Living on Winter’s Edge: Snow Ecology of Large Mammals in Northern Ecosystems

Abstract

Climate change is rapidly reshaping snowpacks around the world, with dramatic impacts on wildlife. My dissertation explored the role of snow in shaping predator–prey relationships and movement ecology, focusing on large mammals in the northern United States of America and Canada. Chapter 2 used winter fieldwork to quantify how specific snow properties influence the sink depths of tracks made by predators (bobcats, cougars, coyotes, and wolves) and their sympatric ungulate prey (caribou, deer, moose). Using generalized additive models, breakpoint regression, and animal morphology, I found that near-surface snow density is the most significant predictor of sink depth, with different thresholds for predators and prey. I identified "danger zones" where snow conditions impede ungulates’ movement more than predators, providing a framework to predict the impacts of climate-driven snow changes on predator–prey dynamics. Chapter 3 applied an entirely different methodological approach to investigate whether these danger zones factor into apex predator habitat selection. Using GPS collar data for cougars, wolves, mule deer, and white-tailed deer, I found that snow depth and density significantly affect predator movement, with predators selecting for areas with either shallow or with deep, dense snow. Kill-site investigations for cougar-killed deer indicated similar snow preferences. In Chapter 4, I examined winter movements from an ungulate perspective, using GPS collared data from five moose populations across their entire latitudinal range. I found that, even in winter, high temperatures cause moose to adjust behavioral patterns and alter habitat selection, potentially forgoing foraging opportunities in favor of thermal refuge. These changes were most dramatic in southern moose and may be a contributing factor to recent moose range contraction along their southern margins. Finally, Chapter 5 integrated physically based snow modeling with climate projections to predict how changing snowpacks will alter predator–prey interactions. I focused on two apex predators, cougars and wolves, and their primary prey, mule deer and white-tailed deer, under two mid-century climate change scenarios (SSP2-4.5 and SSP5-8.5). My results showed significant declines in snow-mediated advantages for predators, with a reduction in both the duration and extent of snowpack conditions that favor predator movement (as identified in Chapter 3). These changes are expected to have disproportionate impacts on apex predators, potentially leading to shifts in predator home ranges, prey densities, and ecosystem dynamics. Collectively, these chapters underscore the profound effects of climate-driven changes in snowpack dynamics on large carnivore and ungulate behavior, with potential for significant shifts in predator–prey interactions across the Northern Hemisphere.