Fitness implications of seasonal climate variation in Columbian ground squirrels

Abstract

The influence of climate change on the fitness of wild populations is often studied in the context of the spring onset of the reproductive season. This focus is relevant for climate influences on reproductive success, but neglects other fitness‐relevant periods (e.g., autumn preparation for overwintering). We examined variation in climate variables (temperature, rainfall, snowfall, and snowpack) across the full annual cycle of Columbian ground squirrels (Urocitellus columbianus) for 21 years. We investigated seasonal climate variables that were associated with fitness variables, climate variables that exhibited directional changes across the study period, and finally observed declines in fitness (−0.03 units/year; total decline = 37%) that were associated with directional changes in climate variables. Annual fitness of adult female ground squirrels was positively associated with spring temperature (r = 0.69) and early summer rainfall (r = 0.56) and negatively associated with spring snow conditions (r = −0.44 to −0.66). Across the 21 years, spring snowmelt has become significantly delayed (r = 0.48) and summer rainfall became significantly reduced (r = −0.53). Using a standardized partial regression model, we found that directional changes in the timing of spring snowmelt and early summer rainfall (i.e., progressively drier summers) had moderate influences on annual fitness, with the latter statistically significant (ρ = −0.314 and 0.437, respectively). The summer period corresponds to prehibernation fattening of young and adult ground squirrels. Had we focused on a single point in time (viz. the onset of the breeding season), we would have underestimated the influences of climate change on our population. Rather, we obtained a comprehensive understanding of the influences of climate change on individual fitness by investigating the full lifecycle.

Fitness Estimation for Ecological Studies: An Evaluation in Columbian Ground Squirrels

“…Annual fitness can also be applied to longer-term studies, such as those examining influences of changing climate. In Columbian ground squirrels, variations in spring and summer climates have significant influences on annual fitness (Lane et al, 2012;Dobson et al, 2016). Over a 20-year period, when adult females emerged from hibernation earlier, their annual fitness was greater (Figure 2).…”

“…In this sense, trait groups can be considered alternative adaptations that respond to environmental conditions. Good examples are studies of the influence of global changes in climate on the timing of reproductive events or other elements of a species' lifecycle (e.g., Visser and Both, 2005;Chamaillé-Jammes et al, 2006;Parmesan, 2006;Lane et al, 2012;Tafani et al, 2013;Dobson et al, 2016;Radchuk et al, 2019). An important caveat is that individuals have many traits, some of which may be genetically correlated through linkage disequilibrium or pleiotropy (e.g., Duckworth and Kruuk, 2009;Bize et al, 2017;Mullon et al, 2018).…”

“…Herein, our goals were: first, to review our longterm studies of reproduction and survival in a comparison of annual and lifetime estimates of fitness, and lifetime reproductive success and individual fitness estimates from matrix models. We expected that annual and lifetime estimates of fitness might not correspond closely, since environmental conditions may vary among years (Lane et al, 2012;Dobson et al, 2016). Second, to test this expectation, by comparing annual and alternative lifetime fitness measures for correspondence.…”

Phenotypic Plasticity and Local Adaptation in a Wild Hibernator Evaluated through Reciprocal Translocation

“…Brady, D. Bolnick, R. Barrett, et al, unpublished manuscript). Such an effect could potentially be due to a shortened growing season resulting from the delay and is likely exacerbated by a separate trend for warmer and dryer conditions occurring later in the season (leading, presumably, to desiccation of the ground squirrels' plant food resources; Dobson et al 2016).…”

“…We calculated daily mean ambient temperature (T a ) and mean precipitation based on daily recordings taken at the weather stations and used a sliding window approach to determine the relevant periods over which to represent annual values. The sliding window approach is a commonly used technique to identify relevant weather periods (van de Pol et al 2016) and has previously been used in this context for Columbian ground squirrels in this area (Lane et al 2012;Dobson et al 2016). We varied the windows from 2 to 227 days and considered all possible windows from November 1 to June 15.…”