Environmental Predictability, Variability, and Spermophilus columbianus Life History over an Elevational Gradient

Abstract

We quantitatively tested various aspects of the theory of r— and K—selection for six populations of Columbian ground squirrels from Alberta, Canada. Three measures of environmental predictability (maximum and minimum temperatures, precipitation) and a presumed measure of food resource levels supported the prediction that environments at lower elevations were less predictable, and had greater, more variable food resource levels than environments at higher elevations. Columbian ground squirrels in more predictable environments (i.e., at higher elevations) had higher adult survival rates, later ages at maturity, and possibly lower genetic variabilities than did squirrels in less predictable environments (at lower elevations). Body mass was greater at lower elevations than at higher elevations. Litter size showed no trend with respect to elevation, but it tended to be most variable in unpredictable environments at lower elevations. Although they were more predictable, the daily minimum temperatures at higher elevations tended (P = .06) to show wider variation than the minimum temperatures at lower elevations. Previous apparent problems with r—K theory may be attributable in part to the assumption that predictability and stability should covary. The major difference between the pattern emerging from our study and that predicted by traditional theory is that predictability of environments, and concomitantly the occurrence of K—strategists, was found to increase with movement up an elevational gradient. Portions of r—K theory may be found to be useful once all of its parameters are consistently measured.