home range size was independent of population density. Studies of nutria dispersal and home range have been facilitated by use of radiocollar telemetry (Coreil and Perry 1977). Nutria (Myocaster coypus) in Louisiana 19 Interactions with Other Species Because nutria and muskrats are so similar ecologically, there is no question that they compete for food and space in areas where they co-occur. However, co-occurrence (at least in large densities) is not common because nutria are most abundant in freshwater situations, whereas muskrats seem to prefer salt or brackish waterways and marshes (Lowery 1974). In addition, muskrats prefer marshes dominated by chairmaker’s bulrush (Scirpus olneyi = Schoenoplectus americanus), whereas nutria feed extensively in marshes dominated by cordgrass (Spartina patens), which is not a preferred food of muskrats (Chabreck et al. 1981, Nyman et al. 1993). Although direct evidence of competition between nutria and muskrats is only anecdotal, indirect evidence of competition is illustrated by the fact that removal of nutria populations from areas of coexistence result in rapid expansion of muskrat populations (Evans 1970). Where they co-occur, nutria are behaviorally dominant over muskrats, probably by virtue of their larger body size. Muskrat nests are sometimes taken over by nutria to be used as nests or resting platforms (see Habitat Use above). Possible Limiting Factors As discussed in the previous section, competition with native semi-aquatic mammals, such as the muskrat, does not appear to be a major limiting factor for nutria populations. In fact, absence of competition from native species may explain the ease with which nutria are introduced to suitable habitats worldwide. Annual mortality estimates for nutria populations range from 53% (Chapman et al. 1978) to 74% (Newson 1969). These estimates include both natural and trapping Nutria (Myocaster coypus) in Louisiana 20 mortality. Natural predation is an important limiting factor for nutria populations. In South America, caymans (Caiman longirostris, C. niger, and C. sclerops) are reported to be the major natural predator of nutria (Aliev 1966b). Similarly, in North America, the American Alligator (Alligator mississippiensis) is known to consume large numbers of nutria regularly. A study of the diet of the American Alligator in southeast Louisiana revealed that nutria constitute approximately 60% (by weight) of the alligator’s diet. In alligators over 1.7 meters in length, mammals are the most important food group based on prey mass or volume in stomach analyses (Wolfe et al. 1987). Other major predators of the nutria in South America include the jaguar (Panthera onca), mountain lion (Puma concolor), ocelot (Leopardus pardalis), the little spotted cat (Leopardus tigrinus), and other medium-tolarge sized predatory mammals (Dennler 1930). In North America, the red wolf (Canis rufus), red fox (Vulpes vulpes) and ermine (Mustela erminea) have been reported to consume nutria regularly (Willner 1982). According to Aliev (1966a), the major mammalian predators of nutria in Eastern Europe include domestic dogs (Canis familiaris), golden jackals (C. aureus), gray wolves (C. lupus), and the jungle cat (Felis chaus). Young nutria, as well as smaller adults, are often eaten in large numbers by birds of prey, including the bald eagle (Haliaeetus leucocephalus) (Dugoni 1980, Jeb Linscombe, pers. comm.), red-shouldered hawk (Buteo lineatus), the marsh harrier (Circus aeruginosus), and the tawny owl (Strix aluco) (Ellis 1965, Aliev 1966b, Warkentin 1968). Nutria (Myocaster coypus) in Louisiana 21 Young nutria are also consumed by large snakes, such as the cottonmouth (Agkistrodon picivorous), the gar (Lepisosteus sp.), and turtles of several species (Warkentin 1968, Evans 1970). Human predation on nutria (trapping and shooting) takes a major toll on nutria populations in certain areas of their introduced range. In the mid-1970s, the number of nutria pelts taken in Louisiana reached an all-time peak of approximately 1.9 million pelts per year. Today, ?30,000 nutria are trapped annually by the Louisiana fur industry (data for 2000-2001 provided by the Louisiana Department of Wildlife and Fisheries) (Linscombe, 2001). Microbial infections and endoparasites can cause considerable mortality, especially in times of high population densities. Nutria populations are known to be susceptible to rabies (Matouch et al. 1978), equine encephalomyelitis (Page et al. 1957), paratyphoid (Evans 1970), salmonellosis (Safarov and Kurbanova 1976), pappilomatosis (Jelinek et al. 1978), leptospirosis (Twigg 1973, Howerth et al. 1994), toxoplasmosis (Holmes et al. 1977, Howerth et al. 1994), richettsia (Kovalev et al 1978), coccidio sis (Michalski and Scheuring 1979), and sarcoporidiosis (Scheuring and Madej 1976). Diseases caused by microbial infections can result in significant mortality in nutria populations, especially in times of high population densities. At least a dozen kinds of microbial infections have been reported in nutria populations (see Breeding Biology and Population Density), but estimates of actual mortality caused by these diseases are unavailable. Endoparasites rarely kill their host, but they can reduce the fitness of nutria populations and thereby retard population growth. Internal parasites reported from nutria Nutria (Myocaster coypus) in Louisiana 22 populations include the nematode Strongyloides myopotami, which infects most, perhaps all, populations of nutria along the Gulf Coast of the United States (Babero and Lee 1961). According to Lowery (1974) fur farmers in Louisiana claim that this nematode is responsible for occasional periods of low reproduction and mass mortality in nutria. Strongyloides myopotami also is known to cause "marsh itch" or "nutria itch," which is a severe rash caused by larval roundworms that enter the skin of trappers who handle nutria fur (Burk and Junge 1960, Lee 1962, Little 1965). Other endoparasites reported in nutria populations (Babero and Lee 1961) include 11 species of trematodes (including Echinostoma revolutum, Heterobilharzia americana, and Psilostomum sp.), 21 cestode species (including Anoplocephala sp.), one acanthocephalan (Neoechinorhynchus sp.), and 31 nematode species (including Trichostrongylus sigmodontis, Longistriata maldonadoi, Strongyloides myopotami, and Trichuris myocastoris). The most prevalent endoparasites in South American populations of nutria include the trematode Hippocrepis myocastoris, the cestode Rodontolepis sp., and the nematodes Dipetalonema travassoso, Graphidioides myocastoris, and Trichuris myocastoris (Babero et al. 1979). External parasites of nutria include the chewing louse (Pitrufquenia coypus), the flea (Ceratophyllus gallinae), and the tick species Dermacentor variabilis, Ixodes arvicolae, I. hexagonus, I. ricinus, and I. trianguliceps (Newson and Holmes 1968 and Willner 1982). Nutria (Myocaster coypus) in Louisiana 23 CHAPTER 2 Chronology of Nutria Establishment, Historic Harvest Levels, and Population Fluctuations in Louisiana Nutria were reportedly first released in Louisiana in the marshes near New Orleans in the early 1930’s. The animals from this release were recovered by trappers and did not establish a breeding population (Lowery 1974). During the 1930’s, a series of accidental and perhaps intentional releases along the Gulf Coast quickly resulted in the establishment of feral populations. The origins and numbers of the founding stock or stocks are not known with any certainty at this time. Nutria were found at Lake Arthur in 1940 (Ashbrook 1948). Sportsmen and trappers had begun trapping and transplanting feral nutria into marshes from Port Arthur, Texas, to the Mississippi River by 1941. A hurricane in Texas in 1941 is credited with further dispersing nutria in southeast Texas and southwest Louisiana (Evans 1970). By 1941-42, nutria were being trapped on the Sabine and Laccasine National Wildlife Refuges in Cameron Parish of western Louisiana (Ashbrook 1948). Nutria continued to expand their range in succeeding years through natural dispersal and stocking efforts. By 1947 nutria were found at the Delta National Wildlife Refuge at the mouth of the Mississippi River (Ashbrook 1948). In the late 1940’s, nutria were being promoted as a biological agent for the control of aquatic weeds (primarily water hyacinth, Eichhornia crassipes at that time), and were transplanted throughout the southeast (Harris 1956; Lowery 1974; Evans 1970). Nutria (Myocaster coypus) in Louisiana 24 Historic Harvest Levels Nutria were well established throughout the coastal areas of Louisiana by 1943, and exhibited rapid population growth for a number of years thereafter. Indications of nutria population levels in Louisiana since 1943 are largely indirect, and comes from two sources: 1) annual pelt harvest records as reflected in state severance tax records, and 2) incidence and degree of nutria damage to crops, levee systems and native marsh habitats. Local nutria population levels have been estimated in Louisiana in a number of studies using direct methods such as mark-recapture (Robicheaux 1978, Linscombe et al. 1981), night counts (Spiller and Chabreck 1975), and indirect indexes including as scat counts and active trail counts (Spiller and Chabreck 1975, Davidson 1984). Historically, the primary indictor of the state-wide nutria population has been the annual fur trapping harvest level derived from severance tax records. The records show the first nutria being marketing during the 1943-44 trapping season, with 436 pelts (Lowery 1974). Table 2 summarizes the annual harvest levels, average pelt prices, and trapping license sales for the trapping seasons from 1950-51 to 2000-01 (Mouton et al. 2001). Figure 3 shows the same information graphically. Trapping seasons typically have run from December through February, when pelts are prime. The pelt harvest trend line reflects not only the nutria population but trapper effort, which is in turn driven by the international fur market. By the 1961-62 trapping season the nutria harvest overtook muskrat in Louisiana for the first time (Tarver et al. 1987). From 1962 to 1982, an average of 1.3 million nutria were harvested from the coastal marshes each year (Linscombe 1992). The sustained high harvest over this period and the limited reports of damage problems Nutria (Myocaster coypus) in Louisiana 25 suggests that the annual recruitment rate (primarily births) in the nutria population and the mortality rate (natural losses plus human harvest) were below the carrying capacity of the habitat on statewide level. Due to vagaries of the international fur markets as well as the actions of antifur activists, the demand and price for nutria pelts began to decline in the early 1980’s. The Louisiana nutria harvest declined dramatically in the succeeding years, from over 1.2 million in the 1980-81 season to 134,000 by the 1990-91 season (Linscombe 2001). Population Estimation The other long-term indicator of nutria population levels in Louisiana has been the level of nutria damage to wetlands, coastal agriculture and forestry. There was a rising incidence of complaints of damage to marshes, rice, sugarcane, and levee systems beginning in the mid-1950’s (Lowery 1974, Mouton et al. 2001). High nutria populations and severe over-grazing were noted, particularly in the Mississippi Delta (Linscombe 1992). Biologists described areas where nutria had completely denuded natural levees