摘要:

Ecological research can be misinterpreted by the popular press and misapplied in land management. One example of this concerns the controversial concepts of overcompensation by grazed plants and herbivore optimization of plant productivity (or, as popularly phrased, the idea that plants benefit from being grazed). Although available evidence indicates that whole—plant overcompensation and optimization of productivity rarely occur and may have little or no evolutionary or applied significance, these concepts have been accepted by some popular writers and land managers, and are being used to justify heavy livestock grazing on western North American rangelands. There may be many reasons for this acceptance of unsubstantiated hypotheses, including uncritical reading and overly broad extrapolations by writers and managers, as well as failure by scientists to clearly and accurately communicate results and conclusions.

ISSN:1051-0761    

DOI:10.2307/1941780

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

We argue that herbivores often induce nonlinear or biphasic growth and development in plants. Collectively these individual responses translate into a system—level optimization curve wherein at low levels of herbivory overall community responses show increases in production potential, whereas extreme herbivory causes extreme reduction in productivity. The transition between these two states defines a point of optimal herbivory in respect to C and N processes.We present four case examples from the literature demonstrating such nonlinear responses, suggesting a widespread existence for this herbivore—plant phenomenon. The nonlinear responses appear to demonstrate temporal and spatial scale dependencies.

ISSN:1051-0761    

DOI:10.2307/1941781

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

The evidence that pure ecological science has influenced grazing management practices employed in livestock husbandry on the North American Great Plains is tenuous at best. Those practices arose from critical technological innovations in the 19th century that modified the essential properties of grazing ecosystems when applied to livestock husbandry. Ecological research has identified many significant differences as well as similarities between natural and human—defined grazing systems, and how husbandry can lead to overgrazing. There is considerable evidence that moderate grazing can increase primary productivity and unequivocal evidence that some plants benefit from the presence of grazing animals.

ISSN:1051-0761    

DOI:10.2307/1941782

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Grazing is more than just defoliation of plants. The impact of herbivory affects ecosystem structure and function, both above and below ground. Ultimately, effects of herbivory are expressed to varying degrees at many levels of the ecosystem. Herbivory has been shown to affect plant physiology, morphology, and genetics. Plants have evolved many ways to avoid or tolerate herbivory. Whether plant overcompensate, equally compensate, or undercompensate to herbivory depends on pre— and post—harvest conditions of the plants and their environment. To be important to the manager, the magnitude of compensation must be greater than the inherent "noise" in the system. Natural resources managers use scientific information about herbivory to reduce ambiguity in decision—making in an environment of uncertainty. If an ecological response like compensation is to have practical application for the manager, the meaningful effects must occur on time and spatial scales that the manager can respond to with available resources.

ISSN:1051-0761    

DOI:10.2307/1941783

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Compensatory plant growth may be a significant ecological process that minimizes the reduction of primary production in direct proportion to the severity of defoliation in some species and systems given the appropriate combination of environmental variables. However, the potential benefits of compensatory growth should not obscure the well—established ecological processes governing the sustainability of grazed systems in the face of large scale environmental degradation and a rapidly expanding human population. The sustainability of grazed systems is a more fundamental issue than grazing optimization.

ISSN:1051-0761    

DOI:10.2307/1941784

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

The central assumption for management of range condition–that plant response to selective grazing drives changes in plant community structure–is only weakly supported by evidence from semi—arid rangelands. Most of the vegetation changes attributed to selective grazing can instead be explained through proper interpretation of grazing intensity. Specialized livestock grazing systems, which assume that selective seasonal grazing controls ecosystem function, work poorly on semi—arid rangelands when compared to simpler grazing methods based on managing grazing intensity. Compensatory growth has been well linked to ecosystem processes in highly productive and intensively managed pastures, but not on semi—arid rangelands.

ISSN:1051-0761    

DOI:10.2307/1941785

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Criticism of the basis and use of the herbivore optimization theory is discussed. It is argued here that evidence from theory and agricultural practice support the view that compensation and overcompensation of total primary production can occur in plant populations and communities subjected to grazing. However, whether this will occur depends on specific circumstances. Modeling and carefully designed field studies should be used to determine the responses of rangeland systems under a variety of environmental conditions and grazing intensities.

ISSN:1051-0761    

DOI:10.2307/1941786

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

There are several well—known mechanisms by which grazing can reduce the subsequent growth rate of plants, and several other well—documented mechanisms by which grazing can enhance plant growth rate. The net effect of single or repeated grazing events on the cumulative growth of plants may thus be zero, negative, or positive, depending on availability of leaf area, meristems, stored nutrients, and soil resources, and on the frequency and intensity of defoliation. Plants are preadapted to compensate, up to a certain point, for losses due to grazing, by virtue of their modular structure and development. Reports of "overcompensation" to grazing, as one extreme of a wide range of responses observed in natural grasslands, need not be treated with special skepticism; neither are they a solid base for a general theory of evolved grass–grazer mutualism. The question of compensatory growth is of relevance to management of Western rangelands for livestock production, but of relatively little relevance to conservation goals. No region—wide answers can be expected. Any drastic change in grazing intensity in either direction requires evaluation by community—specific and long—term research.

ISSN:1051-0761    

DOI:10.2307/1941787

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Recent articles in Ecological Applications and a response by Painter and Belsky (this issue) illustrate the controversy over theories of plant response to grazing, such as herbivore optimization and overcompensation. Observations of elk grazing in Yellowstone National Park and mixed cattle and elk grazing in the Southwest are presented as case studies to test these theories. Heavy grazing by elk in Yellowstone caused some areas to be dominated by exotic grasses, while shrubby species were heavily browsed. High—elevation rangeland conditions deteriorated in the Southwest where elk and cattle both grazed. Theories of herbivore optimization and overcompensation do not appear to be supported by the response of western rangelands to grazing by native herbivores.

ISSN:1051-0761    

DOI:10.2307/1941788

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

There has been a tendency for ecologists to present results from ecological studies and then be critical of resource managers because the best available scientific information was not incorporated into decisions about natural resources. This unfortunate situation must be remedied by ecologists, who should explicitly consider the ways in which results can be applied to management issues. Ecologists and resource managers should collaborate to establish the topical framework in which concepts such as growth compensation/overcompensation should be considered. Finally, under presettlement conditions, the tallgrass prairie probably produced overcompensation under lightly to moderately grazed conditions.

ISSN:1051-0761    

DOI:10.2307/1941789

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY