Variation in life history and demography of the American black bear

Abstract

Variation in life history and demography across a species' range informs researchers about regional adaptations and affects whether managers can borrow information from other populations in decision‐making. The American black bear (Ursus americanus) is a long‐lived game species whose continued persistence depends on management of harvest and removal of habituated bears that come into conflict with humans. Understanding the demography of black bears guides efforts at management and conservation, yet detailed knowledge of many populations is typically lacking. I performed a hierarchical Bayesian meta‐analysis of black bear demographic studies across the geographic range of the species to explore how vital rates vary across the range, what information they give us about population growth, and whether managers can justify borrowing information from other studies to inform management decisions. Cub, yearling, and adult survival and fecundity varied between eastern and western North America, whereas subadult survival did not show geographic structuring. Adult survival and fecundity appeared to trade off, with higher survival in the western portions of bears' range and higher fecundity in the east. Although adult survival had the highest elasticity, differences in reproduction drove differences in population growth rate. Mean population growth rate was higher in the east (0.99; 95% credible interval [CrI]: 0.96, 1.03) than the west (0.97; 95%CrI: 0.93, 1.01). Despite declining trends in the west, 34% of the distribution of population growth rate was >1, compared to 55% in the east. Further work needs to be done to address the cause of the apparent trade‐off between adult survival and fecundity and explore how the estimated growth rates are likely to affect population status of black bears. Because population growth rates are close to 1 and small deviations could impact whether a population is considered increasing or decreasing, managers need to employ caution in borrowing vital rates from other populations. © 2011 The Wildlife Society.

Modeling black bear population dynamics in a human-dominated stochastic environment

“…This model structure incorporated the following life history traits: ∼16 months maternal investment in offspring leading to a 2-year litter production cycle (Lee and Vaughan, 2004), age of primiparity ranging from 3 to 7 years (Beston, 2011), and litter production in consecutive years is possible when females breed after loss of an entire litter (Barber and Lindzey, 1986). We developed a sequence of stochastic matrices in two steps.…”

“…First, we defined as the frequency of good natural food years and generated an independent identically distributed (iid) environmental sequences of good and poor natural food years using random draws from a binomial distribution. Second, we used vital rate estimates from Aspen (Baruch-Mordo et al, 2014) in conjunction with mean and 95% credible intervals for Western bear populations determined by Beston (2011) as a guide for selecting vital rate means and standard deviations for good and poor natural food years in the three scenario described in Section 2.3. We used these vital rate means and standard deviations to calculate parameters for beta and gamma distributions (see Appendix A).…”

“…We created a Baseline scenario to establish a reference for comparing a population free from the influence of urban food sources and conflict-bear management with two urban removal scenarios (see Table 2 for vital rate values used in simulation). There was little information about vital rates calculated during good and poor natural food years in the literature; we thus parameterized the Baseline scenario vital rates such that the vital rate mean taken across good and bad years was near values calculated in a meta-analysis for Western black bears (Beston, 2011). We assumed that poor natural food production more negatively influenced younger stage class survival rates than adult survival (Eiler et al, 1989;Schrage and Vaughan, 1995;Noyce and Garshelis, 1994;Kasbohm et al, 1996).…”

“…We used black bears (Ursus americanus) as a model species for this exercise because they are well studied across their range (see Beston (2011) for summary of population studies), will readily take advantage of anthropogenic resources, and wildlife managers are challenged with trends of increasing human-bear conflict rates and maintaining productive bear populations (Hristienko and McDonald, 2007). Bear populations that use urban food sources potentially have vital rates that differ in comparison to populations relying solely on natural food sources that vary seasonally and annually (Beckmann and Lackey, 2008;Elowe and Dodge, 1989;Inman and Pelton, 2002;Rogers et al, 1976).…”

“…We parametrized our models using demographic data from a 6-year study of urban bears (Aspen study), supplemented with vital rates from a meta-analysis of Western black bear populations (Beston, 2011). We developed a Baseline scenario where the bear population did not have access to urban food sources or experience management removal of conflict bears and used computer simulation to compare this with two management scenarios where bears utilized human foods and managers removed conflict bears.…”