Population dynamics and life history trade-offs of moose (Alces alces) in south-central Alaska

A common framework toward understanding the ecological forces affecting herbivore population dynamics is to partition those forces according to their origin higher (top-down),or lower (bottom-up) on the trophic chain. I employed that framework to describe phenotypic life history trade-offs in a large herbivore, moose (Alces alces). The relative importance of top-down and bottom-up ecological constraints to female moose in south-central Alaska was appraised at the population level by analyzing population size, growth, and winter habitat of moose, and the size and impact of their principal predator populations (wolves [Canis lupus] and brown bears [Ursus arctos]). At the individual level, longitudinal mortality and reproductive patterns of radio-collared individuals were studied to estimate vital rates and life history trade-offs that could be attributed to nutrition and predation, respectively. The population was essentially stable through the 1980s and early 1990s, and began declining in the period of this study (1994-2000). Nutritional constraints on productivity were evident in low twinning rates (9-24%) and delayed age of first reproduction (3.4 yr) relative to other moose populations. However, the decline of the population was due principally to high rates of summer mortality of calves, resulting in low recruitment in the fall (0.11-0.22 offspring/female) and increasing adult mortality. Bear and wolf densities were high and moderate, respectively, while human harvest of moose was limited to males. Modeled population growth indicated a very limited capacity for population recovery through increased reproduction when compared to the effects of reducing predation. Constraints on population growth were mirrored in life history trade-offs. Trade-offs that were attributed to bottom-up effects included reduced rates of both parturition and twinning as a consequence of maternal investment in the prior year. Important trade-offs also were detected between top-down factors and reproductive effort through significantly increased predation on adult females with calves and increased survival of yearlings when adults skipped or abandoned reproductive attempts and extended the duration of maternal care.