Grazer Ecology

by

Anthony Joern, Edward J. Raynor

• LAST REVIEWED: 27 June 2018
• LAST MODIFIED: 27 June 2018
• DOI: 10.1093/obo/9780199830060-0201

Introduction

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.

General Overviews

This section provides a fundamental description and recognition of the importance of grazing systems as described by several seminal papers, and we also provide critical references on the ecology and feeding physiology of large herbivore grazers. After providing descriptions of the general system and grazer participants, we include a subsection of alternate conceptual views of grazing ecosystems with implications for grazer management and conservation. Bell 1971 provides an overview synthesis of grazer interactions and species behavioral ecology based on native species within the Serengeti, which is then further developed in McNaughton 1985, McNaughton and Georgiadis 1986, and Sinclair and Norton-Griffiths 1995 provides foundational insights into the stochastic nature of grazer populations and ecosystem ecology of grazing systems, and the role grazers have on ecosystem function and structure within the Serengeti. Frank, et al. 1998 builds on insights from the African Serengeti and compares the quintessential African grazing system to Yellowstone National Park in North America. Burkepile 2013 makes a persuasive and call for the consideration of insights learned from both aquatic and terrestrial grazing systems to be synthesized to improve conservation and restoration of grazing systems generally. Given the central place of large herbivores for understanding the topic, additional references provide a framework for large mammalian herbivores and their ecology. Owen-Smith 1992, van Soest 1994, Gordon and Prins 2008, and Prins and van Langevelde 2008 are foundational books that give overviews of the importance of large herbivores to ecosystem ecology in grazing systems; provide insight on nutritional ecology of ruminants, behavioral ecology, and evolutionary biology of ungulates occupying grazing ecosystems; and introduce key questions on the interaction of grazers and their resources at varying spatial and temporal scales.

• Bell, R. H. V. 1971. A grazing ecosystem in the Serengeti. Scientific American 225:86–93.

  DOI: 10.1038/scientificamerican0771-86

  Bell provides an important synthesis of a charismatic system on what was known about grazer communities at the time, especially regarding their synchrony with forage resources in space and time. Initial insights into forage maturation and nutritional ecology of wild ungulates are provided, triggering the development of key studies in this unique Serengeti ecosystem and elsewhere.

• Burkepile, D. E. 2013. Comparing aquatic and terrestrial grazing ecosystems: Is the grass really greener? Oikos 122:306–312.

  DOI: 10.1111/j.1600-0706.2012.20716.x

  The provision of examples of how herbivory in aquatic systems facilitates primary production, herbivore richness, competition, and facilitation as seen in terrestrial ecosystems is crucial for understanding grazer ecology. By showcasing studies that highlight our understanding of top-down versus bottom-up control and alternative states, this review establishes a framework in wet or dry ecosystems for determining drivers of grazer foraging choices, which inevitably should facilitate ecosystem conservation and restoration.

• Frank, D. A., S. J. McNaughton, and B. F. Tracy. 1998. The ecology of the Earth’s grazing ecosystem. BioScience 48:513–521.

  DOI: 10.2307/1313313

  Functional components of grazing ecosystems in the Serengeti (East Africa) and Yellowstone National Park (North America) are compared. Primary topics include energy dynamics, seasonal migrations and the role of nutrition-rich green waves, grassland structure and grazing efficiency, feedbacks, regulation of above-ground net primary productivity (ANPP) by grazers, sustainability of grazing systems, and human transformations and conservation challenges. Similarities and differences between these systems are examined in detail to seek general underlying patterns.

• Gordon, I. J., and H. H. T. Prins. 2008. The ecology of browsing and grazing. Berlin: Springer.

  DOI: 10.1007/978-3-540-72422-3

  An evolutionary biology–focused book that introduces evolutionary history, morphophysiological adaptations, ruminant nutritional ecology, comparative feeding behavior and population dynamics, and management techniques of browsers and grazers as well as their effects on plant evolution and ecology.

• McNaughton, S. J. 1985. Ecology of a grazing ecosystem: The Serengeti. Ecological Monographs 55:259–294.

  DOI: 10.2307/1942578

  The grazing ecosystem in protected East African savanna (Kenya and Tanzania) is described with an emphasis on linking primary production and herbivory, including their critical interactions. Six general features emerge: (a) the stochastic nature of precipitation, grazing, nutrient availability, and fire; (b) fluctuations of primary production that produce an unpredictable food source for grazers; (c) nomadic lifestyles of mobile dominant grazers facilitate exploitation of constantly shifting resource base; (d) carrying capacity is not fixed independently of grazers because of their effect on primary productivity; (e) grazers influence the composition and diversity of vegetation; and (f) interactions lead to a more deterministic grazing food web than expected in such a variable environment.

• McNaughton, S. J., and N. J. Georgiadis. 1986. Ecology of African grazing and browsing mammals. Annual Reviews in Ecology and Systematics 17:39–65.

  DOI: 10.1146/annurev.es.17.110186.000351

  A crucial resource for understanding the differences between African browsers and grazers and their interactions with resources. Distribution, life-history strategies, and abundance of these mammals in relation to biotic and abiotic factors common in Africa are discussed. Authors also touch on the role of grazers as ecosystem engineers.

• Owen-Smith, R. N. 1992. Megaherbivores: The influence of very large body size on ecology. Cambridge, UK: Cambridge Univ. Press.

  This seminal book synthesizes studies in large herbivore ecology and most importantly raised questions on the role of body size in ungulate biology and the influence of megaherbivores on ecosystem processes.

• Prins, H. H. T., and F. van Langevelde, eds. 2008. Resource ecology: Spatial and temporal dynamics of foraging. Wageningen UR Frontis Series 23. Dordrecht, The Netherlands: Springer Science & Business Media.

  A book about the ecology of trophic interactions between grazers and resources. Key researchers in the fields of spatial ecology of herbivore resources, foraging ecology, movement ecology, and nutritionally ecology inform readers about resource ecology, and critical questions in grazer ecology are remaining to be answered.

• Sinclair, A. R. E., and M. Norton-Griffiths, eds. 1995. Serengeti: Dynamics of an ecosystem. Chicago: Univ. of Chicago Press.

  This is the first volume of a comprehensive, foundational series on the Serengeti grazing ecosystem and synthesizes the ecology, management, and conservation of the Serengeti ecosystem. The series progresses over a twenty-year period to continually update the areas of critical need and expands into the need to view the problem as a coupled human-natural system if the Serengeti is to be sustainable as a grazing system.

• van Soest, P. J. 1994. Nutritional ecology of the ruminant. Ithaca, NY: Cornell Univ. Press.

  This is a foundational text on nutritional ecology that highlights the importance of nutritional assessments to the ecology and biology of ruminants and other nonruminant herbivorous mammals. In turn, nutritional studies are placed in a historical context to highlight the effectiveness of nutritional approaches and their fundamental importance to issues of grazer conservation. Mechanistic causal interrelationships between plants and animals are emphasized.