Population Ecology of Uinta Ground Squirrels

Abstract

Abstract. The behavior and ecology of Uinta ground squirrels (Spermophilus armatus) at the Utah State University Forestry Field Station northeast of Logan, Utah, were studied 1964-71 to determine the role of behavior in population regulation. In 1968 the population was reduced experimentally to about one-half the previous density.The study area consisted of a lawn area where resident squirrels more than replaced themselves, and an area of mixed shrubs and grasses where they did not. Surplus squirrels from the lawn raised the population density off the lawn. Also, habitat suitabilities varied with population densities. Hence, densities in the two areas were not proportional to habitat preferences.Before the reduction the population density fluctuated widely but the mean change for 5 yr was approximately zero. A potentially high rate of increase was curtailed by disappearance of juveniles and yearlings; juveniles tended to disperse from the natal burrow. The principal changes following the reduction were increases in percentage of yearling squirrels breeding and of juveniles remaining on the study area. Litter size and proportion of adult females breeding did not change significantly, but losses during hibernation, due in part to badger predation, decreased. Dispersal of squirrels from the study area played a key role in population regulation.Dispersers probably suffered higher mortality than sedentary squirrels, but dispersal was not necessarily maladaptive for the individual. Dispersers may have been unable to produce young at high densities; thus emigration offered the only opportunity for reproduction.

Effect of Density Reduction on Uinta Ground Squirrels: Analysis of Life Table Response Experiments

“…Virtually all squirrels in each habitat were captured, marked, and followed until disappearance from the study area. Originally, a life table was constructed for the female segment of the population in each habitat (Slade and Balph 1974; hereafter ''pre-reduction life tables''). During the second phase of the study (1968)(1969)(1970), population density in each habitat was reduced by ϳ60% using a stratified random sampling protocol such that sex and age composition remained unchanged.…”

“…However, ␣ in some species inhabiting highly seasonal temperate and arctic environments shows little or no change even under experimental perturbations in the environment (e.g., Dobson 1988, Dobson and Michener 1995, Jouventin et al 1999. Conversely, survival and fertility rates may be extremely labile from habitat to habitat or year to year (e.g., Slade and Balph 1974, Dobson and Murie 1987, Dobson 1988. LTRE analysis combines sensitivity of to life history variables with observed demographic changes and thus quantifies the actual influence of demographic variables when population growth rate changes over time or space (Caswell 1989a, b, 2000, Levin et al 1996, Horvitz et al 1997, Ehrlèn and van Groenendael 1998.…”

“…Such experiments may involve manipulations that artificially cause changes in population growth rate, such as food supplementation (e.g., Cole and Batzli 1978, Jansson et al 1981, Dobson 1988, 1995, Guyer 1988, Boutin 1990), decrementing population size (e.g., Slade and Balph 1974, Arcese and Smith 1988), manipulation of predators (e.g., Marcströ m et al 1988, Krebs et al 1995, exposure to pollutants (e.g., Levin et al 1996), or a combination of several factors (e.g., Birch 1953, Taitt and Krebs 1983, Rao and Sharma 1986, Krebs et al 1995. The majority of such studies, particularly those of terrestrial vertebrates, have not applied LTRE analyses (but see Brault and Caswell 1993), although several have documented changes in demographic variables that could cause changes in population growth rate.…”

“…The purpose of our study was to apply LTRE analysis in a reexamination of an experimental reduction in population density that was applied to Uinta ground squirrels (Spermophilus armatus) by Slade and Balph (1974). This species was particularly appropriate for our study, not only because of the experimental treatment, but also because habitat-and age-specific life table data were gathered.…”

Uinta Ground Squirrel Demography: Is Body Mass a Better Categorical Variable Than Age?

“…However, we could not compare the relative importance of age and body mass in affecting fecundity and survival because the study animals could not be accurately aged. Slade and Balph (1974) constructed traditional life tables for female Uinta ground squirrels (Spermophilus armatus) near Logan, Utah. Because both age and body mass are known for most of these squirrels, compari- sons of age and mass effects on litter size and survival are possible, and there is evidence that both might be important.…”