Ecological and demographic effects on intraspecific variation in the social system of prairie dogs

Ecology, Sept, 1995 by Steven E. Travis, C.N. Slobodchikoff, Paul Keim

By considering population density as an additional factor associated with variations in social group size and composition, we have increased the predictive power of the habitat variability-mating system model (Slobodchikoff 1984). As originally formulated, this model predicted the initial formation of social groups in patchy environments, with the maintenance of monogamous pairs in more uniform environments. Not only was this model supported by evidence from several studies of prairie dogs (Slobodchikoff 1984, Travis and Slobodchikoff 1993), but it has also proven consistent with the findings of numerous investigations of other vertebrate species. These include studies of marmots (Marmota spp.), which have allowed comparisons of resource distributions and social systems across species (Merriam 1971, Barash 1974, Andersen et al. 1976, Armitage 1977), African ungulates (Jarman 1974), and primates (Southwick and Siddiqi 1974, Milton and May 1976, Clutton-Brock and Harvey 1977).

Our results further suggest that levels of inbreeding displayed by Gunnison's prairie dogs undergo predictable changes with increases in population density. While we have discussed reasons for expecting high levels of inbreeding among the members of stable populations at low densities, and a switch to outbreeding at densities approaching environmental saturation, evidence from previous studies in support of these predictions has yet to be advanced. Few studies have dealt directly with the relationship between environmental stability and inbreeding levels. However, several studies provide evidence suggestive of such a relationship. Foremost among these is a study by Wool and Bergerson (1986) on the flour beetle (Tribolium castaneum), in which both inbred and outbred lines were created and then subjected to two environmental treatments. Inbred lines were found to perform better under the constant environmental treatment, as determined by variation in several fitness characters, than under the randomly varying environmental treatment; the situation for outbred lines was reversed. Two other studies provide additional evidence suggesting the advantages of alternative mating tactics under changing environmental conditions. These studies, conducted at sites in the southern Appalachian mountains, attempted to contrast genetic heterogeneity of natural populations under stressful and nonstressful conditions. The first study dealt with two species of land snails, Mesomphix subplanus and M andrewsae (Stiven and Bruce 1988), while the second dealt with a stream salamander, Desmognathus quadramaculatus (Stiven 1989). In both studies, elevated levels of genetic heterogeneity were found to represent populations from commercially logged areas rather than undisturbed sites. Whether these differences were due to a switch from inbreeding to outbreeding with increasing habitat instability, to increases in additive genetic variance (as suggested by Bryant et al. 1986a, b, and Goodnight 1987, 1988), or to increased immigration relative to disturbed areas, was not determined.