Ecological studies investigating niche breadth and overlap often have limited spatial and temporal scale, preventing generalizations across varying environments and communities. For example, it is not clear whether species having restricted diets maintain such patterns relative to closely related species and across their geographic range of co-occurrence. We used stable isotope analysis of hair and fur samples collected from four regions of sympatry for Canada lynx (Lynx canadensis) and bobcat (Lynx rufus) spanning southern Canada and the northern United States, to test the prediction that the more generalist species (bobcat) exhibits a wider dietary niche than the more specialist species (Canada lynx) and that this pattern is consistent across different regions. We further predicted that Canada lynx diet would consistently exhibit greater overlap with that of bobcat compared to overlap of bobcat diet with Canada lynx. We found that Canada lynx had a narrower dietary niche than bobcat, with a high probability of overlap (85–95%) with bobcat, whereas the bobcat dietary niche had up to a 50% probability of overlap with Canada lynx. These patterns of dietary niche breadth and overlap were consistent across geographic regions despite some regional variation in diet breadth and position, for both species. Such consistent patterns could reflect a lack of plasticity in species dietary niches. Given the increasingly recognized importance of understanding dietary niche breadth and overlap across large spatial scales, further research is needed to investigate the mechanisms by which broad-scale patterns are maintained across species and systems.

Spatial and temporal variation in prey availability can affect predator foraging strategies, but it is unclear how large‐scale gradients in prey availability shape predator diets in space and time. The facultative specialist hypothesis predicts that niche expansion by specialist predators is advantageous when preferred prey are limited and alternate prey are available, as might be expected following the collapse of primary prey populations in species‐poor environments or at range margins where species assemblages shift. Canada lynx (Lynx canadensis) are broadly distributed and prey predominantly on snowshoe hares (Lepus americanus), with other species serving as alternate prey when hares are limited. We used stable isotope analysis of lynx, snowshoe hare, and red squirrel hair during periods of high/low snowshoe hare availability from five regions throughout their distribution, to measure spatiotemporal changes in diet and test the facultative specialist hypothesis. Hares were the primary food item in all lynx diets, irrespective of space and time. Core lynx populations consumed the greatest proportion of hares overall and increased the use of alternate prey when hares were limited. Despite fluctuations in hare availability, lynx toward the southeastern extent of their distribution invariably consumed alternate prey in similar proportions to those exhibited by core populations during hare lows. The temporal consistency of lynx diets in these regions indicates that broader diets are obligate along this distribution edge. Our results provide new evidence for the facultative specialist hypothesis across a broad spatial scale and emphasize that, as specialist–generalist paradigms operate along a continuum, so too can obligate–facultative strategies depending on environmental context.