Characteristics of a Naturally Regulated Grizzly Bear Population

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Keay, J. A., C. T. Robbins, and S. D. Farley. 2018. Characteristics of a naturally regulated grizzly bear population. Journal of Wildlife Management 82(4):789-801.

ABSTRACT: Knowledge of density-dependent responses in grizzly bears is largely circumstantial yet critical to managing populations near carrying capacity. We studied grizzly bear (Ursus arctos) body condition and demographics in Denali National Park, Alaska, USA, 1991–1998, and compared our results with data from a nearby heavily hunted population to better understand the characteristics of a naturally regulated bear population. We captured grizzly bears just after den emergence in spring (May) and just before den entrance in fall (Sep). We measured body fat using bioelectrical impedance analysis, estimated bear ages, and fitted adult, subadult, and 2-year-old dependent bears with telemetry-collars. We used capture-mark-resight techniques to estimate density. We estimated grizzly bear density at 37 bears/1,000 km2; density of adult and subadult bears was 27 bears/1,000 km2. Adult females that produced cubs had 40% more fat and 10% more lean mass pre-denning than non-reproductive females but had similar lean mass and similar but very low body fat (8%) post-denning. Adult females that produced cubs lost more fat and lean mass over winter than non-reproductive adult females. The physiological demand on adult females producing cubs was 29 kg of total body mass split evenly between fat mass and lean mass. Females that did not lose cubs during the cubs’ first summer averaged more body fat the previous fall than those that lost partial or entire litters. Reproductive rate (0.34) and average litter size (2.1) were high and comparable to increasing populations (λ > 0). Age of dependent young at family breakup (2.9 yr) and age of first reproduction (7.5 yr) were high and comparable to declining populations (λ < 0). Cub (0.34), cub litter (0.43), and yearling survival rates (0.46) were low. Subadult and adult survival rates were high (0.94–1.00) for both sexes. The female age structure was bimodal, suggesting episodic recruitment. The percentage of subadult males was low (3% of population and 10% of male segment). We conclude that the grizzly bear population in Denali National Park was near or at carrying capacity relative to available nutritional resources, adult female access to nutritional resources determined whether cubs were born and the likelihood of their survival, and high dependent bear mortality was the primary factor limiting recruitment and regulating the population. © 2018 The Wildlife Society.