at the mouth of the Mississippi River. The nutria population was estimated to peak at 20 million animals during the years 1955-59. Many once dense stands of cattail (Typha spp.) were largely destroyed. “Eat-outs”, areas of open water, appeared in many areas along the coast. Aggressive non-native plants, as well as unpalatable native plants filled the open water areas in places, but the structural integrity of the marsh had been weakened. Hurricane Audrey (June, 1957) made landfall in southwestern Louisiana. The value of the native marsh vegetation in buffering storm surges became apparent. The weakened marsh structure was unable to prevent a huge wave of seawater from inundating the interior marshes and the Chenier Plain (Lowery Nutria (Myocaster coypus) in Louisiana 26 1974). Although thousand of nutria are reported to have drowned in the storm surge, hurricane Audrey is also credited with pushing thousands of nutria inland (Harris and Chabreck 1958). Soon after, reports of agricultural damage increased. Nutria were found to live in the rice fields year-round if not controlled. Damage to rice occurred in southwest Louisiana and consists of: grazing on plants which retards or prevents the production of mature grain and burrowing into levees which interferes with water management at various stages of cultivation which are vital to rice production. The burrowing problem is exacerbated when cattle grazing on large levees step into and enlarge nutria burrows, or become injured (Evans 1970). Sugarcane damage occurs when nutria damage mature canes by gnawing or completely cutting the stalks. Young, transplanted canes may be completely uprooted. Many more plants are destroyed than are eaten. In contrast to the rice field situation, nutria typically visit rather than live in the sugarcane fields (Evans 1970). In response to damage problems, the nutria was taken off the list of protected wildlife in 1958. A $0.25 bounty was authorized but the funds were never appropriated (Mouton et al. 2001). In addition, the Denver Wildlife Research Center (DWRC), attached to the Bureau of Sport Fisheries and Wildlife at that time, began a nutria damage control research program in 1963 that continued through 1967 (Evans 1970). The program identified and evaluated existing damage management techniques (trapping, shooting), and developed new methods including the use of toxicants and agricultural habitat management. While the DWRC program had some success in identifying and developing damage control methods, the pest status of nutria was at odds with the state fur industry efforts to promote the nutria as a wildlife resource. A compromise between Nutria (Myocaster coypus) in Louisiana 27 competing interests was reached in 1965 when the nutria was returned to the protected wildlife list (Linscombe 1992; Mouton et al. 2001). However, control of any nutria determined to be an agricultural nuisance was still allowed without a permit (Evans 1970). With the increasing economic benefits of trapping nutria the annual harvest climbed steadily during the 1960’s and complaints of nutria damage to crops diminished (Linscombe 2001, Fowler 1992). By the late 1970’s over 10,000 trapping licenses were being purchased per year in Louisiana (see Table 2). Most trappers operated on leased sections of privately held marshlands. Typical leases are about 2,000 ha (5,000 ac.) A trapper will usually set an average of 150 traps and is required to check them daily. Victor #2 leghold traps of Victor #11 double longspring traps were used most commonly. Traps are placed openly in nutria trails. Nutria are also harvested by shooting, although there is a risk of damaging the pelt, and visibility in some habitats limits this method. (Kinler et al. 1987). At the same time harvest levels were increasing, the Louisiana nutria population was reduced by a severe freeze event in February 1962, in which the temperature dropped to 12o F (-10.4o C). The freeze is thought to have killed perhaps millions of nutria. Survivors with missing tails and feet were trapped for a number of years (Lowery 1974). In the succeeding years damage to sugarcane was localized and usually controlled by trapping or shooting nutria around the perimeter of fields. Rice production has gradually shifted to underground irrigation, which has had the benefit of limiting nutria damage as well (D. Reed, pers. comm.). Nutria (Myocaster coypus) in Louisiana 28 In 1987-88, at the same time the trapping harvest had dramatically decreased, reports of significant nutria damage to the wetlands were coming from coastal land managers (Linscombe 1992, Mouton et al. 2001). Aerial flights by the Louisiana Division of Wildlife and Fisheries in 1988 confirmed damage was occurring, particularly in the southeastern marshes, in Terrebonne and LaFourche Parishes. Funding was not available for further flights and systematic aerial surveys for the next several years. Qualitative and anecdotal evidence of marsh damage due to nutria herbivory continued to mount and in 1992 a Nutria and Muskrat Management Symposium was organized. The conference participants, including state and federal wildlife biologists, wetlands scientists, agricultural extension service personnel, and private land managers, concluded that nutria herbivory (as well as muskrat to a lesser extent) was having substantial adverse effects on the agriculture, forestry and native wetland resources (Linscombe and Kinler 1997). The symposium findings provided the documentation and impetus to secure funds from the Barataria-Terrebonne National Estuary Program (BTNEP) to conduct systematic region-wide aerial wetland damage surveys. Aerial flights resumed in 1993. Additional surveys were conducted in 1995 and 1996 (Linscombe and Kinler 1997). Coast-wide aerial surveys were conducted in 1999, 2000, and 2001 under the Nutria Harvest and Wetland Demonstration Project (Mouton et al. 2001). The objectives of the surveys were to “1) determine the distribution of damage along the transect lines as an index of damage region wide, 2) determine the severity of damage as classified according to a nutria relative abundance rating, 3) determine the Nutria (Myocaster coypus) in Louisiana 29 species of vegetation being impacted and 4) determine the status of recovery of selected damaged areas” (Mouton et al. 2001). The 1993 flights identified 90 damaged sites along transects, amounting to 15,000 acres of impacted marsh. Extrapolating from this figure, based on the transect swath width (1/4 mile) and distance between transects (1.8 miles), the damaged acreage in the survey area can be multiplied by a factor of approximately four, resulting in an estimated 60,000 acres impacted by nutria herbivory in the survey area. The 1996 survey found the impacted area on flight transects had grown to 20,642 acres, or 82,568 acres in the survey area (Linscombe and Kinler 1997). The flights conducted in 1998-2001 followed the same transect patterns used earlier. Table 3 summarizes the results of four years of coast-wide nutria damage surveys, by coastal parish. The data is arranged by parish from west to east. The survey results clearly show nutria herbivory damage in recent years is concentrated in the Deltaic Plain in southeastern Louisiana. The most severely impacted Parishes are Terrebonne, LaFourche, Jefferson, and Plaquemines. Terrebonne and Lafourche Parishes, both in the inactive delta, were the number one and two nutria pelt producers, respectively, for many years, and have the most nutria damage as well. Table 4 summarizes the same data set sorted by marsh type. These data demonstrate the impacted areas are primarily found in the freshwater marshes (48%). The freshwater, floating mat marshes provide the most productive marsh habitat for nutria, since the floating mat vegetation provides a productive food resource as well as a stable Nutria (Myocaster coypus) in Louisiana 30 habitat that rises and fall with fluctuations in the water level. The consistent water level is conducive to nutria reproduction (Kinler 1992). Further analyses of the flight survey data from 1998-2001 show that while the areas damaged by nutria declined somewhat from 2000 to 2001, and the number of sites classified as having severe vegetative damage has declined as well. The area of marsh converted to open water from 2000 to 2001 increased from zero to 4,726 acres (Mouton et al. 2001). This suggests intense and sustained nutria herbivory in parts of the freshwater marsh, which in turn indicates high nutria populations that are exceeding the local carrying capacity. Harvest data and damage indexes are only general indicators of nutria population densities. Harvest data are not based on equal effort over time, thus limiting the applications of such information. Flight damage survey results may vary with time of year, observer experience, and many other factors in addition to varying nutria populations. There is no accurate means of converting damage indexes to nutria density. However, if the surveys are carefully applied to minimize experimental error, repeated surveys can provide a reliable index for land managers to monitor habitat response to herbivory pressure. When analyzed with harvest records that can be keyed to particular areas and habitat types, these data provide a reliable basis for formulating management plans (Linscombe and Kinler 1984). Nutria populations have been monitored using a number of indirect and direct indexes, usually applied to local populations only. Many of these studies have shown that populations at the same site can vary tremendously from year to year (e.g., Linscombe et al. 1981), and that nearby populations may be very different (Kinler et al. Nutria (Myocaster coypus) in Louisiana 31 1987). Therefore the results of small-scale population estimates cannot be reliably generalized to outside the study area. Survey methods provide an index of activity that can be repeated over time to estimate population changes on a fixed plot or area. Survey methods for nutria such as night counts, scat counts, and active trail counts were used by Spiller and Chabreck (1975), and Davidson (1984). However, the correlation between the activity counts and the population size or density is rarely known. Without validation studies, these indexes cannot be used to generate population density estimates. Fagerstone (1983) was able to calculate the correlation equation and describe the necessary conditions to use visual counts of ground squirrels in Colorado to reliably estimate population densities on circumscribed plots. A similar approach might be used to validate survey methods for nutria in some circumstances. Mark–recapture studies can be used to estimate density if the assumptions of the model are met. These include a “closed” population (no recruitment or loss to the population during each trapping period) and equal “catchability” of individuals during the study period. To approximate the stable population assumption, trapping periods for nutria are usually limited to 8-12 days (Ryszkowski 1966, Doncaster and Micol 1989). Simpson and Swank (1979) found a population under study in Texas to violate the equal catchability assumption. Adults and sub-adults became trap-shy and skewed the population estimate upwards by 45%. The actual density was determined by shooting and trapping out the entire population at the end of the study. Reggiani et al. (1995) analyzed the results of a nutria mark-recapture study in Italy using the program