Resource Partitioning By Sympatric Brown And American Black Bears

Abstract

The fundamental niche of a species is rarely if ever realized because the presence of other species restricts it to a narrower range of ecological conditions. Additionally, distribution theory predicts that for two competing species living in sympatry, the subordinate species will be constrained from optimal resources. This constraint would result in use of lower quality resources by the subordinate species and possible spatial segregation from the dominant species. I evaluated diet in relation to body condition and reproduction for sympatric brown bears (Ursus arctos) and American black bears (U. americanus) in southcentral Alaska during 1998-2000, and assessed spatial segregation and habitat selection in 2000. Based on isotopic analysis, salmon (Onchorhynchus spp.) predominated in brown bear diets (>53% annually) whereas black bears assimilated 0-25% salmon annually. Black bears did not exploit salmon during 1998, a year with below average spawning numbers, probably because brown bears deterred black bear access to salmon. Enhanced body condition (as indexed by increased percent body fat) from salmon consumption resulted in better body condition the following spring. Further, black bear reproduction was directly related to body condition; reproductive rates were reduced when body condition was poorer. Analyses of radio location data confirmed that 24-hour monitoring of bears was necessary to determine habitat use and that habitat use varied seasonally. Black bears avoided areas occupied by brown bears during summer, supporting the ideal despotic distribution model. In contrast, black bears selected areas where brown bears were present during spring, presumably because of spatially-restricted (i.e., restricted to low elevations) but dispersed availability of food. Similarities in preferred and potentially limited resources resulted in co-occupancy of areas at intermediate to coarse spatial resolutions; however, spatial avoidance of brown bears and black bears influenced population-level use of resources. Further, the realized niche of black bears was constrained by brown bears through partitioning of food resources, which varied among years. Reduced access to salmon caused black bears to forage more extensively in areas containing less nutritious food, resulting in lowered body condition and subsequent lowered reproduction. Coexistence of these species in this study area appears dependent on the distribution, abundance, and availability of salmon and berries.