|Abstract: Livestock grazing is the most widespread land management practice in western North America. Seventy percent of the western United States is grazed, including wilderness areas, wildlife refuges, national forests, and even some national parks. The ecological costs of this nearly ubiquitous form of land use can be dramatic. Examples of such costs include loss of biodiversity; lowering of population densities for a wide variety of taxa; disruption of ecosystem functions, including nutrient cycling and succession; change in community organization; and change in the physical characteristics of both terrestrial and aquatic habitats. Because livestock congregate in riparian ecosystems which are among the biologically richest habitats in arid and semiarid regions, the ecological costs of grazing are magnified in these sites. Range science has traditionally been laden with economic assumptions favoring resource use. Conservation biologists are encouraged to contribute to the ongoing social and scientific dialogue on grazing issues.||Costos ecológicos del pastoreo de ganado en el oeste de Estados Unidos|
Resumen:El pastoreo de ganado es la práctica de manejo de la tierra más ampliamente utilizada en el oeste de Norte América. El setenta por ciento del oeste de Estados Unidos se utiliza para pastoreo, incluyendo areas silvestres; refugios de vida silvestre, bosques nacionales e inclusive algunas parques nacionales. El costo ecológico de esta forma ubicua de uso de la tierra puede ser drámatico. Ejemplos de este costo incluyen pérdida de la biodiversidad; decrecimiento de las densidades de población para una amplia variedad de taxones; alteraciones en las funciones del ecosistem, incluyendo ciclos de nutrientes y sucesiones; cambios en la organización de la comunidad y cambios en las caracteristicas fisicas de habitas terrestres y acuáticos. Dado que el ganado se congrega en ecosistemas ribereños; los cuales están entre los hábitas biológicamente más ricos dentro de las regiones áridas y semi-áridas; los costos ecológicos del pastoreo se magnifican en estos sitios. Tradicionalmente, la ciencia de pastizales, ha estado cargada de suposiciones económicas que favorecen el uso del recurso. Se alienta a los biológos conservacionistas a contribuir al diálogo social y cientifico en los problemas del pastoreo.
Aldo Leopold (1953) once said that to be an ecologist is to "live alone in a world of wounds." The spectacular groundswell of interest in conservation biology is heartening evidence that we no longer work alone. But what about a world of wounds? The wounding of natural processes accelerates, but some wounds are more conspicuous than others. Recognizing a clearcut forest is easy, but it often takes a trained eye to comprehend damage to rangelands. The destruction caused by livestock grazing is so pervasive and has existed for so long that it frequently goes unnoticed. Livestock grazing has received far less attention from conservation biologists than its widespread influence would suggest is appropriate. When I recently surveyed the first six volumes of this journal, for example, I found almost three times as many articles on deforestation as on grazing-related topics.
Livestock grazing is the most widespread influence on native ecosystems of western North America (Wagner 1978; Crumpacker 1984). Grazing by livestock, primarily cattle, is nearly ubiquitous throughout this region. Approximately 70% of the 11 western states of the United States (Montana, Wyoming, Colorado, New Mexico, and westward) is grazed by livestock (Council for Agricultural Science and Technology 1974; Longhurst et al. 1984; Crumpacker 1984), including a broad diversity of ecosystem types and virtually all types of land management designations. Grazing occurs in creosote bush deserts, blackbrush deserts, slickrock mesas, sagebrush flats, pinyon-juniper woodlands, chaparral, ponderosa pine forests, and alpine meadows above timberline.
Grazing occurs on the majority of federal lands in the West, including most of the domains of the U.S. Bureau of Land Management (BLM) and the US. Forest Service, as well as in many national wildlife refuges, federal wilderness areas, and even some national parks. In 16 western states, approximately 165 million acres of BLM land and 103 million acres of Forest Service land are grazed by 7 million head of livestock, primarily cattle (U.S. General Accounting Office 1988a). Of the BLM lands in these states, 94% is grazed. Of federal wilderness areas, 35% have active livestock grazing allotments (Reed et at 1989; this figure is from a nationwide survey - the percentage for the West is probably higher). Urbanized areas, some dense coniferous forests, and a few rock-and-ice peaks are about all that is free from the influence of livestock. Given the ubiquity of livestock, it behooves us to understand the consequences of its presence on the Western landscape.
