COMPENSATORY RESPONSE TO CHANGES IN CALF SURVIVORSHIP: MANAGEMENT CONSEQUENCES OF A REPRODUCTIVE COST IN MOOSE

Abstract

Life history tradeoffs are a well-documented feature in many large mammal species but the management consequences of such tradeoffs are not explored. A cost to present reproduction, in terms of future reproductive success, for females moose was implied in recent work by Testa and Adams (unpubl.). In that paper, rump fat thickness differed in moose with and without a calf at heel in autumn, and was correlated in logistic regression models to subsequent calving. This suggests an energetic link that results in lower reproductive success for female moose in years after successfully rearing a calf to autumn. In the present paper, a model of their results linking present and future success through rump fat changes was favorably compared to a second sample of female moose for which reproductive histories in successive years was known. This individual cost of reproduction in moose may play a role in populations having high and variable rates of additive perinatal mortality due to predation. The cost for individual moose of having and rearing a calf to autumn was estimated, and incorporated into a population model in which perinatal mortality was manipulated to simulate managed reduction of predation rates on neonates. The expectation was that the tradeoff between current and future reproductive success in individuals could reduce the harvest benefits expected from reducing calf mortality. The estimated cost of successfully rearing a calf to the fall in this study was a 44% reduction in fecundity, which led to modeled reductions of 10-13% in the gains expected from better calf survival. This effect could be greater in years of unusually low reproduction, or after an increase in population density.