CAPTURE (Otis et al. 1978) that allows corrections for trap shyness or trap happiness. Nutria (Myocaster coypus) in Louisiana 32 Linscombe et al. (1981), used a variation of the mark-recapture method in which they captured and tagged nutria, then recovered tags from commercial trappers during the following trapping season to generate a population estimate. Some nutria density estimates reported from mark-recapture studies are: 138/ha in Oregon (Wentz 1971); 1.3 to 6.5/ha in Louisiana (Robicheaux 1978); 21.4/ha in Maryland (Willner et al. 1979); 2.1 to 24/ ha over three years in a Louisiana brackish marsh (Linscombe et al. 1981); 24/ha in a Mississippi agriculture-marsh ecotone (Lohmeier 1981); 43.7 ha in Louisiana freshwater marsh (Kinler et al. 1987); and 0.72 – 3.7/ha in a riparian area in Italy (Reggiani 1995). Nutria are live-trapped on floating rafts (Evans et al. 1971) or on land. Carrots are the most common bait used. Nutria have been anesthetized with ketamine hydrochloride (Lohmeier 1981), sodium pentobarbital, and diazepam (Evans et al. 1971) during marking and measuring procedures. Marking methods have included ear or web tagging with metal tags (e.g. monel #3) (Simpson and Swank 1979, Lohmeier 1981, Willner 1982, Reggiani et al. 1995), ear punch codes (Lohmeier 1981) and web clip codes (Reggiani et al. 1995). Gosling (1981) used a technique of retrospective census combined with population simulation to reconstruct the population of nutria in England during the period 1973-1979. The method assumes all nutria deaths are recorded, and that all animals killed are randomly sampled and accurately aged. If adequate resources are available, this method may be applied for estimating limited populations in limited areas, but would not be practical to apply on a scale needed to estimate statewide populations in Louisiana (Kinler et al. 1987). Nutria (Myocaster coypus) in Louisiana 33 Finally, an indirect method of assessing nutria habitat quality that might be linked to population pressure is through the analysis of blood chemistry of nutria at a particular site. Ramsey et al. (1981) found that certain nutria blood parameters were effective indicators of habitat deterioration. Estimating population growth rates is difficult and expensive. Gosling et al. (1980, 1981) and Kinler et al. (1987) describe the techniques and necessary information to be collected to estimate population growth in a study area. Data must be collected on age, reproductive condition, pregnancy rates, and embryo counts. Juvenile survivorship must also be determined or estimated. While useful for characterizing a given population, the results may not apply beyond the local and the time period of the study. Landscape and Climate Effects on Nutria Populations in Louisiana Marshes To briefly summarize the history of nutria populations in Louisiana since 1937: following the introduction of one or a few small founding populations imported from fur farms the state witnessed the rapid establishment and spread of nutria throughout the coastal marshes. Stocking efforts as well as periodic hurricanes, which, while at times causing high mortality among nutria also serve to disperse survivors, accelerated the rate of spread. For the first 20 years following their introduction, the growth trend followed a classic logistic or sigmoid pattern of ecological release of a colonizing species into a favorable habitat that encounters little environmental resistance. The statewide population was estimated to have peaked at about 20 million nutria before hurricane Audrey hit in 1957 (O’Neil 1968). From 1962 until 1982, two primary factors kept the populations below the carrying capacity of the marshes. These were 1) Nutria (Myocaster coypus) in Louisiana 34 the annual trapping harvest and 2) periodic severe weather events which are believed to have drastically reduced the populations in portions of the state and sometimes the entire state. Since 1982 the annual trapping harvest has declined to a fraction of previous levels, and damage to the marshes in the Deltaic Plain has increased to the point that local populations appear to be exceeding the carrying capacity of the habitat. However, populations in the Chenier Plain have caused little damage since the decline of the trapping trade. What might account for the difference? There is evidence that the differing topography of the Chenier Plain and the Deltaic Plain make the nutria more susceptible to severe weather events. Landscape The Chenier Plain is the area west of Vermilion Bay. It was formed from river sediments being swept westward by shoreline currents in the Gulf of Mexico. The deposition of silt and clay sediments from the Mississippi River against the shoreline created mudflats that eventually became covered with salt-tolerant vegetation, creating new marsh. This process continued during times when the active Mississippi delta was to the west of it’s current location (Chabreck 1972). Two periods of delta building activity have occurred near Vermilion Bay and contributed to the building of the Chenier Plain in the last 7,000 years: the Teche Delta period, about 2500 B.C., and the LaFourche Delta period about 1300 B.C. Sediments from the river were picked up by the gulf currents during these periods and carried westward. Between the Teche and LaFourche Delta periods, the Mississippi moved it’s coarse far to the east and formed the St. Bernard Delta. During that period and the Nutria (Myocaster coypus) in Louisiana 35 Modern Delta period (the last 700 years), little sediment has entered the gulf currents. Consequently, the forces of wave action have eroded the marshes while simultaneously forming local beaches. When sediment deposition resumed during the LaFourche Delta Period, the marshes again extended the coastline, leaving stranded beaches or cheniers. The resulting east-west orientation of the region affects slows drainage when floods occur. The Deltaic Plain, east of Vermilion Bay, consists of the four deltas described above. Only one, the Modern Delta, is currently active, or still growing. However, due to construction of levees as flood control structures all along the Mississippi River, there is very little delta building today. An exception is in Atchafalaya Bay, where 30% of the Mississippi systems flow is diverted to the Atchafalaya River. Sediments carried by the river are currently extending the delta into the bay. The Deltaic Plain has subsided much more than the Chenier Plain over the centuries, leading to saltwater intrusion and a much larger band of salt marsh than is found in the Chenier Plain. The freshwater, floating mat marsh is also more extensive in the Deltaic Plain (nearly 1 million acres (397 ha) vs. less than 0.5 million acres (191 ha) in the Chenier Plain) (Chabreck 1972). As a result of the historic geologic deposition patterns, drainage in the Deltaic Plain is oriented north-south. Climate It has been well established that severe or prolonged cold temperatures can cause high mortality in nutria. Aliev (1965, 1973) cited instances of mass mortality due to temperatures of –27oC for 40 days in Russia. Axell (1963) attributed a nutria die-off in England to a particularly harsh winter in 1962-63. Gosling et al. (1981) reported a sharp decline in nutria numbers in England following a continuous 12-day freezing period in Nutria (Myocaster coypus) in Louisiana 36 1975. Doncaster and Micol (1990) described frostbite to nutria tails and feet following 20 days in which an ice sheet covered canals in France. It is common to find nutria with missing appendages following freezing weather in Louisiana and Maryland (Willner 1982). Reggiani et al. (1995) reported population decreases of 44-64% in Italy following two consecutive cold winters. Ehrlich (1962) and Doncaster and Micol (1990) determined that the presence of unbroken ice sheets which prevent entering the water, and lack of thick vegetative cover above ground contribute to more severe impacts of cold events on nutria. Under cold stress in winter, nutria may shift to a diurnal feeding pattern to maintain adequate food intake (Gosling et al. 1980). Doncaster et al. (1990) found that territorial behavior of dominant males in winter limits access of (mostly juvenile) subordinates to open water when partial ice sheets form. Subordinate individua ls therefore are more exposed to lower air temperatures, as well as being restricted from aquatic forage. This social interaction, along with the smaller body size of juveniles, likely accounts for the observations that juvenile mortality is disproportionately high during freeze events (Aliev 1973). Gosling et al. (1983) developed a mathematical expression or index (dubbed “CRS”) based on the cumulative weighted sequences of freezing days in a winter. The expression includes the length of a run of freezing weather and the number of runs each winter. Freezing days are defined as “24 h periods where temperature minima are = 0°C and maxima =5°C.” (Gosling et al. 1983). The report concludes that nutria are most affected by continuous runs of freezing days, and that the effect of freezing runs is cumulative over the winter. Presumably this is because of the effects of freezes on food Nutria (Myocaster coypus) in Louisiana 37 quantity as well as quality, and on diminishing nutria fat reserves as the season progresses. Mild to moderate cold weather impacts include reduced birth rates due to abortion, increased mortality of juveniles and, to a lesser extent, adults. Significant adult mortality due to cold is common only in severe circumstances. In Louisiana, records of unusually severe or prolonged freezes have not been analyzed in terms of the Gosling winter severity index. However there may have been at least three freezes sufficient to have had impacts on some nutria populations in parts of the state in the 1980’s (Greg Linscombe, pers. comm.). Prolonged floods can also cause high mortality in nutria populations in certain circumstances. Doncaster and Micol (1990) reported no nutria mortality resulted from flood events in France with durations of 5 – 37 days. Foraging was re-directed toward stripping bark of trees during the floods. The habitat in these cases was riparian zones connected to canal networks. Ehrlich (1967) found that nutria in their native range in South America and in Poland, Greece, and Israel were well adapted to changing their feeding and nesting habits in response to seasonal flooding. If waters do not rise too quickly, nutria may be driven out of earthen burrows but are able to create floating nesting platforms from emerging vegetation instead. In Louisiana, flooding of the marshes associated with hurricanes, inland rainfall and high tides can have catastrophic effects on nutria. Hurricane Audrey is credited with killing or driving inland 70% of the nutria on Marsh Island in 1957 (Harris and Chabreck 1958). Thirteen hurricanes made landfall in Louisiana in the period from 1950 – 1996. Rapid flood and rainfall events, especially if lasting a prolonged time, can leave nutria without resting sites and exposed to the elements. If cold temperatures occur at the same Nutria (Myocaster coypus) in Louisiana 38 time, weather-caused mortality is very possible (Greg Linscombe, pers. comm.). Nutria in the Chenier Plain are probably more vulnerable to such events due to the topography and relatively limited floating marshes. Floodwaters tend to back up behind the locks and drain much more slowly than they would in the