Livestock grazing is the most widespread land manageraent pracPice in western North America Seventy percent of the western United States is grazea~ including wilderhess area~ tvlldlife refuge& national forest~ and even some national park~ The ecological costs of this nearly ubiqui. tous form of land use can be drarr~pi~ Examples of such costs include loss of biodiversity; lowering of population densities for a wide variety of taxa~ disruption of ecosystem funcPion~ including nutrient cycling and succession,, change in community organization; and change in the physical characteristics of both terrestrial and aquatic habitat~ Because livestock congregate in riparian ecosysten~ which are among the biologically richest habitats in arid and semiarid region~ the ecological costs of grazing are magnified in these site~ Range science has traditionally been laden with economic assumppions favoring resource use Conservation biologists are encouraged to contribute to the ongoing social and scientific dialogue on grazing issuex Costus ecol6gicos del pastoreo de ganado en el oeste de Estados Unidos Resumen: El pastoreo de ganado es la prdcpica de manejo de la tierra mds ampliamente upilizada en el oeste de Norte Am~xt E1 setenta por ctento del oeste de Estados Untdos se upiliza para pustoreo, incluyendo dreas silvestreg refugios de vida silvestre, bosques nacionales e inclusive algunos parques nacionale& El costo ecol6gico de esta forma ubicua de uso de ia Pierra puede set dramdpico. Ejemplos de este costo incluyen pdrdida de ia biodiversidad; decrecimiento de las densidades de poblaci6n para una amplia variedad de taxone~, alteraciones en ias funciones del ecosistem, incluyendo ciclos de nutrientes y sucesiones; cambios en ia organizaci6n de la comunidad y cambios en las caracteris. Picas flsicas de hdhitas terrestres y acudpico~ Dado que el ganado se congrega en ecosistemas ribere~og los cuales estdn entre los hdbitas biol6gicamente mds ricos dentro de ias regiones dridas y semi.dridag los costos ecol6gicos del pastoreo se magnifican en estos siPio& Tradicionalment~ la ciencia de Pastizale~ ha estado cargada de suposiciones, eco-n6micas que favorecen el uso del recurso. Se alienta a los bi61ogos conservacionistus a contribuir al didlogo social y ctenttflco en los problemas del pastoreo.

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Climate change affects public land ecosystems and services throughout the American West and these effects are projected to intensify. Even if greenhouse gas emissions are reduced, adaptation strategies for public lands are needed to reduce anthropogenic stressors of terrestrial and aquatic ecosystems and to help native species and ecosystems survive in an altered environment. Historical and contemporary livestock production-the most widespread and long-running commercial use of public lands-can alter vegetation, soils, hydrology, and wildlife species composition and abundances in ways that exacerbate the effects of climate change on these resources. Excess abundance of native ungulates (e.g., deer or elk) and feral horses and burros add to these impacts. Although many of these consequences have been studied for decades, the ongoing and impending effects of ungulates in a changing climate require new management strategies for limiting their threats to the long-term supply of ecosystem services on public lands. Removing or reducing livestock across large areas of public land would alleviate a widely recognized and long-term stressor and make these lands less susceptible to the effects of climate change. Where livestock use continues, or where significant densities of wild or feral ungulates occur, management should carefully document the ecological, social, and economic consequences (both costs and benefits) to better ensure management that minimizes ungulate impacts to plant and animal communities, soils, and water resources. Reestablishing apex predators in large, contiguous areas of public land may help mitigate any adverse ecological effects of wild ungulates.

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Livestock grazing is the most ubiquitous land use in western North America, yet it rarely has been studied in a controlled manner because of the lack of large areas free of grazing. We compared the ecological effects of three grazing treatments-long-term protection, short-term protection, and currently grazed-at Chaco Culture National Historic Park in northern New Mexico. Chaco has a long history of human habitation and is now one of the largest grazing exclosures in the American West. We studied the effects of livestock grazing on the cover of plants, soil crusts, and plant species richness at six sites with different potential natural vegetation. Species richness was higher under long-term protection than under current grazing at all six sites. Trends in shrub and grass response varied significantly across the six sites. Shrub cover increased with long-term protection at four upland sites, and grass cover increased with protection at four sites. The response of Chaco vegetation to release from grazing varied significantly according to each site's ecological potential, determined in part by edaphic and topographic characteristics. These nuances in vegetation response represent natural ecological variation and contrast with the notions of widespread shrub "invasion" often inferred in the past.Efectos del Pastoreo Histórico de Ganado sobre la Vegetación en el Parque Histórico Nacional Cultura Chaco, Nuevo México Resumen: El pastoreo de ganado es el uso de suelo más generalizado en Norte América occidental, sin embargo rara vez ha sido estudiado de manera controlada debido a la ausencia de extensasáreas libres de pastoreo. Comparamos los efectos ecológicos de tres tratamientos de pastoreo -protección a largo plazo, protección a corto plazo y pastoreo actual -en el Parque Histórico Nacional Cultura Chaco, en el norte de Nuevo México. Chaco tiene una larga historia de presencia humana y ahora es una de lasáreas libres de pastoreo más extensas en el oeste americano. Se estudiaron los efectos del pastoreo de ganado sobre la cobertura de plantas, corteza de suelo y riqueza de especies de plantas en seis sitios con diferente potencial de vegetación natural. La riqueza de especies fue mayor bajo protección a largo plazo que bajo pastoreo actual en los seis sitios. Las tendencias en la respuesta de arbustos y hierbas variaron significativamente en los seis sitios. La cobertura de arbustos incrementó con la protección a largo plazo en cuatro sitios elevados, y la cobertura de pasto incrementó con la protección en cuatro sitios. La respuesta de la vegetación de Chaco a la ausencia de pastoreo varió significativamente conforme al potencial ecológico de cada sitio, determinado en parte por características edáficas y topográficas. Estos matices en la respuesta de la vegetación representan una variación ecológica natural y contrastan con las nociones de "invasión" generalizada de arbustos, inferida a menudo en el pasado.