High Mortality of Piping Plovers on Beaches with Abundant Ghost Crabs: Correlation, Not Causation

Ghost crabs (Ocypode quadrata) have been implicated in mortality of eggs and chicks of the beach-nesting Piping Plover (Charadrius melodus) whose Atlantic Coast populations are listed as threatened. Through observation and experimentation, we investigated the interactions between ghost crabs and plovers on Wild Beach, a Piping Plover nesting area on Assateague Island, Virginia. This site has a high abundance of ghost crabs and historically low fledging success compared to adjacent areas with fewer crabs. We observed encounters of crabs with plover eggs, chicks, and adults in the field, but never predation. In staged encounters of crabs with eggs and chicks (using hatchery reared quail as plover surrogates), we were unable to elicit predatory behavior either on the beach or in the lab. We conclude that although instances of ghost crab predation on Piping Plover eggs and chicks occur, they are rare and cannot account for the high mortality frequently reported on beaches where ghost crabs are abundant. Adult plovers behave toward crabs as if they were dangerous to eggs and chicks, and their young broods in the study area did not forage along the foreshore. Hence, ghost crabs may increase mortality indirectly. Frequent responding to crabs by parents may attract more deadly brood predators. Brood nutrition may suffer as adult plovers direct chicks away from areas where forage is reportedly richer but crabs are abundant, such as the foreshore. Nutrient intake may be further reduced on more southerly breeding grounds where high temperatures on backshores force chicks to stop foraging and take shelter during mid-day. Although high mortality cannot be attributed directly to predation by crabs, it may be due to factors that covary with crab abundance, such as high temperature, behavioral responses of adult birds, and poor forage. Received 28 April 1998, accepted 7 Feb. 1999 Anecdotal and published reports of ghost crab predation on Piping Plovers (Charadrius melodus; Loegering et al. 1995, Watts and Bradshaw 1995) have led to concern that crab predation may hamper recovery of plovers on the Atlantic Coast, where the species is listed as threatened (Loegering and Fraser 1995; U.S. Fish and Wildlife Service 1993, 1996). To assess the extent of crab-caused mortality, we investigated interactions between ghost crabs (Ocypode quadrata) and Piping Plovers during incubation and chick rearing on Wild Beach on Assateague Island, Virginia, within the Chincoteague National Wildlife Refuge. Compared to other portions of this barrier island that are used for nesting areas by the Piping Plover, Wild Beach has higher abundances of ghost crabs (B&ton 1979) and lower rates of fledging success (U.S. Fish and Wildlife Service 1994). Piping Plovers breed from eastern Canada to North Carolina, as well as in the Great Lakes region and the Great Plains of Canada and the U.S. (Haig 1992). On the Atlantic Coast, plovers typically lay four eggs in a 1 Dept. of Marine, Earth and Atmospheric Sciences, Box 8208, North Carolina State Univ., Raleigh, NC 27695.8208. * Corresponding author; E-mail: [email protected] shallow scrape in the sand, usually well beyond the high-tide mark, or in shelly stormflattened areas (washouts) between and behind the primary dunes. Chicks are precocial and forage in moist backshore areas where available, or on the foreshore (the area between the tides; Loegering and Fraser 1995). Ghost crabs, named for their cryptic coloration, range along the Atlantic Coast from Rhode Island to Brazil and throughout the Caribbean (Chace and Hobbs 1969). They are among the fastest terrestrial invertebrates (Full and Weinstein 1992) and formidable predators with acute sensory receptors for vision, vibration, taste, and smell (Cowles 1908, Wellins et al. 1989). They are most abundant on high energy beaches, where they obtain over 90% of their diet preying on intertidal invertebrates (Wolcott 1978). They are extremely flexible foragers, also scavenging, deposit feeding, consuming seeds and insects, and are documented predators of turtle hatchlings (Amdt 1994, Robertson and Pfeiffer 1982). At dusk crabs move from their burrows on the backshore and among the dunes to feed in the swash zone. Crabs seek out and take refuge in burrows as dawn approaches (Wolcott 1978). Poor fledging success on beaches where crabs are abundant, coupled with extensive