Niche theory in its various forms is based on those environmental factors that permit species persistence, but less work has focused on defining the extent, or size, of a species’ environment: the area that explains a species’ presence at a point in space. We proposed that this habitat extent is identifiable from a characteristic scale of habitat selection, the spatial scale at which habitat best explains species’ occurrence. We hypothesized that this scale is predicted by body size. We tested this hypothesis on 12 sympatric terrestrial mammal species in the Canadian Rocky Mountains. For each species, habitat models varied across the 20 spatial scales tested. For six species, we found a characteristic scale; this scale was explained by species’ body mass in a quadratic relationship. Habitat measured at large scales best-predicted habitat selection in both large and small species, and small scales predict habitat extent in medium-sized species. The relationship between body size and habitat selection scale implies evolutionary adaptation to landscape heterogeneity as the driver of scale-dependent habitat selection.

Co-occurring species are rarely considered as a factor infl uencing habitat selection. However, niche theory predicts that sharing resources, predators, and other interspecifi c interactions can limit the environmental conditions under which a species may exist. How does the spatial distribution of one species aff ect that of another within shared landscapes? We tested whether sympatric marten Martes americana and fi shers M. pennanti in a mountain landscape in Alberta, Canada exhibit local-scale spatial segregation, beyond diff erential habitat selection. We modelled marten and fi sher distribution in relation to remotely-sensed habitat data and species co-occurrence, using generalized linear models and informationtheoretic model selection. Marten and fi shers selected diff erent habitat types and showed diff erent responses to habitat fragmentation. Even after accounting for these diff erences, the absence of one species signifi cantly explained the occurrence of the other. We conclude that the spatial distribution of marten and fi shers infl uences habitat selection by each other at landscape scales, and hypothesize that this pattern may result from competition in a spatially heterogeneous environment. Species-habitat models that consider only resources may fail to capture key predictors of species ' occurrence. Reliable prediction and inference requires that ecologists expand from landscapes to also include species-scapes: a spatial plane of species interactions that combines with resources to drive species ' distributions.