We hypothesized that the relative availability of meat, indicated by contribution to the diet, would be positively related to body size and population productivity of North American brown, or grizzly, bears (Ursus arctos). Dietary contributions of plant matter and meat derived from both terrestrial and marine sources were quantified by stable-isotope analysis (δ13C and δ15N) of hair samples from 13 brown bear populations. Estimates of adult female body mass, mean litter size, and population density were obtained from two field studies of ours and from other published reports. The populations ranged from largely vegetarian to largely carnivorous, and food resources ranged from mostly terrestrial to mostly marine (salmon, Oncorhynchus spp.). The proportion of meat in the diet was significantly correlated with mean adult female body mass (r = 0.87, P < 0.01), mean litter size (r = 0.72, P < 0.01), and mean population density (r = 0.91, P < 0.01). Salmon was the most important source of meat for the largest, most carnivorous bears and most productive populations. We conclude that availability of meat, particularly salmon, greatly influences habitat quality for brown bears at both the individual level and the population level.
Is the relationship between population density and body size consistent across independent studies? A meta-analytical approach