The ecology of plants, large mammalian herbivores, and drought in Yellowstone National Park

Net aboveground primary production (ANPP), large herbivore consumption (C), and dung deposition (D), an index of nutrient flow from herbivores to the soil, were measured in grassland and shrub-grassland habitat on winter, transitional, and summer range used by herds of elk Cervus elaphus and bison Bison bison. ANPP, C, and D varied widely among sites: ANPP range; 16-589 g/m², C range: 0-306 g/m², and D range: 0-68 g/m². An average of 45% of ANPP was consumed by herbivores. Production and consumption, and consumption and dung deposition were positively correlated across all sites. Sites were grazed when plants were growing. There was a 19% reduction in ANPP from 1988 to 1989, likely caused by death or injury to plants during the 1988 drought. Drought also appeared to be partially responsible for reductions in elk and bison from 1988 to 1989, which were coincident with declines in C and D. Results indicate direct effects and suggest indirect effects of a single-season drought on grassland function that will persist for several years after the event. -from Authors