Abstract
Understanding the ultimate drivers of evolution and population regulation are core goals of ecology. Climate change can affect the evolution and persistence of free-ranging animals via myriad mechanistic pathways, and so is of particular interest to researchers, policy makers, and the public. Negative changes in abundance and demographic vital rates in response to climate change have been documented in some animals, but the mechanisms underpinning these relationships are often not well understood such that we lack the ability to make generalized predictions about the effects of future climate change on animal behavior, abundance, and evolution. Furthermore, species and individual level traits likely mediate the effects of climate on animals. For example, species with strongly seasonal annual cycles – like obligate hibernators – may be particularly sensitive to climate change. A better understanding of how climate interacts with and influences variation in behavior and life-history is needed to explain patterns observed in nature and inform the conservation of species, biological communities, and ecological processes.
We investigated mechanisms shaping behavior and demography of the northern Idaho ground squirrel (Urocitellus brunneus), a federally threatened species endemic to a small portion of west-central Idaho, USA. The squirrels suffered population declines in the late 20th century and now persist in a fragmented metapopulation consisting of <3000 individuals. These small herbivores hibernate for 8.0-9.5 months per year, leaving just few months during which they must reproduce and attain sufficient fat reserves to survival their long hibernation period and reproduce in future years. Food limitation resulting from anthropogenic fire suppression and/or competition with the larger, co-occurring Columbian ground squirrel may constrain northern Idaho ground squirrel populations. Climate, therefore, potentially influences northern Idaho ground squirrels in at least 3 ways: by 1) altering time and energy allocated to thermoregulatory behavior, 2) influencing forage availability directly, and 3) modulating competitive interactions with Columbian ground squirrels. We sought to test predictions regarding the effects of climate, hibernation, competition, and endogenous state on northern Idaho ground squirrel foraging activity and survival based on these hypothesized mechanistic pathways.
Chapter 1 tests predictions of Optimal Foraging Theory to explain variation in the amount of time northern Idaho ground squirrels allocate to aboveground foraging activity. Specifically, we sought to elucidate the effects of thermal environment, predation risk, and endogenous state on squirrel foraging activity. We also considered the effects of hibernation behavior and the squirrel’s strongly seasonal annual cycle on their foraging behavior – a novel aspect of this study. Foraging time increased as the aboveground active season progressed and hibernation approached, suggesting energetic thresholds imposed by that extreme adaptation to seasonally unsuitable conditions drive foraging behavior during the active season. Furthermore, heavy squirrels foraged less relative to lean squirrels as hibernation approached, indicating that squirrels in poor body condition accepted increasingly greater predation risk compared to their heavier counterparts as residual foraging opportunities diminished. Extreme temperatures also negatively influenced time spent foraging, meaning that an increase in overall temperature as well as increases in extreme weather events expected as a result of climate change may constrain foraging opportunities for these threatened squirrels. Management actions that improve forage abundance and quality, therefore, are likely to increase squirrel fitness and so populations by allowing squirrels to better avoid aboveground risks posed by extreme thermal conditions and predation while still attaining the necessary fat reserves to survive their long hibernation period and reproduce in the future.
Chapter 2 considers the effects of seasonal temperature and precipitation metrics on survival of northern Idaho ground squirrels as well as two coexisting species – Columbian ground squirrels (Urocitellus columbianus) and yellow-pine chipmunks (Tamias amoenus) – with an eye toward predicting the effects of climate change on populations of these imperiled squirrels and herbivorous and hibernating animals more broadly. We also tested for hypothesized negative effects of competition with the larger, socially dominant Columbian ground squirrel on northern Idaho ground squirrels and whether changes in climate have implications for competitive interactions between the species. Northern Idaho ground squirrel survival was negatively density dependent, suggesting the squirrels are, in fact, food or space limited. Moreover, Columbian ground squirrel density had a sharp negative effect on northern Idaho ground squirrel survival during the active season, compelling evidence of competition between the two ground squirrel species. Winter snowfall negatively influenced northern Idaho ground squirrel survival, but positively and strongly influenced Columbian ground squirrel survival, suggesting that climate modulates competition between the congeners by influencing Columbian ground squirrel survival and so abundance and distribution. Changes in forage availability related to winter snowpack likely underpin these patterns, and this is one of the first studies to demonstrate such a mechanistic pathway by which climate can influence the demography of coexisting species. The apparent negative effects of competition on northern Idaho ground squirrel survival suggest that management actions aimed at benefiting the threatened squirrels are most likely to be successful in areas where they will not be subsequently outcompeted by Columbian ground squirrels or when coupled with control of the larger squirrels.