Understanding the influence of domestic livestock upon native ecosystems is a problematic process. Ascertaining the potential natural vegetation of most Western ecosystems is difficult because ungrazed land is extremely rare. Ecologists have gained insight into the effects of grazing primarily in three ways: (1) Historic records provide perspective on the dramatic changes that have transpired since the introduction of livestock to the West (see Cooper 1960). As Hastings (1959) pointed out, however, one must be cautious in interpreting historical records, due to the subjectivity of different observers. Historic photographs have also been used in an attempt to recreate an ecological baseline (see Hastings & Turner 1965); Bahre (1991) reviewed the necessary cautions in interpreting historic photographs. (2) Areas excluded from grazing through serendipity, such as isolated mesa tops, provide startling contrast to adjacent areas that have been continuously grazed (see Rummell 1951). (3) Areas that intentionally exclude livestock (exclosures) provide a before-grazing and after-grazing comparison. Exclosures can be monitored as they recover from the effects of grazing and can be compared with adjacent grazed sites. Almost all exclosures share two characteristics: (1) their areas are usually quite small (Bock et al. 1993a), often less than 50 ha; and (2) they have been grazed prior to exclosure. In other words, very few studies of truly ungrazed landscapes exist. Most recreational impact studies concur that the original impact upon a pristine site is the most severe (Cole 1981; Cole & Marion 1986); thus, exclosure studies probably underestimate the true extent of grazing effects because they cannot monitor the most drastic damage, which occurred long ago. In addition, virtually all exclosure studies examine areas too small to encompass landscape-level diversity. In summary, we lack a clear ecological benchmark for determining the effects of grazing.
Attempts to discern grazing effects are also hampered by the difficulty in distinguishing between different range management practices. Management variables include grazing intensity ("stocking rate"), livestock species, seasonality of grazing, and degree of active management, such as movement of livestock between pastures. Unfortunately, the management history of many sites is unknown. Many studies do not describe grazing intensity (see, for example, Glinski 1977; Reynolds & Trost 1980; Crouch 1982). Furthermore, standardized terminology is lacking for different grazing intensities. Relative terms, such as "heavy," "moderate," and "light" grazing, may be undefined (see Jeifries & Kiopatek 1987) or qualitatively defined in very different ways. Among the criteria used are presence of livestock, presence of trails, range condition (see Jones 1981), and amount of herbage remaining after a grazing season (see Welch et al. 1991). Studies that have quantified grazing intensity have done so inconsistently. For example, two studies (Mosconi & Hutto 1982; Baker & Guthery 1990) analyzing the effect of "heavy" grazing differed in their definition by a factor of seven. The much-used term "overgrazing" is wrought with controversy and lack of clarity; even specific discussions of overgrazing fail to define it (see Menke & Bradford 1992). This rudimentary state of knowledge interferes with analysis of the role of different grazing practices on biodiversity.
Available evidence indicates that livestock grazing has profound ecological costs. Autecological, synecological, and geomorphological studies have confirmed that native ecosystems pay a steep price for the presence of livestock. Three primary attributes of ecosystems have been elucidated: composition, function, and structure (Franklin a al. 1981). Livestock grazing has a profound impact on all three. The ecological costs of livestock grazing can be summarized as follows:
I ) Alteration of species composition of communities,including decreases in density and biomass of individual species, reduction of species richness, and changing community organization.
(2) Disruption of ecosystem functioningincluding interference in nutrient cycling and ecological succession.
(3) Alteration of ecosystem structure,including changing vegetation stratification, contributing to soil erosion, and decreasing availability of water to biotic communities.
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