Deltaic Plain, where the natural flow is directed south to the Gulf. The more extensive floating marsh in the Deltaic Plain also offers more protection from floods, because the mat rises and falls with the water level. As with cold weather events, the severity and frequency of catastrophic flood events may have cumulative effects on nutria populations over time. It has been suggested that the combination of more frequent, prolonged, and severe freezing and flooding events during the1980’s to 2001 may partially account for the lower population densities of nutria in portions of the Chenier Plain (Greg Linscombe, pers. comm.). Other climatic events that may have an impact on nutria populations are excessive heat and drought. Aliev (1965) attributed nutria mortality during a drought in the Caucasus region to heat stroke. This occurred at temperatures of 35-40°C. Drought may shrink or eliminate bodies of water and concentrate nutria in higher densities than the local habitat can support. Under these conditions nutria are also more susceptible to predation, hunting and trapping. Ecological Genetics and Nutria Populations in Louisiana As would be expected with translocated populations of any organism derived from a small number of founders, most nutria populations outside South America are relatively monomorphic genetically. Ramsey et al. (1985) reported only three of 22 presumptive loci were polymorphic in introduced feral nutria populations. Comparisons Nutria (Myocaster coypus) in Louisiana 39 of overall individual genetic heterozygosity in introduced populations found relatively high variation in Louisiana (5%) compared to other areas. For example, average individual heterozygosity was only 1% in Maryland nutria (Morgan et al. 1981), 0.2% in England, and 0% in Washington state populations (Ramsey et al. 1985). Low genetic diversity in introduced populations is often due to founder effects and genetic drift associated with population “bottlenecks”, in which a population is reduced to a small size for one or more generations. The relatively high heterozygosity in Louisiana populations may be a result of highly heterozygous founders, multiple introductions, or both. In general terms, loss of genetic diversity is negatively correlated with the minimum population size and positively correlated with the duration of the bottleneck (Nei et al. 1975). Although the numbers and origins of the nutria that founded the Louisiana populations are not known, little loss of genetic diversity is expected when a population passes through a short bottleneck of only a few generations. This assumes that most or all of the transplants contributed to the founding gene pool. Inbreeding depression refers to a loss of individual (and population) fitness associated with a lack of genetic diversity. It is attributed to the increased homozygosity of rare, recessive alleles (alternate gene forms) and is often expressed most strongly in traits related to fitness such as fecundity. The converse of inbreeding depression is heterosis, a term describing hybrid vigor or increased fitness in the progeny of different genotypic parents, and is often expressed in greater fecundity and survival (Bodkin et al. 1999). Heterosis may be most pronounced when relatively genetically monomorphic or inbred demes (local populations) meet and interbreed. Nutria (Myocaster coypus) in Louisiana 40 There were likely multiple introductions in Louisiana. Later introductions of nutria from different sources could have contributed to a heterotic-effect that in turn might have hastened the spread and proliferation of nutria in the state. The rate of spread of fire ants (Solenopsis saevissima) in the southeast U.S. was observed to increase rapidly following a second introduction 11 years after the first (Carson 1968). Studies of old field mice (Peromyscus polionotus) have shown that highly heterozygous females are more aggressive, have higher reproductive rates, and are more active dispersers (Smith et al. 1975, Garten 1976). Among some species of voles (Microtus spp.) dispersing females tend to be more heterozygous than non-dispersers (Krebs et al. 1973). There is little information available on genetic diversity levels of wildlife populations and the associated fitness and population growth. A study of remnant and translocated populations of sea otters (Enhydra lutris) found lower growth rates in remnant populations as opposed to translocated populations, although no relation was found between growth rates and haplotype diversity of the different populations (Bodkin et al. 1999). Haplotype diversity was correlated with the minimum population size and the number of years at the minimum population size. This result confirms the earlier work by Nei et al. (1975) regarding the effects of size and duration of bottlenecks. Repeated bottlenecks coupled with long population fluctuation cycle length may greatly reduce genetic variability in a population (Motro and Thompson 1982). In regards to ecological-genetic interactions, Bodkin et al. (1999) found that the quality of the habitat was an important determinant of population growth following bottlenecks. In Louisiana nutria, isolated (non-coastal) populations and those periodically reduced by severe winter weather have shown reduced genetic variation, perhaps due to Nutria (Myocaster coypus) in Louisiana 41 founder effects and genetic drift (Ramsey et al. 1985). Linscombe (pers. comm.) has indicated the western Louisiana populations may have been kept below threshold levels in the 1980’s and 1990’s due to a series of climatic setbacks. It is not known if these events created localized bottlenecks or affected the genetic diversity and population growth of western Louisiana nutria as a whole. In terms of management strategies, the impact of efforts to reduce local nutria populations might be maximized by applying control efforts to populations during bottlenecks when they are both numerically and genetically depauperate. Isolated or remnant populations resulting from catastrophic climatic events, or from population crashes due to habitat degradation, may be most vulnerable to control efforts since their recuperative abilities in terms of fecundity and dispersal success may be genetically attenuated. Conversely, strategies to encourage populations with desirable traits as a resource, for example the higher quality pelts produced by western Louisiana nutria, may benefit from exploiting genetic diversity. An appropriate strategy would include translocating animals from different sources, using large numbers of transplants, and managing for optimum nutria habitat (Bodkin et al. 1999). Genetic characterization of potential transplants is also encouraged to prevent possible negative hybridization effects, such as abnormal meiosis resulting from crossing over within inverted chromosomal segments in inversion heterozygotes (Kinler et al. 1987). Nutria (Myocaster coypus) in Louisiana 42 CHAPTER 3 Interactions of Nutria with Other Animal Populations in Louisiana Introduction The experiences with introduced nutria in Louisiana and elsewhere have clearly demonstrated that the species has significant impacts on the flora, fauna, and landscape of the invaded ecological communities. This chapter will focus on the interactions of nutria with other animals in the coastal marshes of Louisiana. While invasions by nonindigenous species often cause significant and deleterious changes in the newly colonized communities, there is no accepted framework for characterizing and evaluating the type of impacts, magnitude of impacts, data quality, and extent of impacts at the community level (Ruiz et al. 1999). By contrast, two-species population interactions have long been characterized by ecologists in terms of costs and benefits to each species. The following list describes many of the most common twospecies interactions. A plus sign (+) indicates a benefit, a zero (0) indicates a neutral effect, and a negative sign (-) indicates a detrimental effect (Table 2). Nutria (Myocaster coypus) in Louisiana 43 Table 2. Categories of interspecies interactions. Adapted from Odum (1971). Interaction Category Species 1 Species 2 1. Neutralism 0 0 2. Competition – direct interference 3. Competition – resource use 4. Amensalism 0 5. Parasitism + 6. Predation + 7. Commensalism + 0 8. Proto-cooperation + + 9. Mutualism + + Most two-species interactions can be readily assigned to these categories. Moving beyond categorical descriptions is often difficult because quantitative aspects of the interaction are lacking. For example, the fact that alligators may prey heavily on nutria does not in itself indicate the relationship will have population level impacts on either species. Interactions of ecological significance are those that cause a significant and measurable change in the abundance or distribution of one or both of the species (Ruiz et al. 1999). Species known or suspected to have significant direct interactions with nutria are American alligator, muskrats, waterfowl, raptors, and to some extent other bird life. American Alligator The American alligator (Alligator mississippiensis) is the largest reptile in North America. Ancestors of the American alligator appeared about 200 million years ago. They currently range from Texas eastward to North Carolina. Louisiana has the highest population, estimated at nearly 2 million in the nearly 4.5 million acres of suitable habitat in the state. While found in cypress-tupelo swamps and lakes, and canals and rivers, the highest population are found in the freshwater coastal marshes. American alligators lack Nutria (Myocaster coypus) in Louisiana 44 the buccal salt-secreting glands present in crocodiles, and therefore can tolerate only moderate salinity levels. This restricts their distribution to freshwater, intermediate, and brackish marshes, and to mangrove swamps with limited salinity (Anonymous, FAAC 2002). Alligators may live to about 70 years. They are slow growing, gaining about a foot in length per year. Those reaching about 6-8 feet become breeders. Mortality among the young is high, with only 10-20 % surviving to reproduce. Many young are eaten by other, larger alligators (Wolfe et al. 1987). Alligators have been harvested in Louisiana for at least two hundred years. In the early 1800’s skins were used for boots and saddles, while their oil was used in steam engines and cotton gins. Over the years demand waxed and waned. There was an increased use of hides occurring during the Civil War. In the late 1800’s, the development of commercial tanning processes in New England led to the production of more durable hides. In 1962, the alligator season was closed in Louisiana due to low numbers. The USFWS listed the American alligator as endangered in 1967. Populations in the state slowly rebounded, and beginning with Cameron Parish in 1972, a harvesting season was reinstated. Other parishes followed and the season was opened statewide in 1981. The American alligator was removed from the USFWS endangered species list in 1987. It was removed from the International Union for the Conservation of Nature (IUCN) “Red List” of threatened species in 1996. The IUCN Crocodile Specialist Group (CSG) Action Plan for the species currently considers the availability of population survey data to be “Good”, the need for wild population recovery to be “Low”, and the potential for sustainable management “High”. Research Nutria (Myocaster coypus) in Louisiana 45 priorities are rated as “Moderate”, and include investigations of population biology and husbandry techniques (CSG 2000). The principle threats to American alligators are considered to be habitat destruction and environmental contamination (IUCN 2001). In Louisiana, a 30-day annual harvest season now takes place each fall. A special spring harvest has been allowed at times on Marsh Island in May, June and July (Anonymous 1990). An experimental “Bonus Tag” program was initiated in 1999 and continued through the 2001 season. Trappers are issued 10% more tags than they would normally receive. This program is intended to encourage harvesting of smaller, 4 – 5 feet long alligators, which are occurring in greater numbers than the normally harvested 6 – 7 feet alligators (Elsey 2000). In 2001, 34,583 wild alligators were harvested. The average length of those taken with regular tags was 7.25 ft. Those taken with bonus tags were 5.85 ft in average length. The total commercial value of wild hides and meat was $9.02 million. Additional non-consumptive alligator revenue is generated in Louisiana from swamp tours, valued at $4 million in 2001 (Linscombe 2001). In 1986, the state of Louisiana began an alligator-farming program. Alligator farmers are issued permits to collect wild alligator eggs and hatch them under artificial conditions. The farmers raise the alligators in captivity. Currently they are releasing approximately 14% of the 48 inch long alligators each year to augment the wild breeding stock in the marshes. The percentage released is adjusted periodically based on breeding success in the wild, survival rates of various size alligators and other factors. Egg production in the wild is subject to fluctuations. Drought conditions led to low egg collections in 1998, but with higher water levels in 1999 production increased dramatically (Elsey 2000). Currently about 300,000 eggs are collected annually, and the Nutria (Myocaster coypus) in Louisiana 46 hides and meat produced by the alligator farming industry were valued at over $12 million in 2001 (FAAC 2001). Both the wild and farmed alligator industries are tied to the success of wild alligator populations. This provides a strong incentive to public and private land managers in the state to maintain or improve marsh habitats. Nutria-Alligator Interactions The relationship between alligators and nutria is obviously that of predator and prey. Valentine et al. (1972) reviewed alligator food habit studies reported from 1929 to 1964, all conducted at the Sabine National Wildlife Refuge in southwest Louisiana. By combining the results of four studies from 1946 to 1964, they were able to analyze a sample of 731 alligators. They determined that crustaceans were the most important food class for alligators of all ages at the Sabine N.W.R. Fish, birds and mammals were also taken in varying proportions. Muskrats were found in 33-52% of alligator stomachs examine in two studies in the early 1940’s (O’Neil 1949). By 1961, a survey of 25 alligator diets found nutria in 56% of the stomachs examined, comprising 46% of stomach volume. However, the following year nutria dropped to 5% occurrence in alligator stomachs analyzed. Nutria population estimates and fur harvest records for the Sabine N.W.R. indicated the nutria population was declining quite rapidly during the period 1961 – 1965, from an estimated population of 74,000 to 9,000. (Valentine et al. 1972). There is no data to suggest that alligator predation was driving the nutria decline. Fur harvests removed about 30% of the nutria each year through this period (Valentine et al. 1972). Valentine et al. (1972) considered the alligator to be an opportunistic feeder that will eat anything that moves. Prey items are limited only by size. In their review, nutria Nutria (Myocaster coypus) in Louisiana 47 were found only in stomachs of alligators over 3 feet in length. Wolfe et al. (1987) found a similar size threshold. Chabreck (1996) reported that cannibalism by alligators is common, and at times even adult alligators may be taken as prey by larger alligators. Wolfe et al. (1987) examined stomach contents of alligators collected from southeastern Louisiana, and summarized previous work on alligator diets. Basing diet composition on actual or estimated live weights of intact, undigested prey items refined the diet analysis. Earlier studies often relied on occurrence and weight of partially digested remains to estimate the levels of diet components. In their survey of 100 alligator stomachs, muskrats and nutria together accounted for 83% of the diet weight and occurred in 77% of the stomachs. While muskrats occurred with greater frequency than nutria, the nutria accounted for more than two thirds of the mammalian flesh weight in the samples. Crustaceans, mostly blue crabs (Callinectes sapidus) and crawfish (Procambarus spp.) had a high rate of occurrence but comprised only about 1% of the diet weight. As alligator size increased, there was a trend towards replacing the numbers of muskrats in the diet with fewer nutria. Feeding efficiency appeared to be maximized by selecting the largest practical prey item, which will be the nutria in areas where muskrats and nutria occur together, are equally abundant, and equally “catchable” (Wolfe et al. 1987). The same study also concluded that alligators are opportunistic feeders and diet composition is largely determined by availability and vulnerability of the prey. In the past, trappers have expressed concern that they were competing with alligators for nutria and muskrat (McNease and Joanen 1977). No published information has been found to verify this. While it has been shown that nutria and muskrats are often Nutria (Myocaster coypus) in Louisiana 48 taken by alligators in large numbers, it is unlikely that alligators have significant impacts on either prey species due to the high reproductive rates of both rodents (Wolfe et al. 1987). Nor is there any information available on the possible dependence of the alligators on muskrat and nutria populations. The nature of these relationships represent a significant research need that should be pursued since populations of both nutria and alligator have large impacts on the Louisiana marsh ecology and economy. Wild harvested nutria have been used as a feed supplement for the alligator farming industry, and for other animal feeds. In New Orleans, nutria collected during recent population reduction campaigns have been used for animal feed at the Audubon Zoo (A. Ensminger, pers. comm.). In order to expand these uses a number of logistical and economic obstacles must be overcome. Some of these are addressed in the accompanying report “Marsh Dieback and Nutria Control Research: Socioeconomic and Cultural Analysis” (Brown 2002). Nutria meat is currently being used in limited quantities to feed young, farm-raised alligators (D. Ledet, pers. comm.). Nutria meat is a high quality food for this purpose, whether used fresh or processed into a meal (Coulson et al. 1987). There are significant practical concerns about using nutria as animal food, including spoilage, contamination, and the need to develop a processing and storage infrastructure. There have been previous incidents of contamination with toxicants used to control nutria populations (Evans and Ward 1967), and lead. Secondary lead poisoning of farm-raised alligators has been attributed to bullet fragments processed with the nutria carcasses (Camus et al. 1998). Nutria (Myocaster coypus) in Louisiana 49 Muskrat The historical record of the muskrat (Ondatra zibethicus rivalicious) in Louisiana has been reconstructed to the extent possible by Lowery (1974). There is very little mention of muskrat by the early explorers and naturalists, although the fossil record confirms their presence as far aback as the Pleistocene. Lowery concludes that muskrat probably occurred in low numbers up until about 1910. Earlier, populations were likely held down by a limited suitable food supply and an abundance of predators. The rise in muskrat populations in the early 20 century is tied to the alligator hunting trade at the turn of the century. As alligators became scarcer, hunters discovered they could more easily find the remaining reptiles by burning the marshes. Repeated burning held the marsh in an earlier successional stage dominated by three-cornered grass (Scirpus americanus) (formerly S. olneyi) and also called chairmaker’s bulrush (Schoenoplectus americanus). This is prime food for muskrats, and soon thereafter populations exploded (Lowery 1974). The muskrat trapping industry became established by about 1910. By the 19131914 season about 4.25 million muskrat were trapped in the state (Lowery 1974). There were radical swings in the harvest, and presumably the muskrat populations, over the next 40 years. The fluctuations are attributed to a large extent to the ability of muskrats to rapidly reproduce when conditions are good, and their susceptibility to climatic catastrophes. Female muskrats in Louisiana may produce up to nine litters a year, with 4-6 young per litter (Chabreck 1992). On the other hand, many muskrats may drown during floods or hurricanes, or perish because of drought or disease (Chabreck 1992). Louisiana muskrats also appear to go through population cycles of 10-14 years or longer. Nutria (Myocaster coypus) in Louisiana 50 The cycles may be driven by density-dependent factors relating to the local carrying capacity. In addition to foraging, much more vegetation is destroyed for use in building muskrat houses. “Eat-outs,” denuded areas resulting from overgrazing, are associated with over-population and may lead to population crashes. Following a crash the marsh vegetation gradually recovers and a new muskrat boom follows. Population cycles are local and rarely synchronized across the state. This is reflected in the harvest records. From 1923 to 1960 the annual harvest always exceeded 1 million pelts (Lowery 1974, Chabreck 1992). Muskrat distribution and abundance is tied closely to the distribution and abundance of three-cornered grass. In turn, three-cornered grass is limited to areas with a proper range of water level fluctuations and salinity. The appropriate conditions are found along estuarine shorelines and in brackish marsh (Lynch et al. 1947, Palmisano 1972, Kinler et al. 1987, Chabreck 1992). Water management and annual burning in the brackish marshes maintain the three-cornered grass stands. These practices benefit muskrats as well as wintering snow geese (Chen caerulescens) (Kinler et al. 1987). Prior to the early part of the 20 century, only freshwater floating marsh consistently produced dense populations of muskrat in southeast Louisiana (Lynch et al. 1947). The muskrat populations in Louisiana peaked in 1945-46, then went into a gradual decline which has persisted to this day, in spite of large vacillations at times. The decline has been attributed to a number of factors, including a decrease in the abundance and distribution of three-cornered grass, catastrophic weather events, industrialization of the marshes, and at times, over-population (Lowery 1974, Kinler et al. Nutria (Myocaster coypus) in Louisiana 51 1987). As saltwater intrusion has changed the salinity regime of the coastal marshes in the last few decades, brackish marsh and the associated plant communities have been reduced in area. Increasing salinity is unfavorable not only to the vegetation on which muskrats depend, but also has direct negative