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.

Introduction of Dorper sheep into Australian rangelands: implications for production and natural resource management

The growing popularity of the Dorper breed of sheep potentially may have implications for the ecological sustainability of the semiarid and arid rangelands of southern Australia. The implications are heightened by forecasts of a warming and drying climate in these rangelands, which may in itself place native vegetation under increasing stress. While the Dorper breed of sheep offers important production advantages, little is known from research under Australian conditions about their grazing ecology and management requirements from a natural resource perspective. Key factors identified from this review of literature from other countries include a high fertility and fecundity, a generalist feeding strategy, a high growth rate and a capacity to survive and reproduce under low-rainfall conditions. The wider range of plant species selected by the Dorper compared with the traditional Merino breed of sheep potentially creates both opportunities and risks for rangeland condition. Less selective grazing may reduce pressure on some species but the capacity to harvest sufficient nutrients over a smaller area could concentrate grazing and promote resource degradation. High reproductive efficiency under a wide range of seasonal conditions may lead to more rapid onset of overgrazing and will require close attention to both natural resources and animal marketing if resource degradation is to be avoided.

The spatial dimension of pastoral herding: A case study from the northern Negev

Sustainable management of heterogeneous grazing systems requires better understanding of the spatial dimension of grazing ecology. We deployed animal-borne GPS devices to map the daily foraging excursions of a shepherded flock of 200 goats under traditional Bedouin management, on a study site in the semiarid region of the northern Negev, Israel. A total of 88 daily foraging excursions were mapped during the herbaceous growing seasons of 2002 and 2003, and the spatial dimension was analyzed with GIS tools. A typical foraging excursion lasted 5.5 h, during which the flock moved across the landscape at an average speed of 0.3 m/s (1.1 km/h) and traveled 5.4 km. The foraging route was highly elongated in shape and reached a distance of 1.5 km from the night corral. Flock movement speed was affected significantly by distance from the night corral, being greatest at shortest and longest distances. Speed decreased with increasing slope angle and differed between aspects. The frequency of flock presence across the study site deviated significantly from random with respect to aspect but not to slope angle and distance from the corral. The effect of aspect changed slightly between the study years. The product of daily distance traveled and flock path width yielded a rate of area coverage of 0.122 km2 per day. The average amount of herbage dry matter removed on any one pass of the flock is therefore in the order of just a few grams per square meter. The use of the area available to the flock was highly non-uniform, which suggests that better planning of grazing management could mitigate negative effects of grazing.