Princes and Paupers: Effects of Annual and Early-life Resource Variability on Demography in Populations of Common Garter Snakes, Thamnophis Sirtalis
Resource availability, acquisition, and assimilation drive trade-offs and shape life-history strategies within individuals and across populations. With limited dispersal ability and notable life-history variation, garter snakes provide unique model systems for understanding population responses to variable environments. We investigated the relationship between demographic parameters and the environment for montane populations of Common Garter Snakes (Thamnophis sirtalis) using a 16-year mark-recapture data set. We quantified patterns in survival, reproduction, and growth and how they vary between sexes, among populations, and in response to annual variability in resource conditions. We found that reproductive output increased in years when prey availability was greater. In contrast, survival varied among populations and between males and females but not with respect to annual differences in environmental conditions. We found an interaction between early-life resource conditions and sensitivity to resource availability in later life. Growth was accelerated in years of higher prey availability for individuals that experienced poor prey conditions in their first year of life. Overall, our findings reveal spatiotemporal variation in growth and reproduction that are consistent with fluctuations in local resources and are complemented by trait plasticity in populations of a sympatric garter snake. Our results provide important insights regarding demography of ectothermic vertebrates with indeterminate growth and elucidate influences of annual variation in prey abundance on survival, reproduction, and growth and effects of early-life conditions. Importantly, our results suggest that efforts to predict how populations will respond to fluctuating environments should incorporate early-life environment to account for context-dependent variation in demographic processes.ABSTRACT
Study area near Eagle Lake, California, USA. Collection sites for mark-recapture study in northern California. Common Garter Snakes (Thamnophis sirtalis) were hand-captured, processed, and released.
Estimates of litter size across prey-year conditions. Mean estimates and 95% confidence intervals for litter size in low-prey and high-prey years for four disparate populations (M2, M3, M4, M8) of Common Garter Snakes (Thamnophis sirtalis) during both high- and low-prey conditions.
Structural growth analysis 1—prey-year (low-prey, high-prey) growth rate for four montane populations (A–D) of Common Garter Snake (Thamnophis sirtalis) in northern California.
Mean estimates and 95% credible intervals for sex-specific average growth coefficient 
presented as 
such that higher values can be interpreted as faster growth.
Structural growth analysis 2—early-life cohort (resource-poor, resource-rich) across high- and low-prey year growth for four combined populations of Common Garter Snake (Thamnophis sirtalis) in northern California.
Mean estimates and 95% credible intervals for early-life cohort average growth coefficient presented as such that higher values can be interpreted as faster growth.
Body condition analysis 1—prey-year condition. Relative body condition across four montane populations of Common Garter Snakes (A–D) in low and high-prey environments.
Effect size and 95% confidence interval for body condition predicted by the top model as determined by Akaike's Information Criterion for small sample size (AICc) with the form Body Condition ∼ PreyYear × Sex + Population.
Body condition analysis 2—cohort condition. Relative body condition across four montane populations of Common Garter Snakes (A–H) grouped by resource-rich and resource-poor cohorts and low- and high-prey years between males and non-gravid females.
The top model as determined by Akaike's Information Criterion Criterion corrected for small sample size (AICc) is the full model with interactions between nutritional cohort (resource-rich and resource-poor), sex, prey-year (high-prey and low-prey) and population (M2, M3, M4 and M8).
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