Environmental influences on spatial and temporal patterns of body-size variation in California ground squirrels (Spermophilus beecheyi)

Aim In order to understand how ground squirrels (Spermophilus beecheyi) may respond to future environmental change, we investigated five biotic and environmental factors potentially responsible for explaining body-size variation in this species across California. We examined the concordance of spatial patterns with temporal body-size change since the last glacial maximum (LGM). Location California, western North America. Methods We quantified body size of modern populations of ground squirrels (n = 81) and used a model-selection approach to determine the best variables (sex, vegetation, number of congeners, temperature and/or precipitation) explaining geographical variation in body size among modern populations. We also quantified body size of one fossil population in northern California (n = 39) and compared temporal body-size change in S. beecheyi at this location since the LGM with model predictions. Results Body size of modern populations conformed to Bergmann's rule, with larger individuals in northern (wetter and cooler) portions of California. However, the models suggest that precipitation, rather than temperature or other variables, may best explain variation in body size across modern spatial gradients. Our conclusion is supported by the temporal data, demonstrating that the body size of S. beecheyi has increased in northern California since the LGM, concordant with precipitation but not temperature change in the region. Main conclusions Precipitation, rather than temperature, vegetation or number of congeneric species, was the main factor explaining both spatial and temporal patterns of body-size variation in S. beecheyi. The integration of space and time provides a powerful mechanism for predicting how local populations may respond to current and future climatic changes.