Responses by grizzly bear population dynamics to certain environmental and biosocial factors

Based on analysis of previously published data, comparison among grizzly populations and among years within the Yellowstone population reveal that essentially all parameters of reproduction and recruitment (survivorship - net emigration) were positively correlated with food supply and nutrient-energy balance, but negatively correlated with densities of adult males and females. Although densities of cub litters and of cubs would be positively correlated with density of adults when adults are scarce, all available data is for cases where densities of adults were so high that the correlations were negative--indicating strong density dependence. The forms of density dependence exhibited by Yellowstone grizzlies are exceedingly complex. When density of adult males was high, few cubs were produced, mainly daughters; these cohorts had low rates of recruitment to adulthood. By contrast, when adult males were scarce, many cub litters and cubs were born, mostly sons; these cohorts had high rates of recruitment to adulthood. There were also negative correlations between recruitment of recently weaned subadults vs. concurrent density of adult males. Those findings are interpreted in terms of competitive reproductive strategies of adult males vs. females. (1) Aggression by adult males against immatures might benefit the males through (a) nutritive value of immatures which are eaten, (b) reducing current and future competition from victims for resources or genetic representation, or (c) increasing opportunities to mate with mothers of the immatures. (2) Females might minimize wastage of investment in offspring likely to be killed or prematurely exiled by the adult males. This could explain the negative correlations between (a) number, sex ratio, and recruitment rate (manifesting investment) per offspring vs. (b) adult male abundance and adult sex ratio. (3) The Fisher (1930) and Trivers-Willard (1973) hypotheses, respectively, also predict reciprocal adult-offspring sex ratios and corresponding levels of investment per cohort. Presentation of findings on grizzlies is accompanied by review and analysis of comparative information on black and polar bears. An attempt is made to integrate the bulk of current knowledge on factors governing dynamics of bear populations in order to more clearly reveal its implications for theory and management, and to facilitate development of theoretical stock-recruitment and population models--models in which dynamics are controlled by food supply, densities of adult males and females, adult sex ratio, and age. Although these statistical results should be most applicable to grizzly populations where bears frequently aggregate in large numbers at food concentrations, basic features of the descriptive and theoretical models should be applicable to bear populations in general and perhaps to other taxa.