effects on muskrat litter production and survival of young (Dozier 19**). Muskrat-Nutria Interactions The decline of the muskrat beginning in the 1940’s coincided with the tremendous increase in nutria seen in the late 1940’s and through the 1950’s. However, there is little evidence that the trends are related (Ensminger 1955, Evans 1970, Lowery 1974). The two species do occur together. However, there is a degree of niche separation due to different feeding habits and habitat preferences. Whereas the density of muskrat is tied closely to S. olneyi production, nutria are able to thrive on a variety of plants in addition to S. olneyi. In their native S. America as well as in the U.S., the highest quality habitat for nutria is freshwater environments with stable water levels (Atwood 1950, Coreil 1984, Kinler et al. 1987). In Louisiana this describes the freshwater marsh, characterized by floating mat vegetation and species such as maidencane (Panicum hemitomon), bulltongue (Sagittaria falcate), spikerush (Eleocharis spp.), and alligatorweed (Alternanthera philoxeroides) (Kinler et al. 1987). High densities of nutria also occur at times in intermediate and brackish marshes. Where nutria and muskrat co-occur, the nutria appear to be behaviorally dominant. There are a few anecdotal reports of harassment and direct confrontations. For example Lowery (1974) describes incidents of nutria attacking muskrats held in traps. However, Evans (1970) concluded from field and pen studies Nutria (Myocaster coypus) in Louisiana 52 that direct confrontations are rare and insignificant. However, he documents instances of nutria damaging or destroying muskrat houses in the course of feeding and burrowing. Evans (1970) also suggested there may be competition for high spots in marshes during floods. Perhaps the strongest evidence of competition for resources or through direct interference comes from Evans (1970), who reported that removal of nutria from an areas of co-occurrence was often followed by a surge in the muskrat population. Bird–Nutria Interactions The primary direct interaction between birds and nutria is a predator-prey relationship. Young nutria in particular are taken by bald eagles (Haliaeetus leucocephalus) (Dugoni 1980, Jeb Lincombe, pers. comm.), red-shouldered hawks (Warkentin 1968), and even magpies (Pica pica) (Willner 1982). Jemison and Chabreck (1962) found no evidence that owls fed on nutria in an area where nutria were abundant. However, in Europe and Asia, predation has been reported by tawny owls (Strix aluco) as well as great blue herons (Ardea cinerea), harriers (Circus spp.), and crows (Corvus spp.) (Ellis 1965, Aliev 1966). Nutria interact indirectly with waterfowl and wading birds through habitat modification. Ponds of open water in the marsh resulting from nutria eatouts may actually be beneficial to waterbirds, including lesser snow geese, mottled duck and blacknecked stilts (Himantopus mexicanus). Quality duck food plants may quickly become established in these open areas (Ensminger 1955). An annotated listing of the USFWS Breeding Bird Survey trend results for Louisiana, 1966 – 2000, is listed in Table 3. In interviews conducted in February 2002, Nutria (Myocaster coypus) in Louisiana 53 ornithologists at Louisiana State University identified some bird species that could be adversely affected by changes in the marsh habitats as a result of nutria herbivory (Mark Hafner, pers. comm.). None of the species listed are currently considered as threatened or endangered by the USFWS. The king rail (Rallus elegans), purple gallinule (Porphyrula martinica), least bittern (Ixobrychus exilis), pied-billed grebe (Podilymbus podiceps), and common yellowthroat (Geothlypis trichas), all displayed downward trends in Louisiana for the period 1966 – 2000. The mottled duck (Anas fulvigula) is declining on a nationwide basis, although not in Louisiana. It is listed on the National Audubon Society “WatchList” (formerly the “Blue List”), which identifies birds undergoing non-cyclic declines on a region-wide or nation-wide basis. The resident seaside sparrow (Ammodramus maritimus) and a species that does not breed in Louisiana but only winters in the coastal marshes, Nelson’s sharp-tailed sparrow (Ammodramus nelsoni) are also on the Audubon WatchList. Valentine et al. (1972) reported that alligators prey on herons, egrets, rails gallinules, and mottled ducks. While birds were not high in occurrence in alligator stomachs, they were high in volume. The role of nutria as an alternative prey source for alligators and positive or negative effects on bird predation by alligators has not been investigated. Nutria (Myocaster coypus) in Louisiana 54 Other Nutria Predators Young nutria are also preyed upon by turtles, gar (Lepisosteus sp), and the cottonmouth (Agkistrodon picivorous) (Warkentin 1968, Evans 1970). Little information is available on the extent of interaction or the effects on the species involved. Indirect Relationships Between Nutria and Other Animal Populations As mentioned earlier, interactions of ecological significance are those that cause a significant and measurable change in the abundance or distribution of one or both of the species. Beyond the information cited above, there is little data available concerning direct nutria impacts upon the abundance or distribution of other marsh animals. However, nutria herbivory impacts on the habitat may have even greater influences on the marsh ecology (Grace and Ford 1996). The distribution and abundance of many marsh dwelling animal species is closely linked to the plant communities. In turn, the distribution and productivity of plant communities depends on salinity regimes, water level fluctuations, water turbidity, and rates of tidal exchange (Chabreck 1976). The vulnerability of the coastal marshes to both gradual and catastrophic processes that alter these characteristics at a given locale will impact all the animals that cannot relocate easily or quickly enough to avoid changing conditions. Storm surges may extend farther into the freshwater marshes now than in the past due to nutria grazing. The resulting saltwater intrusion will have an impact on both abundance and distribution of alligators locally. In the longer term, muskrats, which rely on Scirpus americanus in the brackish marsh, will be affected by saltwater intrusion. Resident and over-wintering Nutria (Myocaster coypus) in Louisiana 55 birds can be expected to be affected by habitat changes as well, in some cases positively and in some cases negatively. The coastal wetlands also provide structural shelter for aquatic organisms, serving as a nursery ground for marine fishes and crustaceans (Valentine et al. 1972). The wetlands cycle nutrients out of detritus and water, promoting the growth of microorganisms at the bottom of the fishery food chain. The Gulf of Mexico is America’s most productive shrimp fishery. Ninety-eight percent of the harvest of fish and shellfish from the Gulf comes from inshore areas (Anonymous 2001). Continued coastal wetland losses and habitat alterations, which are in part attributable to nutria herbivory, are expected to decrease the productivity of the Gulf’s commercial and sport fishery industry (Kendrick 1998). Nutria (Myocaster coypus) in Louisiana 56 Table 3. North American Breeding Bird Survey Louisiana Trend Results. Adapted from: Sauer, J. R., J. E. Hines, and J. Fallon. 2001. The North American Breeding Bird Survey, Results and Analysis 1966-2000. Version 2001.2, USGS Patuxent Wildlife Research Center, Laurel MD Credibility Categories: VL: Very Low. Data has an important deficiency, such as very low abundance (< 0.1 birds/route), very small samples (< 5 routes), or very imprecise (5% change/year would not be detected). L: Low. Data has a deficiency, such as low abundance (< 1.0 birds/route), small sample size (< 14 routes), quite imprecise (3%/year change would not be detected), or sub-interval trends are significantly different (p<0.05, z-test) suggesting inconsistency in trend over time. M: Moderate. At least 14 samples of moderate precision, and of moderate abundance on routes. Trend: Estimated Trend, summarized as percent per year. P: Probability that trend is significantly different from 0. “*” indicate significant difference. N: Number of survey routes in the analysis. 95% C.I.: 95% confidence interval for the trend estimate. R.A.: Relative abundance for the species, in birds per route. VL Pied-billed Grebe -12.6 0.32 4 -26.0 0.9 0.08 ----22.0 0.33 4 VL Least Bittern -0.7 0.94 5 -18.8 17.4 1.25 72.4 0.23 3 -5.4 0.16 4 VL Mottled Duck 3.6 0.60 12 -9.4 16.6 4.55 17.6 0.55 4 6.7 0.29 10 VL Clapper Rail 1.5 0.86 4 -14.2 17.3 0.85 ---2.0 0.86 4 VL King Rail -12.9 0.06 9 -24.6 -1.2 2.64 21.5 0.78 3 -11.1 0.34 7 VL Purple Gallinule -18.9 0.17 6 -42.0 4.2 0.39 ----17.3 0.25 4 VL Common Moorhen 7.3 0.24 11 -4.2 18.8 4.74 2.7 0.95 4 5.9 0.58 8 VL Marsh Wren 12.1 0.02 3 8.4 15.7 2.31 ---11.0 0.00 3 M Common Yellowthroat -1.0 0.42 63 -3.3 1.4 8.39 -3.3 0.12 24 -1.0 0.44 57 VL Seaside Sparrow 71.1 0.61 3 ***** 305.5 0.18 ---232.5 0.20 3 L Boat-tailed Grackle 1.4 0.66 17 -4.8 7.6 37.31 -2.6 0.91 5 0.3 0.93 15 Nutria (Myocaster coypus) in Louisiana 57 CHAPTER 4 Impacts of Nutria on Louisiana Wetland Habitats Introduction Herbivory can have significant effects on both the physical aspects and plant communities of ecosystems (Ford and Grace 1998). In general, the impacts of introduced nutria on habitats have been considered neutral when population densities are low (Hillbricht and Ryszkowski 1961, Ehrlich and Jedynak 1962, Ellis 1963, Wentz 1971). At high densities nutria activity can have significant impacts in the fo rm of reduced plant biomass, changes in plant species composition, and altered physical structure of the marshes (Llewellyn and Shaffer 1993, Johnson and Foote 1997, Ford and Grace 1998, Evers et al. 1998, Mouton et al. 2001). Nutria have long been recognized as having the ability to alter vegetative communities when occurring at high densities (Hillbricht and Ryszkowski 1961, Ehrlich and Jedynak 1962, Harris and Webert, 1962, Ellis 1963, Wentz 1971, Litjens 1980). They are voracious eaters, consuming about 25% of their body mass in vegetation each day (Gosling 1974). For average adult nutria this amounts to about 1,100 grams per day (Christen 1978). Nutria direct their feeding efforts at the bases of some marsh plants, including the roots and rhizomes (Ellis 1963, Gosling 1974, Willner et al. 1979, Chabreck et al. 1981, Taylor et al. 1997). This destroys the whole plant, while as little as 10% may be eaten (Taylor et al. 1997). Nutria, like other semi-aquatic mammals such as beaver and muskrat, have additional impacts on the physical structure of marshes beyond their foraging activities. Intense grazing by nutria can have negative effects on soil Nutria (Myocaster coypus) in Louisiana 58 building processes in the marshes (Ford and Grace 1998). These impacts may in turn lead to increased erosion, and setback or maintain the successional stages of plant communities (Ehrlich and Jedynak 1962, Myers et al. 1995). It has been shown that intense nutria herbivory can override plant community interactions such as mutualism and competition (Taylor et al. 1997). Nutria also manipulate the habitat by constructing resting platforms and nests from vegetation and burrowing into spoil banks and levees (Atwood 1950, Ehrlich 1962). Nutria grazing and excretion stimulates growth of some plants such as the