Interspecific Resource Partitioning in Sympatric Ursids

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. The effects of this narrower range of conditions define how resources are partitioned. Resource partitioning has been inferred but not demonstrated previously for sympatric ursids. We estimated assimilated diet in relation to body condition (body fat and lean and total body mass) and reproduction for sympatric brown bears (Ursus arctos) and American black bears (U. americanus) in south-central Alaska, 1998-2000. Based on isotopic analysis of blood and keratin in claws, salmon (Oncorhynchus spp.) predominated in brown bear diets (> 53% annually) whereas black bears assimilated 0-25% salmon annually. Black bears did not exploit salmon during a year with below average spawning numbers, probably because brown bears deterred black bear access to salmon. Proportion of salmon in assimilated diet was consistent across years for brown bears and represented the major portion of their diet. Body size of brown bears in the study area approached mean body size of several coastal brown bear populations, demonstrating the importance of salmon availability to body condition. Black bears occurred at a comparable density (mass:mass), but body condition varied and was related directly to the amount of salmon assimilated in their diet. Both species gained most lean body mass during spring and all body fat during summer when salmon were present. Improved body condition (i.e., increased percentage body fat) from salmon consumption reduced catabolism of lean body mass during hibernation, resulting 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 lower. High body fat content across years for brown bears was reflected in consistently high reproductive levels. We suggest that the fundamental niche of black bears was constrained by brown bears through partitioning of food resources, which varied among years. Reduced exploitation of salmon caused black bears to rely more extensively on less reliable or nutritious food sources (e.g., moose [Alces alces], berries) resulting in lowered body condition and subsequent reproduction.

The role of forage availability on diet choice and body condition in American beavers (Castor canadensis)

“…2). reconstructed beaver diets using stable isotope analysis with similar results, and the method has been employed using claws (Belant et al, 2006) and IsoError (Phillips, 2012). Either our vegetation survey failed to detect aquatic vegetation, or beavers were foraging beyond our defined 400-m radius foraging area.…”

“…Isotopic signatures can be used to separate plants into broad categories based on such qualities as photosynthetic pathway or carbon source (e.g., aquatic macrophytes from terrestrial plants; LaZerte and Szalados, 1982;Keeley, 1998;Cloern et al, 2002;. Keratin (in claws, hooves, or nails) provides less variable results than soft tissue when used to estimate assimilated diet (Kielland, 2001;Struck et al, 2002;Belant et al, 2006), and can be sampled to represent a temporal record of assimilation (Mizukami et al, 2005).…”

“…Reduced foraging time can limit energy gained by an animal, adversely affecting body condition and subsequent reproductive success (McNamara and Houston, 1992;Belant et al, 2006). For example, American beaver (Castor canadensis) growth rates depend on available forage, as well as climate, and degree of population exploitation (Baker and Hill, 2003).…”

Isotopic evidence for omnivory among European cave bears: Late Pleistocene Ursus spelaeus from the Peştera cu Oase, Romania

“…Large adult males also appear to be more carnivorous than females or subadult bears (28,29). North American black bears (U. americanus) appear to have similar plant/meat dietary proportions as brown bears (29), except that the larger brown bears are frequently more carnivorous when the prime meat is maritime (e.g., salmon) (46). This ecological flexibility of modern brown bears therefore makes an appropriate model to understand the range of isotopic values now available for European cave bears, both within and between site-specific samples.…”

Grizzly bear monitoring by the Heiltsuk people as a crucible for First Nation conservation practice

“…Available salmon biomass declined up to 2008, which supports this hypothesis. When salmon availability is low, both physiological, i.e., nutritional, (Boulanger et al 2004a, Belant et al 2006, Bryan et al 2013) and social, i.e., dominancy hierarchies, (Gende and Quinn 2004) factors have been implicated http://www.ecologyandsociety.org/vol19/iss2/art70/ in lower reproductive output. This coupling between salmon and grizzly abundance is consistent with observations ) and modeling (Levi et al 2012) across larger, cross-population spatial scales.…”