Habitat Partitioning by Two Sympatric Species of Chipmunk (Genus: Neotamias) in the Warner Mountains of California

—In the Warner Mountains of California, two sympatric species of chipmunk partition their habitat primarily through the mechanism of competitive exclusion by social dominance and aggressive interactions. Forests are optimal habitats for both Neotamias amoenus and N. minimus. In this study N. amoenus actively excludes N. minimus from the forest through successful aggressive interspecific interactions, leaving N. minimus to occupy primarily the arid sagebrush scrub. Neotamias amoenus was observed to be the more social species. Neotamias minimus appears to lack the level of social structure of N. amoenus and was not observed to win any aggressive encounters. Neotamias minimus avoids interaction with N. amoenus. It is generally accepted that two species cannot occupy the same area, utilizing the same resources during the same time (Savage 1958; Whittaker et al. 1973). The question then arises as to how can two or more closely related species occupy the same locality. Competitive exclusion is a mechanism that has been proposed to explain the utilization of the same area by two similar species (Brown 1971; Connell 1961). Competitive exclusion is the exclusion of one species by another from mutually desirable resources (Armstrong and McGehee 1980). One species can exclude another from a resource through a variety of mechanisms that may include social and physical dominance. Although dominance is usually discussed within a particular species, the same principles can apply to interspecific interactions between morphologically similar species (Morse 1974). Chipmunks (genus Neotamias) are ecologically diverse rodents that can be found in a wide array of habitats across North America. Most species are contiguously allopatric and actively partition common habitat areas (Heller 1971). A number of species may share a relatively small area, but distributions tend only to be abutting with overlapping ranges kept to a minimum (Patterson 1980). Chipmunks achieve nearly non-overlapping ranges through various mechanisms of competitive exclusion such as aggression (Brown 1971; Chappel 1978; Meredith 1976; Sheppard 1971) and avoidance (Morse 1974; Sheppard 1971). The importance of interspecific dominance, as a mechanism of habitat partitioning, through interspecific aggression between chipmunk species has been supported and well documented by Brown (1971), Chappell (1978) Heller (1971), Meredith (1976), Sheppard (1971), and others. For example, it has been documented that in a laboratory setting N. amoenus is dominant over N. minimus through aggressive interactions (Sheppard 1971; Meredith 1976). Neotamias minimus is the most widely distributed of all North American chipmunks and occupies a wide spectrum of habitats such as sagebrush scrub, woodland, and alpine * Corresponding Author ([email protected]) Bull. Southern California Acad. Sci. 106(3), 2007, pp. 208–214 E Southern California Academy of Sciences, 2007