invasive aquatic fern Salvinia spp. (Ehrlich 19**). Nutria have been introduced to some areas at least in part because of their potential ability to reduce undesirable vegetation (Barabash and Morozova 1952, Ensminger 1955, Ehrlich 1962). In Poland and Israel, nutria activity converted plantclogged lakes and waterways to open water, which improved conditions for irrigation, local fisheries, and boat travel (Ehrlich 1957, Ehrlich and Jedynak 1962). Ehrlich and Spielberg (1960) concluded that the stocking of nutria to clear reeds from ponds and waterways in Israel was preferable to chemical and mechanical methods. The increased water turbidity attributed to nutria foraging prevented the growth of filamentous algae, which furnish shelter for Anopheles mosquito larvae, vectors of malaria. In Oregon, creation of open water and increased nutrient cycling due to nutria herbivory were considered beneficial to inland marshes (Wentz 1971). Ensminger (1955) noted that in some cases, areas of Louisiana marsh converted to open water by nutria activity were quickly colonized by plants favored by waterfowl. Nutria were stocked in some parts of Louisiana with the intention that they might reduce the introduced water hyacinth (Eichhornia crassipes) which clogs waterways (Lowery 1974). However, in many cases Nutria (Myocaster coypus) in Louisiana 59 these efforts have been ineffective because the nutria tend to select native species over invasive introduced plants (Lowery 1974, Woods et al. 1992). The Impacts of Nutria Herbivory in Louisiana The ecology of the coastal marshes of Louisiana is characterized by complex and very dynamic abiotic and biotic processes (Chabreck 1976, Fuller et al. 1985, Evers et al. 1998). Most of the Mississippi delta is now in a long-term phase of degradation driven by natural abiotic processes such as the rising sea level, subsidence, wave erosion, and saltwater intrusion. The effects of these processes may be amplified by man-made modifications to the marsh structure such as dikes, levees and canals. The natural processes of land maintenance and building, which were once fed by sediment deposits from the river, have been curtailed by the levees that now contain the river all the way to the Gulf of Mexico (Sasser et al. 1995). While most of the Louisiana coast is losing land area, two locations are undergoing a net growth in land area. These are Atchafalaya River delta and the Wax Lake delta, both fed by the Atchafalaya River. The Atchafalaya River carries about 30% of the flow of the Mississippi and Red Rivers into the Atchafalaya Bay. Sediment deposits from the river water resulted in the emergence of islands at the river’s mouth beginning in 1973. The site has provided a unique opportunity to study the interactive processes of sediment deposition, wetland development, vegetative succession, and herbivory by nutria, muskrat and waterfowl (Fuller et al. 1985). Biotic processes in the marshes have been manipulated by man for a variety of purposes. In the brackish marshes, burning has been used as management tool for at least Nutria (Myocaster coypus) in Louisiana 60 a hundred years to facilitate activities such a alligator hunting and muskrat and nutria trapping, and to encourage growth of Scirpus americanus (formerly S. olneyi), which is a favored forage of lesser snow geese, muskrat and nutria (Kinler et al. 1987, Ford and Grace 1998). Weirs are used to manage water levels by dampening tidal surges. Ditching allows drainage to be controlled. Both types of structures enable land managers to stabilize or adjust water levels as needed to promote the desired vegetative community composition and abundance (Kinler et al. 1987). Nutria damage first became evident in Louisiana in the 1950’s when the population was estimated to have reached 20 million (Lowery 1974). Eat-outs appeared, areas that were either denuded of vegetation or converted to open water. Early exclosure studies by Chabreck et al. (1959) found that herbivory in the brackish marshes could reduce vegetation biomass by 40%. The rise of the trade in nutria fur led to annual nutria harvests exceeding 1 million per year from 1962 to 1981 (La. Dept. of Wildlife and Fisheries records). The increased trapping pressure combined with nutria losses attributed to severe weather events in the late 1950’s and early 1960’s resulted in reduced populations of nutria. Reports of damage were mostly associated with agricultural crops (Evans 1970, Lowery 1974). A similar situation has been described in the Chesapeake bay area for this same period. Exploding populations of nutria in the 1950’s were associated with marsh damage as identified in aerial photographs. The damage has accelerated since the mid1970’s due to the decline of pelt prices and the trapping harvest (Haramis 1996). In Louisiana, nutria are found in primarily in three types of marshes which are categorized by salinity levels. These are the freshwater, intermediate (= oligohaline) and Nutria (Myocaster coypus) in Louisiana 61 brackish (= mesohaline) marshes. Nutria only occur in low numbers in the saline marsh (Linscombe and Kinler 1992). Additional populations may be found in the bald cypress swamps. The floating freshwater marshes are the best nutria habitat in Louisiana, although the intermediate and brackish marshes can support very high populations at times (Kinler 1992, Linscombe 1992). The freshwater marshes are characterized by floating mats of vegetation. The most abundant plants are maidencane (Panicum hemitomon); bulltongue (Sagittaria lancifolia (= S. falcata)); spikerush (Eleocharis spp.); and alligatorweed (Alternanthera philoxeroides). Intermediate marsh is dominated by wiregrass (Spartina patens); reed (Phragmites communis); and bulltongue (Sagittaria lancifolia). In brackish marsh the most abundant species are wiregrass (Spartina patens); inland saltgrass (Distichlis spicata); and three-cornered grass (Scirpus olneyi (= Scirpus americanus = chairmakers bulrush, Schoenoplectus americanus)) (Chabreck 1970). The Atchafalaya delta is converting from a marine bay to a mainly freshwater marsh due to the infusion of river water (Llewellyn and Shaffer 1993). Southeast Louisiana Studies A number of studies in the Pearl River Wildlife Management Area (WMA), near the Louisiana-Mississippi border and north of Lake Ponchartrain, have investigated the role of herbivory in the marshes. Herbivory at the study sites was attributed primarily to nutria, although feral hogs are found in the area as well. Nutria populations in this area were estimated at about 22/ha. (Ford and Grace 1998). Researchers found herbivory reduction of above-ground biomass of intact stands of plants to be in the range of 30-50% Nutria (Myocaster coypus) in Louisiana 62 (Taylor and Grace 1995). Panic grass (Panicum virgatum) and aster (Aster subulatus) were significantly reduced by herbivory in the freshwater marsh, whereas in the intermediate marsh herbivory significantly increased P. virgatum and hairypod cowpea (Vigna luteola). In the brackish marsh, herbivory had no impact on species abundance. The results supported a conclusion that nutria herbivory had specific effects on some species and has a general plant community effect as well. In other research in the Pearl River WMA, plant neighbor interactions and herbivory effects were studied (Taylor et al. 1997). Species biomass of Spartina patens, Spartina alterniflora, and Panicum virgatum was reduced by 75% due to herbivory in freshwater, intermediate and brackish marshes. Mutualistic plant neighbor effects were found for S. patens in the brackish marsh and for P. virgatum in the freshwater marsh. However, both species suffered from competitive suppression in other salinity regimes (S. patens in fresh and intermediate marshes, P. virgatum in the brackish marsh). In the presence of intense herbivory, the positive and competitive effects of plant neighbors were eliminated. These results applied to herbivory on isolated stands of transplants in the study area. Nutria were observed to feed preferentially on these isolated clumps of vegetation, a behavior which could promote the formation of eatouts in areas of persistent feeding (Taylor et al. 1997). Ford and Grace (1998) conducted studies that focused on the interactive effects of herbivory and fire on the coastal marshes. The results suggested a hypothesis that fire impairs S. patens more than other species, while herbivory impairs other species more than it does S. patens. The hypothesis provides a basis for the long established practice of burning the relatively unpalatable S. patens in the brackish marshes to promote the Nutria (Myocaster coypus) in Louisiana 63 growth and abundance of Scirpus americanus, which is considered a favored food of lesser snow geese, muskrats and nutria (Kinler et al. 1987). Spartina patens is found in Panicum virgatum and Sagittaria lancifolia dominated marshes as well whereas Scirpus americanus is generally restricted to the brackish marsh. However, in this study burning S. patens dominated marsh did not significantly affect the relative cover of S. patens and Scirpus americanus. The authors suggested this may have been due to the fact that nutria herbivory at the study site reduced the fuel load so much that hot, continuous burns could not be sustained (Ford and Grace 1998). Other work by Ford and Grace (1996) at the Southeastern Louisiana University Turtle Cove Environmental Research Station suggested that herbivory can increase the susceptibility of freshwater marshes to damage from saltwater intrusion. Sods of Sagittaria lancifolia, a dominant plant of the freshwater marshes, were subjected to simulated herbivory (clipping), flooding and increased salinity (15%). None of the three treatments applied alone or in pairs caused long-term damage to the sods. But the simultaneous occurrence of all three stressors reduced growth and caused plant death. The experiment simulated conditions created by storm driven saltwater intrusions, which have been associated with vegetation diebacks. While this study simulated tropical storm effects and nutria herbivory, the results show that the combination of these stressors can potentially contribute to habitat loss in the freshwater marshes. In experiments in the brackish marshes of the Pearl River WMA, Ford and Grace (1998) found that herbivore activity decreased above-ground biomass, below-ground production, soil elevation and root zone expansion. They suggest that in areas where mineral sediment deposition rates are high, marshes can withstand herbivory, but in the Nutria (Myocaster coypus) in Louisiana 64 absence of such deposits, herbivore activity can have negative effects on soil-building processes that can lead to habitat destruction. Atchafalaya Bay Studies As noted previously sediment deposits from the Atchafalaya River resulted in the emergence of islands at the river’s mouth, beginning in 1973. The early colonization of the islands by plants and animals as been described by Fuller et al. (1985). By 1980 the newly forming delta had grown to 17 km. About 11 km were covered by vegetation. The islands were quickly colonized by broadleaf arrowhead (Sagittaria latifolia), and delta arrowhead (Sagittaria platyphylla). Nutria and muskrat colonized the islands within two years of their appearance. Researchers soon noticed that previously vegetated areas were reverting to un-vegetated mudflats. Year-round grazing by nutria and muskrat, and by fall and winter concentrations