Population ecology and survivorship of San Joaquin antelope squirrels in grazed and control plots in the San Joaquin Desert of California

Abstract

The San Joaquin antelope squirrel (Ammospermophilus nelsoni) is endemic to the San Joaquin Desert of California. It has been listed as Threatened by the state of California since 1980 due to profound habitat loss, but a paucity of information could limit conservation efforts for this species. We examined data collected each August during 1997–2006 to determine whether A. nelsoni population attributes differed between grazed and ungrazed study plots. We found that sex ratios, mean weights, percentage of reproductive males, number of young, yearly survival, longevity, overall survivorship, and population growth trajectories all were similar between grazed and ungrazed plots. In general, sex ratios were even, males were heavier than females, some males were reproductive in August (although most females were not), and the number of young was inversely related to residual dry matter. We also found that we captured most individuals only once, but we captured a few squirrels for up to for 5 years, and the populations on both grazed and ungrazed plots were growing during the 10-year study. Our study was only the second long-term investigation of this species and the information is needed for further conservation and recovery efforts.

Are Shrubs a Necessary Component of the Habitat for Ammospermophilus nelsoni in the San Joaquin Desert?

“…Similar rates of yearly survival were found in the 10-year grazing study on the Lokern, where 77.3% of SJAS were caught in only 1 year during annual trapping (Germano et al 2012). The greatest longevities of SJAS that we found (3 years) were exceeded by both the study by Hawbecker (1958) and the Lokern grazing study (Germano et al 2021): Hawbecker (1958) found several squirrels living to 4 years and 1 lived at least to 5 years 8 months, and during the grazing study, 11 SJAS lived at least to 4 years and 2 lived at least to 5 years. We likely would have found several squirrels living past 3 years if we had continued our trapping.…”

“…For the most part, we found the greatest number of young in May, perhaps indicating only one bout of reproduction per year, but a few young were found in summer on the shrub plot. Hawbecker (1958) believed that SJAS bred only once per year; however, during our grazing study, we found several years in which a second litter of young was produced (Germano et al 2021). Unlike our current study, the grazing study occurred over 10 years, and second litters did not occur every year.…”

“…Our quarterly estimates of survivorship give a first approximation of annual survivorship of 0.578 to 0.882 across sex, age group, and plots if quarterly survivorship did not vary within a year. Estimates of annual survivorship of SJAS based on 10 years of annual trapping at the grazing study site, however, varied from 0.317 to 0.404 (Germano et al 2021). Our quarterly estimates may indicate greater survivorship of SJAS at this site than during the grazing study, but the differences in the duration of time used to estimate survivorship makes a comparison unreliable.…”

“…A recent range-wide study of SJAS presence/absence (Cypher et al 2021) found that squirrels were not associated with shrubs in general or with shrub densities, and SJAS also were associated with kangaroo rat burrows. Another small squirrel, the Uinta Ground Squirrel (Urocitellus armatus), also appears not to be dependent on shrub cover in its habitat; when shrubs were experimentally removed from a 1.25-ha study plot in Wyoming, the population size, sex ratios, and age structure were not affected (Parmenter and MacMahon 1983).…”

“…burrows are present. While Hawbecker (1953) believed that SJAS in areas without shrubs were nonbreeders, SJAS are widespread and persistent on the Carrizo Plains, where there are thousands of acres devoid of shrubs (USFWS 1998, Cypher et al 2021. For conservation planning purposes, it is important to compare SJAS population size and demographics between shrubland and shrubless areas.…”