The facultative specialist hypothesis predicts that niche expansion by specialist predators is advantageous when preferred prey are limited and alternative prey are available, yet patterns of niche variation differ across spatiotemporal gradients and it remains unclear how environmental characteristics such as prey availability and community structure affect foraging strategies of free-living organisms. Understanding of facultative foraging has been constrained by narrow spatial or temporal scales because of methodological limitations; however, increasing refinement of diet reconstruction using stable isotope analysis enables examination of foraging dynamics across large geographic areas. Canada lynx (Lynx canadensis) are facultative specialists of snowshoe hare (Lepus americanus) and both experience cyclic fluctuations in abundance, but snowshoe hare fluctuations are attenuated towards southern latitudes where availability of alternative resources for predators is higher. Using stable isotope analysis, I tested the effect of biotic interactions on foraging strategies of lynx across spatiotemporal scales to elucidate a broader perspective on adaptive foraging decisions in free-living organisms. In northern populations, where availability of alternative prey in winter is largely restricted, declining hare abundance prompted differential niche expansion among individuals, whereby newly independent lynx used alternative prey but adults maintained highly specialized diets. The relative consumption of alternative prey was consistently higher and less variable in southern lynx populations, regardless of fluctuations in hare abundance, indicating that a more generalized foraging strategy is obligate for all individuals in these areas. Although southern lynx fed more broadly than northern conspecifics, their diet breadth remained narrow when compared with sympatric bobcat (Lynx rufus), a closely-related generalist species that also prefers lagomorphs. Across four different communities, lynx diets overlapped completely with bobcat, but over half of bobcat fed differently from lynx, suggesting high potential for competition and supporting the generality of the specialist-generalist paradigm of foraging strategies at large spatial scales. Finally, I investigated potential consequences of niche expansion to individual stress physiology, but found no relationship between chronic stress and prey choice. Collectively, my thesis demonstrates how prey availability and community complexity differentially affect the facultative foraging strategies of a free-living specialist predator, substantiating the facultative specialist hypothesis at an exceptionally large geographic scale.