Contributions of vital rates to growth of a protected population of American black bears
Analyses of large, long-lived animals suggest that adult survival generally has the potential to contribute more than reproduction to population growth rate (λ), but because survival varies little, high variability in reproduction can have a greater influence. This pattern has been documented for several species of large mammals, but few studies have evaluated such contributions of vital rates to λ for American black bears ( Ursus americanus ). We used variance-based perturbation analyses (life table response experiments, LTRE) and analytical sensitivity and elasticity analyses to examine the actual and potential contributions of variation of vital rates to variation in growth rate (λ) of a population of black bears inhabiting the Pisgah Bear Sanctuary in the southern Appalachian Mountains of North Carolina, using a 22-year dataset. We found that recruitment varied more than other vital rates; LTRE analyses conducted over several time intervals thus indicated that recruitment generally contributed at least as much as juvenile and adult survival to observed variation in λ, even though the latter 2 vital rates had the greater potential to affect λ. Our findings are consistent with predictions from studies on polar bears ( U. maritimus ) and grizzly bears ( U. arctos ), but contrast with the few existing studies on black bears in ways that suggest levels of protection from human-caused mortality might explain whether adult survival or recruitment contribute most to variation in λ for this species. We hypothesize that λ is most strongly influenced by recruitment in protected populations where adult survival is relatively high and constant, whereas adult survival will most influence λ for unprotected populations.