INTERACTION OF TOP-DOWN AND BOTTOM-UP LIFE HISTORY TRADE-OFFS IN MOOSE (ALCES ALCES)

Abstract

Moose (Alces alces) in south-central Alaska have been found to have phenotypic life history trade-offs that can be partitioned into the same “bottom-up” and “top- down” trophic framework often applied at the population level. The potential interaction of these life history trade-offs was studied under varying degrees of predation and nutritional limitation using models based on data from radio-collared moose in south-central Alaska. The interaction was mildly antiregulatory, assuming that bottom-up trade-offs are density dependent. That is, as nutritional constraints increased, a female that skipped reproduction after successfully rearing a calf enhanced the survival of herself and her yearling enough in the second year to overcompensate for the loss of productivity. In a declining population, the reverse could occur, accentuating the decline. A more general, stage projection matrix was used to model a broader range of predation rates in summer and winter, in order to contrast the potential importance of bears (Ursus arctos and U. americanus) and wolves (Canis lupus) in such an interaction. The greatest effect of life history interactions on population growth occurred with moderate predation in summer and heavy predation in winter. Relative to direct ecological effects on survival and reproduction, the influence of such an interaction was relatively weak. However, subtle effects on the stability of predator– prey interactions, particularly those involving bear predation, cannot be excluded because of the antiregulatory nature of the interaction and uncertainties about class-specific vulnerability to predation in other settings. These findings underscore the relevance of understanding the alternative life history choices that arise from interactions in both directions on the food chain, particularly given that the top-down trade-offs are usually poorly known.