Grazer Ecology

Grazing systems, grass-like vegetation interacting with their large mammal grazers, are important globally, where estimates of their potential extent (depending on classifications) range from 30 to 70 percent of the terrestrial land surface and show a major presence on five continents. In grasslands and savanna ecosystems, the grazing energy channel is prominent (~50 percent of energy flows through herbivores), unlike energy flow in more arid ecosystems where the detrital energy channel predominates. While variable, estimates of consumption of above-ground net primary productivity (ANPP) by native large mammal herbivores ranges from 1 (desert grassland) to ~64 percent (mesic grasslands), and cattle remove 15–80 percent (see chapter by J. K. Detling, “Grasslands and Savannas: Regulation of energy flow and nutrient cycling by herbivores,” in Concepts of Ecosystem Ecology: A Comparative View, edited by L. R. Pomeroy and J. J. Alberts [New York: Springer-Verlag, 1988], pp. 131–154). Consequently, in addition to altered aboveground biomass, one expects significant system responses to grazers, including altered plant community species composition, changed plant morphology and population structure, impacted nutrient cycles, and altered habitat structure in turn affecting animal species distributions both native and exotic. Examples of each of these responses are provided in this article. Our bibliography takes a decidedly grazer-centric view. Topics in grazing ecology are wide ranging, where both plant and grazer responses are studied as we attempt to integrate the many moving parts operating at multiple scales to understand responses from multiple perspectives. These include an understanding of the role of disturbances (fire, drought, herbivory), internal dynamics driving fire-grazer interactions, variable environmental conditions (especially primary production and rainfall), resource heterogeneity at multiple spatial scales, variable herbivore body size, different digestive physiologies of herbivores, sedentary presence and migratory movement of large mammalian herbivores in response to variable environmental conditions, and trophic control of food webs including bottom-up/top-down regulation with important roles for direct and indirect species interactions. Combined, many factors contribute to a range of equilibrial and nonequilibrial interpretations of key responses and patterns of grazing ecology with important implications for management and conservation of these systems worldwide. Much of grazing ecology focuses on the interactions of large mammal herbivores with vegetation structure and plant communities. Much less is known about invertebrate grazers, although they can be important participants as well. This article deals primarily with vertebrate grazers, factors affecting grazing dynamics, and examples of the effects of grazing on grassland structure and function.