Abstract
Eight years of age-specific survival data and 6 years of fecundity data from a free-living population of Belding's ground squirrels (Spermophilus beldingi) at Tioga Pass, California, were used to test the hypothesis that time-specific life tables, based on data from individual years, were different from the cohort-specific life table, based on the combined data from all years. The results indicated that neither the age structure of the male nor the female population significantly differed among years (all P > 0.05). Furthermore, the means and the variances in the sizes of weaned litters did not differ among years either in the population at large or within individual age-classes (all P > 0.05). A 27-day snowstorm that occurred in the spring of 1977 increased mortality and reduced reproduction, but it did not change the ground squirrels' age-specific survival or fecundity patterns. Taken together, our analyses revealed that each time-specific life table provided age-specific survival and fecundity estimates that were statistically indistinguishable (P > 0.05) from the composite, cohort-specific life table for each sex, regardless of severe environmental conditions. This is the first demonstration of the equivalence of time- and cohort-specific life tables for a free-living population of mammals.
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