Document: DOU-3-4-1

The response to and control of spatial heterogeneity by large herbivores in Yellowstone National Park.

FRANK, D.A.*, D.J.AUGUSTINE and E.W.HAMILTON

Syracuse University, Syracuse, NY ¹

Abstract:
Large herbivores are highly integrated regulators of grassland habitats that respond to and determine the heterogeneity of ecosystem processes at scales ranging from the region to the forage plant and plant part. In Yellowstone National Park, large herds of elk and bison follow a wave of young, nutritious forage from low elevation winter range to high elevation summer range. The opposite migration later in the year results in animals transporting nutrients from high elevation habitat, where they gain condition, to winter range, where their condition declines. Within landscapes, shoot production at topographic positions, which can vary by 10-fold, and consumption are strongly correlated. Grazers promote variation of N availability among topographic sites, suggesting that grazers also enhance between-site variation in plant production and forage quality. Ungulate effects on grassland heterogeneity at smaller spatial scales, examined with geostatistical analyses on soils inside and outside exclosures, showed that herbivores increased fine scale (< 10 cm) variation of soil N properties. Grazers also increased plant diversity at that scale. A clipping experiment on a common Yellowstone grass demonstrated that defoliation promoted root exudation of carbon, rhizospheric microbial biomass, and N availability for plant uptake. These experimental results suggested that plant responses to grazing were involved in herbivores promoting fine scale heterogeneity of soil processes. Together these findings from Yellowstone indicate that ungulates are not just passive occupants of heterogeneous ecosystems, but, instead, are active participants in shaping spatial variation in grassland habitats at a wide range of scales.

Keywords: Spatial heterogeneity, herbivory, grassland, nitrogen, ungulate, Yellowstone National Park

This abstract is being presented at: 3:45 PM in session:
Symposium # 10: Integrating Ecosystem and Landscape Ecology: Causes and Consequences of Spatial Heterogeneity in Ecosystem Processes.