Herbivore Optimization by North American Elk: Consequences for Theory and Management

Abstract

Understanding herbivore optimization has implications for theories underpinning ecosystem processes, management of large herbivores, and the landscapes they inhabit. We designed an experiment to examine interactions related to density dependence of North American elk (Cervus elaphus) and resulting plant responses to herbivory in the Blue Mountains of Oregon, USA, from 1999 to 2001. We experimentally created high (20.1 elk/km2) and low (4.1 elk/km2) population densities of elk and built exclosures to examine effects of herbivory on productivity and species composition of plants. We hypothesized that if herbivore optimization occurred with increasing density of elk, there should be a concordant increase in plant production, followed by a decline in productivity as grazing intensity continued to increase (i.e., herbivore optimization). Net aboveground primary productivity (NAPP) increased from no herbivory to herbivory by elk at moderate density and then declined as herbivory by elk continued to increase in areas with high NAPP (mesic and logged forests) but not in areas with low NAPP (xeric forests and grasslands). Herbivore optimization occurred across all functional groups of plants, including graminoids, forbs, and shrubs for high‐NAPP areas. Herbivore optimization may be difficult to detect in woody plants compared with graminoids because of their differing structure and growth forms. Although herbivore optimization previously has been reported in grasslands, our study documents this phenomenon in woody plant communities. We hypothesize that such subtle changes in NAPP from herbivory might be more common than previously thought; carefully designed experiments are required to detect those responses to herbivory by large herbivores. Apparent offtake of plants followed a similar pattern to NAPP and was greatest at intermediate levels of herbivory by elk, and then declined as NAPP approached zero. Quality of plants, as indexed by percent nitrogen (N), also exhibited a parabolic function with increasing density of elk. Nonetheless, we observed no changes in species composition or diversity of plants with our density manipulations of elk, probably because of the extensive history of grazing by native and domestic herbivores in the Blue Mountains, the resilience of the remaining plants to herbivory, and the short 3‐year duration of our study. Likewise, we observed no increases in rates of nutrient cycling with changes in densities of elk, perhaps because areas where large amounts of elk urine and feces were concentrated (e.g., grazing lawns) did not occur in this ecosystem. Herbivore optimization could have ramifications for population dynamics of ungulates. We argue that other processes, such as migration, predation, or harvest, might be necessary to maintain areas of increased NAPP because of strong density‐dependent feedbacks and negative effects of ungulates on their food supply would not allow equilibrium between herbivores and their food supply at the low densities where we documented herbivore optimiza...

Habitat Interference by Axis Deer on White‐Tailed Deer

“…Elevations on Starkey range from 1,120 to 1,500 m a.s.l. Starkey encompasses 10,125 ha, and since 1987 has been surrounded by a 2.4-m fence that prevents immigration or emigration of large herbivores (Rowland et al 1997, Stewart et al 2002, Stewart et al 2006. As a consequence, elk no longer migrate from our study area to traditional winter ranges.…”

“…As a consequence, elk no longer migrate from our study area to traditional winter ranges. Animals were, therefore, maintained throughout winter in a holding area where they were fed a maintenance diet of alfalfa hay (Rowland et al 1997, Stewart et al 2006. Elk were concentrated and contained within a winter feeding ground from early December to late April each year (Stewart et al 2006).…”

“…Elk were moved among study areas via a system of fenced alleyways across Starkey, and densities of elk were manipulated by selectively returning animals to study areas via those alleys. Consequently, we manipulated population density non-lethally for this study (Stewart et al 2006).…”

“…We restricted our experiment to the northeast area on Starkey, which encompassed 1,452 ha and was separated from the remainder of our study area by a high fence (Stewart et al 2002(Stewart et al , 2006(Stewart et al , 2009. This northeast area was divided into two study sites with the same 2.4-m high fence, east (842 ha) and west (610 ha), to accommodate experimental comparisons of two populations densities of elk (Stewart et al 2006(Stewart et al , 2009.…”

“…This northeast area was divided into two study sites with the same 2.4-m high fence, east (842 ha) and west (610 ha), to accommodate experimental comparisons of two populations densities of elk (Stewart et al 2006(Stewart et al , 2009. We divided the northeast area in a manner that resulted in plant communities being near equal in proportions in the east and west study areas (Stewart et al 2005(Stewart et al , 2006. Such study sites are sufficiently large to allow natural movements and other behaviours of large herbivores (Hirth 1977, McCullough 1979, Stewart et al 2002.…”