Black Bear Population Dynamics in Yosemite National Park

Keay, J. A. 1990. Black bear population dynamics in Yosemite National Park. Ph.D. Dissertation, Univ. Idaho. 126p.

ABSTRACT: Sex and age composition and reproductive rates of nuisance black bears (Ursus americanus) were examined from 1974 through 1988 in Yosemite National Park to assess the effects of an intensive bear management program. A matrix population model was used to evaluate the relative importance of survival and reproductive rates to population growth rate and age structure and the population’s ability to withstand the impacts of intensive management. Bears captured in wilderness averaged 19% males for adults and 38% males for subadults. Both were the lowest reported for black bears. Bears using developed areas averaged 45% males for adults and 55% males for subadults. Age composition did not differ between management units and averaged 64% adults for females and 46% for males. Mean adult age was 8.8 years. Female age structure resembled an unexploited population. Male age composition resembled a harvested population. Widely scattered concentrations of human food predisposed male bears to conflicts with humans and the effects of intensive manipulation by management. Summer litter size averaged 1.6 cubs in wilderness and declined from 2.1 to 1.3 cubs in developed areas. The proportion of females accompanied by litters averaged 0.33 in wilderness and 0.42 in developed areas. Reproductive rate declined with increased adult female age. Reproductive rate of females using developed areas was high suggesting continued access to human food. Survival rates of subadult and young adult bears were more important than those of older bears in affecting population growth. Survival rates of prime-breeding-age females were 2 to 5 times as important as reproductive rates in affecting population growth. Management actions that affect survival rates have a more significant influence on population growth than factors influencing reproductive rates. Conservation efforts should focus on increasing survival rates of subadult and young adult bears rather than productivity. Mortality would have to favor older females over prime-breeding-age adults for older females with reduced reproductive potential to predominate in adult age structure unless young bear mortality and emigration exceeded reproduction and immigration for 10 to 15 years.