Life-History Patterns Of North American Elk: Effects Of Population Density On Resource Partitioning, Reproduction, And Plant Productivity

Abstract

I examined density dependence in North American elk (Cervus elaphus ) and effects of density dependent processes on resource partitioning, physical condition, reproduction, and ecosystem processes. Specifically, I examined spatial, temporal, and dietary niche partitioning among elk, mule deer (Odocoileus hemionus) and cattle (Bos taurus ). I tested hypotheses related to density-dependent processes in elk by creating populations at high (20.1 elk/km2) and low (4.1 elk/km2) density. I hypothesized that physical condition and fecundity of females would be lower in an area of high population density than in the low-density area. Simultaneously, I tested hypotheses relating to herbivore optimization in response to varying levels of herbivory. I observed differences among elk, mule deer, and cattle in diets and use of space, particularly elevation, slope, and use of logged forest. Those 3 herbivores showed strong avoidance over a 6-h temporal window, but that effect was weaker for the previous 7 days. Changes in habitat use by elk and mule deer in response to addition and removal of cattle indicated competitive displacement. Results of the experiment to examine density dependence in elk indicated reduced physical condition and reproduction in the high-density population compared with low-density population. Pregnancy rates were most affected by body condition and mass of females. Density dependence in elk also had strong effects on plant communities; net aboveground primary productivity (NAPP) increased from no herbivory to moderate grazing intensity, and then declined as grazing intensity continued to increase. Compensatory responses by plants likely are more difficult to detect when responses to herbivory are subtle and occur at relatively low grazing intensity. I observed strong effects of density dependence on physical condition of elk and reductions in NAPP of plant communities with high levels of grazing intensity. At high-population densities resources for elk declined and NAPP was reduced. At low-population density elk were in good physical condition with high rates of reproduction, and NAPP increased, indicating compensatory responses by plants. Density-dependent feedbacks in populations of large herbivores help regulate population dynamics, and those same processes have substantial effects on ecosystem functioning.