Abstract - Grass-grazer Interactions at Wind Cave National Park
Detling, James K., Hobbs, N. Thompson and Gerhardt, Troy. 1993. Grass-grazer Interactions at Wind Cave National Park, Final Report. 52 p.
In grassland, the activity of grazers and fire can alter the structural and nutritional patch-mosaic of the landscape and, thus the grazing patterns of herbivores. We examined the selectivity of large herbivores for previously grazed and ungrazed patches located in areas of contrasting fire history and aboveground net primary production (ANPP) in the mixed-grass prairie of Wind Cave National Park, South Dakota. Landscapes composed of grazed and ungrazed patches (5 m x 5 m) were established in previously burned and unburned areas along catenas showing natural gradients of ANPP. Locally heavy grazing was simulated by mowing. Selectivity of the park's ungulates for these patches was estimated by monitoring offtake (amount of ANPP consumed) and grazing intensity (proportion of ANPP consumed) through the growing season.
Along catenas, ANPP was significantly higher in swales than on hilltops. The interaction of catena position and grazing on patch selection approached signficance for grazing intensity, however, grazed patches received heavier use than ungrazed patches in swales (high ANPP) and on hilltops (low ANPP). Averaged across interactions, mowed patches and swale areas received significantly heavier use than ungrazed patches or hilltop areas. The relatively high production even on hilltops is discussed as a possible explanation for the discrepancy between the predicted response and the observes patterns.
In fire landscapes, the effect of mowing on herbivore patch selection was influenced by the fire history of the area. Both offtake and grazing intensity were significantly affected by the fire x mowing interaction. Mowed patched in unburned areas received heavier use that any other patch type in these landscapes. We suggest that the structural differences between mowed patches and the surrounding matrix in unburned areas and the relative absence of such differences in burned areas may have contributed to the interactive effects of fire and mowing on patch selection.