The importance of meat, particularly salmon, to body size, population productivity, and conservation of North American brown bears

Abstract

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.

Testing for a predicted decrease in body size in brown bears (Ursus arctos) based on a historical shift in diet

“…The femur lengths of males at the subpopulation level were positively correlated with the ␦ 15 N values, and because ␦ 15 N values are higher at the predator trophic level (DeNiro and Epstein 1981;Minagawa and Wada 1984), the mean skeletal size of the male bear subpopulations was strongly correlated with their trophic status, which is consistent with other studies from North America (Hilderbrand et al 1999;Mowat and Heard 2006). The result suggests that the body size of male bears is adapted to regional nutrient availability over several generations but not within a generation, and the geographical variation in diets causes the geographical variation in the body size of brown bear subpopulations.…”

“…female body size remains inconclusive. A larger body size also increases the reproductive success of female bears (Stringham 1990;Hilderbrand et al 1999;Zedrosser et al 2007), but a larger size is not always adaptive for females due to the important role of caring for the offspring. Because female bears with cubs sometimes face the risk of infanticide by male bears (Dahle and Swenson 2003;Ben-David et al 2004;McLellan 2005), larger females should be better at defending their offspring, as they are likely stronger than small females.…”

“…The feeding habits of animals exert strong effects on their body size (Mahoney et al 2001;Eifler et al 2003), and a positive correlation between dietary meat intake and body size of brown bears has been demonstrated in several studies (Hilderbrand et al 1999;Mowat and Heard 2006;Zedrosser et al 2007). In mammals, a larger body size influences various ecological and physiological factors, including selection of available dietary resources (McNab 1971), reproductive success (Andersson and Iwasa 1996), and population growth and density (Atkinson and Ramsay 1995;Post et al 1997;Laundré et al 2007;Wauters et al 2007).…”

“…To our understanding, such a dietary shift from a carnivorous to a largely herbivorous diet in omnivorous wild animals has not been reported previously elsewhere in the world. Because the feeding habits of brown bears are strongly correlated with their body size (e.g., Noyce and Garshelis 1994;Hilderbrand et al 1999;Mowat and Heard 2006;Zedrosser et al 2007), the dietary shift in the Hokkaido population has potentially changed the body size of individuals over time. For this reason, we predict that the change in the feeding habits of the Hokkaido brown bears affected their body size.…”

Linking Wolf Diet to Changes in Marine and Terrestrial Prey Abundance

“…For instance, as salmon continue to colonize streams in Glacier Bay (Milner et al, 2007), they may become an important seasonal food for wolves, as demonstrated in coastal British Columbia (Darimont et al, 2008), other areas of Southeast Alaska (Szepanski et al, 1999), and interior Alaska (Adams et al, 2010). In our study, it is likely that scat analysis resulted in an underrepresentation of salmon in wolf diets not only because these fish are highly digestible (Hilderbrand et al, 1999), but also because scat collection occurred before the peak of spawning season. Thus, wolf use of salmon may have been greater than suggested by our results.…”