of waterfowl, were suspected of affecting the plant communities. Exclosure studies showed that nutria and muskrat herbivory were significantly reducing the biomass of S. latifolia and valley redstem or axil weed (Ammannia coccinea). In addition, the grazing and associated mechanical disturbance appeared to be holding the plant communities in a transitional stage (Fuller et al. 1985). Vegetation succession on the growing delta was predicted to be rapid due to the warm climate, rich soils and unlimited moisture (Shaffer et al. 1992). However, during the period 1980-1986 vegetation surveys and continuing exclosure studies found trends of decreased vegetated area and increased species diversity. The decreased vegetation was attributed to two factors – increased nutria herbivory and prolonged flooding. The Nutria (Myocaster coypus) in Louisiana 65 authors hypothesized that nutria herbivory of above-ground portions of Sagittaria latifolia leaves and the below-ground tubers made the plants more susceptible to the effects of flooding. Nutria populations on the islands were believed to have greatly increased during the study period, although census data were not available. These factors are slowing the initially predicted rates of plant community succession (Shaffer et al. 1992). By 1993, the islands in the bay had grown to 50 km. However, 80% of this area consisted of mudflats. Nutria were identified as the primary cause of vegetation loss on the islands, although waterfowl also grazed the islands. Llewellyn and Shaffer (1993) investigated the potential of the willow species, Justicia lanceolata (Chapm.) Small, for freshwater and intermediate marsh restoration in the presence of nutria herbivory. The species is so unpalatable to nutria that it has been described as “repellent”. The authors concluded that the plant is well suited as a tool for marsh restoration, and can function as a barrier to nutria (Llewellyn and Shaffer 1993). Evers et al. (1998) found in exclosure studies on the Atchafalaya delta that waterfowl and nutria herbivory were having roughly equal impacts on the vegetation. The experiments showed herbivory is having a “major impact on expansion, growth and species composition of emergent vegetation.” (Evers et al. 1998). Barataria and Terrebonne Basin Studies The Barataria and Terrebonne Basins comprise of an area of southeast Louisiana bounded by the Atchafalaya River on the west and the Mississippi River on the east. Changes in structure and vegetative composition of the freshwater marshes of Terrebonne Nutria (Myocaster coypus) in Louisiana 66 parish during the period 1968 to 1992 were described by Visser et al. (1999). They found a large scale shift in the dominant vegetation of the study area. The Panicum hemitomon dominated marsh declined from 51% to 14% of the study area, and was replaced with Eleocharis baldwinii-dominated marsh, which increased from 3% to 41% of the study area. Herbivory, increasing water levels and changing water quality were identified as possible driving factors behind the change. The actual cause or causes of the changes were not determined. Nutria herbivory may be responsible for weakening of the floating mats in freshwater marshes in the Barataria-Terrebonne Basins, which makes the creation of open water areas more likely in the event of storms and storm surges (A. Ensminger, C. Sasser, pers. comm.) Aerial surveys of nutria herbivory damage to the marshes in the Barataria and Terrebonne Basins were conducted in 1993, 1995 and 1996 (Linscombe and Kinler 1997). The 1993 flights identified 90 damaged sites along transects, amounting to 15,000 acres of impacted marsh. Extrapolating from this figure, based on the transect swath width (1/4 mile) and distance between transects (1.8 miles), the damaged acreage in the survey area can be multiplied by a factor of approximately four, resulting in an estimated 60,000 acres impacted by nutria herbivory in the survey area. The 1996 survey found the impacted area on flight transects had grown to 20,642 acres, or 82,568 acres in the survey area (Linscombe and Kinler 1997). The plants most affected were, in the freshwater marsh, Eleocharis spp. and pennywort (Hydrocotyl spp.); and in the intermediate and brackish marshes, Eleocharis spp. and Scirpus americanus (Linscombe and Kinler 1997, Mouton et al. 2001). Nutria (Myocaster coypus) in Louisiana 67 Foote and Johnson (1992) reported on work in the brackish marshes south of New Orleans. The area is losing an estimated 2-4% of the vegetated area per year, attributed to nutria herbivory. By tracking the plant biomass and stem turnover rate of Scirpus americanus and Spartina patens, the authors found that the vegetative community is responding to nutria herbivory with increased production, but vegetation biomass is not increasing. The increased plant production appears to be going into increased nutria production. Jacoby et al., (1999) have developed models of nutria-wetland interactions linking data from the brackish marshes of the Barataria Basin. Among the results, they found the marshloss model is not sensitive to the nutria density at which marsh loss begins, but is sensitive to the biomass destroyed per nutria. Also, nutria numbers do not respond significantly to marsh area loss until the area approaches zero, because marsh loss occurs only during winter when marsh biomass is at it’s annual low (Jacoby et al. 1999). Coast-wide Surveys Coast-wide aerial surveys were conduc ted in 1998, 1999, 2000, and 2001 under the Nutria Harvest and Wetland Demonstration Project (Mouton et al. 2001). The coastwide flights conducted in 1998-2001 followed the same transect patterns used in the Barataria-Terrebonne surveys earlier. The survey results indicate nutria herbivory damage in recent years is concentrated in the Deltaic Plain in southeastern Louisiana. The most severely impacted Parishes are Terrebonne, LaFourche, Jefferson and Plaquemines. Nutria (Myocaster coypus) in Louisiana 68 The greatest damage is occurring in the freshwater marshes (48%). The flight survey data from 1998-2001 show that the amount of area damaged by nutria decreased somewhat from 2000 to 2001, and the number of sites classified as having severe vegetative damage declined as well. However, 44 sites comprising 8,531 acres had “old damage” and were not recovering, and 19 sites containing 4,726 acres had converted to open water. The majority of the damaged sites were predicted to recover partially by the end of the 2001 growing season. The overall decrease in damaged area may be attributable to the effects of drought on nutria populations (Mouton et al. 2001). In spite of the partial recovery predicted for many of the lesser-damaged areas, the development of open water and failure of many areas to show recovery at all suggests nutria damage may be leading to permanent loss of coastal wetlands (Mouton et al. 2001). There is evidence that broken areas seen today in the brackish marshes of Marsh Island are remnants from muskrat eatouts that occurred in the 1940’s (Linscombe 1992). Baldcypress Swamps Nutria are also found in the baldcypress (Taxodium distichum) and tupelo (Nyssa spp.) swamps. Damage is typically of three types: girdling of mature timber, impacts on natural regeneration, and impacts on artificial regeneration (Sparks 1992). Lethal damage to mature trees is not considered a serious problem. However, gnawing on mature trees may make the trees more susceptible to other stressors. Little is known about the impacts of nutria on natural regeneration. Nutria impacts on artificial regeneration are well documented (Blair and Langlinias 1960, Conner and Toliver 1987, Myers et al. 1995). Nutria (Myocaster coypus) in Louisiana 69 Nutria pull young seedlings, and eat the succulent bark from the taproot (Blair and Langlinias 1960). In other cases seedlings are clipped above ground. Some re-seeding efforts have been suspended due to nutria damage. A variety of taste and odor repellents, physical repellents and physical barriers have been used to reduce nutria impacts. Physical (wire) barriers appear to be the most effective (Conner and Toliver 1987, Mike Materne, pers. comm.). Research has shown that fall plantings suffer less damage than spring plantings because of the abundance of other food sources during the fall (Conner and Toliver 1987). Summary There is no question that nutria are playing an important role in the complex marsh ecology in Louisiana. A multitude of studies have shown that marsh damage is positively correlated to nutria density. Deleterious effects of intense nutria herbivory include reduction in marsh biomass, setback of vegetation succession, elimination of mutualistic and competitive plant interactions, and stress to plants. When herbivory stress is combined with prolonged flooding and saltwater intrusion, vegetation die-offs may occur. Reduction of soil-building processes due to nutria grazing may lead to habitat destruction. Grazing may weaken the physical structure of floating mat vegetation, and can create open water which makes the marshes more susceptible to storm surge damage. The Society of Wetland Scientists position paper entitled “Definition of Wetland Restoration” has defined Wetland Restoration as “actions taken in a converted or degraded natural wetland that result in the re-establishment of ecological processes, Nutria (Myocaster coypus) in Louisiana 70 functions, and biotic/abiotic linkages and lead to a persistent, resilient system integrated within its landscape” (Anonymous 2000). The goal of wetland restoration is a persistent, resilient system. The restoration should result in the historic type of wetland but not necessarily the historic biological community and structure. Marsh creation is now a major policy objective of the government of the state of Louisiana and the Federal government (Evers et al. 1998). There is strong evidence that current nutria population levels, particularly in the southeast portion of the state, are having significant impacts on the rate of marsh creation in the Atchafalaya delta and on the rate of marsh degradation elsewhere in the Mississippi delta. Without sustained reduction in nutria impacts there will be little chance of restoring or even slowing the degradation of the coastal marshes in Louisiana. The probable impacts of continued marsh habitat modification and loss include decreases in sport and commercial fisheries production, decreased acreage available to treat pollution inputs to the Mississippi delta and the Gulf of Mexico, increased levels of eutrophication, decreased capacity to buffer storms, and decreased habitat for other species (Anonymous, BTNEP 1998). Nutria (Myocaster coypus) in Louisiana 71 Figure 3. Louisiana nutria harvest, average pelt value and annual trapping licenses for the period 1950-51 to 2000-01. Note the Y-axis is logarithmic. Nutria (Myocaster coypus) in Louisiana 72 Table 4. Number of damaged sites and acres damaged along survey transects, by parish, in coastal Louisiana in 1998, 1999, 2000, and 2001. Parishes are listed from west to east. The extrapolated total is calculated by multiplying the total acreage by four to account for areas between transects. A longitudinal line roughly bisecting Iberia Parish is considered the boundary between “western” and “eastern” nutria populations, and also corresponds with the topographic boundary between the Chenier Plain (west) and the Deltaic Plane (east). Table adapted from Mouton et al. 2001. 1998 1999 200