Fitness implications of seasonal climate variation in Columbian ground squirrels
The impacts of climate change on the fitness of terrestrial vertebrate species are often studied with respect to the spring onset of the mating season and subsequent breeding. However, the effect of climate at other times of the year might also be important, and at least sometimes have a major influence on annual fitness. We examined variation in climate variables (temperature, rainfall, snowfall, and snowpack) during 21 years of annual activity and hibernation of Columbian ground squirrels (Urocitellus columbianus). We examined relationships between climate and annual fitness, and asked whether directional fitness declines (0.03 units/year) were associated with directional changes in climate variables.
The annual fitness of adult female ground squirrels was associated with spring temperature (r = 0.691), spring snow conditions (r = -0.442 to -0.663), and early summer rainfall (r = 0.559). Directional changes in climate variables were found for spring snowmelt (r = 0.474) and summer rainfall (r = -0.525). Using path analysis, we found that directional changes in early summer rainfall, with summers getting drier over time, had a stronger influence on annual fitness than the timing of spring snowmelt (ρ = 0.445 and -0.271, respectively). The summer period corresponds to growth of recently weaned young and pre-hibernation fattening for adult ground squirrels. We concluded that the impacts of climate changes on species should be studied throughout their lifecycle events, and a primary focus only on breeding season climate variables may underestimate these impacts.