Andrew Derocher, James Roth, Geoff Koehler, Kevin Kardynal, David McGeachy, Nicholas Pilfold, Keith Hobson

Intraspecific competition, including conspecific killing, can structure populations by driving niche partitioning through differentiated diet or space use. Using extensive stable isotope and satellite-tracking datasets, we quantified trophic niche variation in polar bears across the Canadian Arctic and, focusing on western Hudson Bay, examined how diet constrained by low prey diversity shapes space use on sea ice. We hypothesized that large-scale spatial variation in environmental conditions and prey assemblages drives multiscale trophic niche variation in polar bears. At the subpopulation level, greater prey diversity should expand dietary niche width, and larger niche width should be attained through partitioning between dominant and subordinate demographic groups, while at the individual level, dietary variability within demographic groups should increase with overall subpopulation niche width. We further hypothesized that in regions of low prey diversity, intensified intraspecific competition for prey hotspots promotes spatial structuring between demographic groups to reduce interactions with competitively dominant conspecifics. We found that environmental heterogeneity promotes trophic differentiation, whereas low prey diversity favors spatial niche partitioning, evidenced by segregation of females with young cubs (the demographic group most vulnerable to conspecific predation), and to a lesser extent, other adult females. This work explicitly links diet and space use, providing new insights into context-dependent niche partitioning in an apex predator on sea ice, improving predictions of the impacts of climate-driven sea-ice loss on Arctic food webs.