Abstract - Modification of Vegetation Structures and Ecosystem Processes by North American Grassland Mammals

Whicker, A.D. and Detling, J.K.. 1988. Modification of Vegetation Structures and Ecosystem Processes by North American Grassland Mammals. Plant Form and Vegetation Structure: 301-316.

Abstract

Historically, prairie dogs (Cynomys spp.), small colonial mammals, and bison (Bison bison), large migratory ungulates, were among the most influential grazers in North American grasslands. Our studies in a northern mixed-grass prairie in Wind Cave National Park, South Dakota, indicate that colonization and intensive grazing by prairie dogs greatly alters the ecosystem patch structure an increases the attractiveness of colonized areas to other herbivores such as bison. The objectives of this research have been to (1) investigate how these native grazers influence structural and functional properties of grasslands over time, and (2) determine how these and other herbivires respond to induced changes in the ecosystem.

Soon after colonizing grassland areas, prairie dog populations usually increase and occupy available habitats where they dig burrows and graze surrounding vegetation to a height of several cm. Thereafter, prairie dogs and other herbivores keep the plants clipped off such that aboveground biomass is maintained at one-third to two-thirds that of adjacent, uncolonized areas. Over time, there is usually a shift in community dominance from grasses to forbs and dwarf shrubs, and belowground plant biomass decreases markedly. As there changes occur, utilization of the colonies by other herbivores increases. Aboveground, bison feed preferentially on graminoid-dominated areas of the colonies, while pronghorn (Antilocapra americana), smaller ungulates, feed principally in areas in which forbs and dwarf shrubs have become dominant. Belowground, populations of root-feeding nematodes often increase on heavily grazed prairie dog colonies, and they consume a greater proportion of the annual net root production than on lightly grazed uncolonized areas. The combined activities of these other aboveground and belowground herbivores likely accelerate structural and functional changes in the vegetation initiated by prairie dogs.