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9 - Large herbivores as sources of disturbance in ecosystems
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- By N. Thompson Hobbs, Colorado State University
- Edited by Kjell Danell, Swedish University of Agricultural Sciences, Roger Bergström, The Forestry Research Institute of Sweden, Patrick Duncan, Centre National de la Recherche Scientifique (CNRS), Paris, John Pastor, University of Minnesota, Duluth
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- Book:
- Large Herbivore Ecology, Ecosystem Dynamics and Conservation
- Published online:
- 16 November 2009
- Print publication:
- 25 May 2006, pp 261-288
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Summary
INTRODUCTION
Understanding the causes and consequences of spatial heterogeneity in ecosystems has emerged as a fundamental challenge for contemporary ecologists worldwide (Levin 1992, Pickett & Cadenasso 1995, Pascual & Levin 1999, Allen & Holling 2002, Ettema & Wardle 2002, Perry 2002, Hobbs 2003). Abiotic sources of heterogeneity, variation in soils for example, are described relatively easily. A much more difficult task is to understand how spatial heterogeneity arises from biotic interactions (Augustine & Frank 2001, Steinauer & Collins 2001). These biotic sources of spatial variation can give rise to complex feedbacks in ecosystems; feedbacks that shape the operation of ecological processes across scales and levels of organization (e.g. Holling 1992, Pickett et al. 1992, Tilman 1994, Pastor et al. 1997, 1998, Polis et al. 1997, Van Buskirk & Ostfeld 1998, Keeling 1999, Illius & O'Connor 2000, Augustine & Frank 2001, Steinauer & Collins 2001, van de Koppel et al. 2002).
Much effort has been invested in identifying biotic sources of spatial variation in grassland, steppe and forested ecosystems, particularly in describing the reciprocal role played by large herbivores in responding to landscape heterogeneity and in creating it (Jefferies et al. 1994, Laca & Ortega 1995, Scoones 1995, Hobbs 1996, 1999, Pastor et al. 1997, 1998, 1999, Fuhlendorf & Smeins 1999, Adler et al. 2001, Augustine & Frank 2001, Steinauer & Collins, 2001). Herbivores respond to spatial heterogeneity in plants across a range of scales by selecting locations for feeding and resting (Senft et al.
10 - The roles of large herbivores in ecosystem nutrient cycles
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- By John Pastor, University of Minnesota, Yosef Cohen, University of Minnesota, N. Thompson Hobbs, Colorado State University
- Edited by Kjell Danell, Swedish University of Agricultural Sciences, Roger Bergström, The Forestry Research Institute of Sweden, Patrick Duncan, Centre National de la Recherche Scientifique (CNRS), Paris, John Pastor, University of Minnesota, Duluth
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- Book:
- Large Herbivore Ecology, Ecosystem Dynamics and Conservation
- Published online:
- 16 November 2009
- Print publication:
- 25 May 2006, pp 289-325
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Summary
INTRODUCTION
The question of how herbivores control various ecosystem processes has had a long history in modern ecology. In a now much‐cited paper, Hairston et al. (1960) proposed that in the absence of predators, herbivore populations increase to the limit set by their food supply and thus control net or actual productivity and energy flow. With the addition of predators, herbivore populations become controlled from above; plant productivity is then released from direct control by herbivores and instead is limited by abiotic processes such as climate. These ideas have been developed further by Oksanen and colleagues (Oksanen et al. 1981, Oksanen 1983, 1988). In order to simplify the concepts and models, these studies have ignored the way that the cycling of nutrients between decomposers and higher trophic levels limits net primary productivity in most ecosystems, and the many mechanisms by which herbivores alter nutrient flows through decomposers and soils.
Early recognition of the roles of herbivores in regulating nutrient cycles focused on phytophagous insects (Mattson & Addy 1975) or phytoplanktivorous zooplankton (Kitchell et al. 1979). Perhaps this was because of their ubiquity, rapid population growth rates, high turnover rates and because (at least in the case of zooplankton) they consume most of the primary production (Macfadyen 1964). With the exception of large herds in grasslands (Sinclair & Norton‐Griffiths 1979) and microtine populations in tundra (Schultz 1964), the possibility that mammals could also regulate nutrient cycles was generally ignored for quite some time.
13 - The role of ungulates and large predators on plant communities and ecosystem processes in western national parks
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- By Francis J. Singer, US Geological Survey, Midcontinent Ecological Science Center, Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colorado 80523-1499, USA, Guiming Wang, Natural Resources Ecology Laboratory, Colorado State University, Fort Collins, Colorado 80523, USA, N. Thompson Hobbs, Natural Resources Ecology Laboratory, Colorado State University, Fort Collins, Colorado 80523, USA
- Edited by Cynthia J. Zabel, Robert G. Anthony, Oregon State University
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- Book:
- Mammal Community Dynamics
- Published online:
- 15 December 2009
- Print publication:
- 18 September 2003, pp 444-486
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Summary
Introduction
Human activities have caused fundamental changes in relationships between predators and large herbivores in ecosystems of western North America. In particular, large predators, especially wolves (Canis lupus) and grizzly bears (Ursus arctos), have been eliminated from most of the region. Human developments, as well as agricultural and livestock grazing activities have altered or eliminated many migration routes and habitats of ungulates. Developments such as towns and paved areas have eliminated habitat. Alternatively, human modifications to the landscape, in some cases, have created new and more fertile habitats for ungulates. Habituated ungulates use lawns, golf courses, and agriculture fields that are rich in nutrients in many of the Rocky Mountain states and provinces (Thompson and Henderson 1998).
No magic formula exists for setting goals for the appropriate number of ungulates in a national park or in a more managed ecosystem. National Park Service policy calls for managing for natural processes within the parks, where high ungulate populations may be managed if the concentrations are due to human effects (National Park Service 2001; NPS-77). Unfortunately, most current-day park ecosystems are extensively altered by many human factors, such as the extirpation of large predators, abbreviated or lost migrations of ungulates, and concentrations of ungulates on artificially rich human habitats, such as restored agriculture sites (in many eastern and mid-western parks) or lawns and golf courses within or adjacent to the parks. Determining what constituted natural conditions and natural processes under these altered states can be difficult.