Over half the earth’s terrestrial surface supports herds of wild and domesticated ungulates. These animals can have profound effects on plant and soil processes that vary markedly depending on the structure, climate, and resource level of the ecosystem. Yet a mechanistic understanding of the direct and indirect pathways that determine how herbivores influence ecosystem processes remains rudimentary. Here I will review research in Yellowstone National Park examining how herbivores affect key plant and soil processes to address the relative importance of biotic and abiotic pathways that drive the positive effects that grazers have on Yellowstone grassland aboveground and belowground production.
Results indicate that Yellowstone ungulates increase leaf assimilation rate, leaf litter quality flowing to the soil, soil net N mineralization rate, and soil organic matter decomposition rate (k) of the active pool of soil organic carbon. Yellowstone grazers also change the soil micro-climate by increasing both soil temperature and moisture. The relative importance of the biotic feedback via improved litter quality vs increased soil microbial activity via ameliorated temperature and moisture conditions in regulating grassland production will be examined.