Asymmetric niche partitioning in large omnivores in response to anthropogenic disturbances within subarctic ecosystems.

Niche partitioning is an evolutionary process that allows the coexistence of multiple species in a landscape. However, human disturbance can interfere with this process. For instance, humans may displace competitors and favour species with a greater tolerance for disturbances. American black bears (Ursus americanus) and grizzly bears (U. arctos) are two large omnivores that share similar ecological niches, but that can also coexist by partitioning resources. Understanding the consequences of anthropogenic disturbance on niche partitioning in these two species is important to preserve biodiversity and ecosystem function. This importance is particularly relevant in landscapes located in subarctic biomes where resource availability is limited. In this study, we used data from wildlife cameras deployed at 194 sites in central Yukon during 2020-2023 to determine whether anthropogenic disturbances facilitated, or disrupted, spatiotemporal niche partitioning in American black bears and grizzly bears. Kernel density functions revealed that grizzly bears modified their diel activity pattern in the higher-disturbance landscape during the afternoon, presumably to avoid interactions with humans. This new peak coincidentally promoted temporal niche partitioning with black bears, whose activity pattern remained static across the gradient of disturbance. The bear species spatially segregate, but anthropogenic disturbances had a much stronger effect on grizzly bear occurrences compared to black bears, signalling an asymmetric response. Anthropogenic disturbances constrained grizzly bears to the lower-disturbance landscape. Black bears could take advantage of reduced competition and partially fill the spatiotemporal niche of grizzly bears, but our results did not show that black bears concentrated their activity in areas avoided by grizzly bears. The displacement of grizzly bears may lead to shifts in prey populations or vegetation communities, with unknown consequences for ecosystem structure.