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To purchase or authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.1523-1739.2006.00606.x Byline: MATTHEW R. R. LOESER (*[double dagger]), THOMAS D. SISK (*), TIMOTHY E. CREWS ([dagger]) Keywords: Bromus tectorum; cheatgrass; climatic variation; exotic plants; livestock grazing; plant community; plant cover; short duration grazing Abstract: Abstract: The ecological benefits of changing cattle grazing practices in the western United States remain controversial, due in part to a lack of experimentation. In 1997 we initiated an experimental study of two rangeland alternatives, cattle removal and high-impact grazing, and compared grassland community responses with those with more conventional, moderate grazing practices. The study was conducted in a high-elevation, semiarid grassland near Flagstaff, Arizona (U.S.A.). We conducted annual plant surveys of modified Whittaker plots for 8 years and examined plant composition shifts among treatments and years. High-impact grazing had strong directional effects that led to a decline in perennial forb cover and an increase in annual plants, particularly the exotic cheatgrass (Bromus tectorum L.). A twofold increase in plant cover by exotic species followed a severe drought in the sixth year of the study, and this increase was greatest in the high-impact grazing plots, where native cover declined by one-half. Cattle removal resulted in little increase in native plant cover and reduced plant species richness relative to the moderate grazing control. Our results suggest that some intermediate level of cattle grazing may maintain greater levels of native plant diversity than the alternatives of cattle removal or high-density, short-duration grazing practices. Furthermore, episodic drought interacts with cattle grazing, leading to infrequent, but biologically important shifts in plant communities. Our results demonstrate the importance of climatic variation in determining ecological effects of grazing practices, and we recommend improving conservation efforts in arid rangelands by developing management plans that anticipate this variation. Abstract (Spanish): Impacto de la Intensidad de Pastoreo durante la Sequia en un Pastizal de Arizona Resumen: Los beneficios ecologicos del cambio de practicas de pastoreo de ganado en el oeste de Estados Unidos aun son controversiales, en parte por la falta de experimentacion. En 1997 iniciamos un estudio experimental de dos alternativas, remocion de ganado y pastoreo de alto impacto, y comparamos las respuestas de la comunidad de pastizal con las de practicas de pastoreo moderadas, mas convencionales. El estudio se llevo a cabo en un pastizal semiarido, de alta elevacion, cerca de Flagstaff, Arizona (E.U.A.). Durante 8 anos realizamos muestreos anuales de plantas en parcelas Whittaker modificadas y examinamos los cambios en la composicion de plantas entre tratamientos y anos. El pastoreo de alto impacto tuvo dos efectos direccionales que llevaron a una declinacion en la cobertura de hierbas perennes y al incremento de plantas anuales, particularmente de pasto exotico Bromus tectorum L. Despues de una sequia severa la cobertura de especies exoticas incremento al doble en el sexto ano del estudio, y este incremento fue mayor en las parcelas de pastoreo de alto impacto, en las que la cobertura de especies nativas declino a la mitad. La remocion de ganado resulto en un leve incremento en la cobertura de plantas nativas y redujo la riqueza de especies de plantas en relacion con el control con pastoreo moderado. Nuestros resultados sugieren que algun nivel intermedio de pastoreo puede mantener mayores niveles de diversidad de plantas nativas que las alternativas de remocion de ganado o de practicas de pastoreo de alta densidad y corta duracion. Mas aun, las sequias episodicas interactuan con el pastoreo de ganado, conduciendo a cambios, poco frecuentes pero biologicamente importantes, en las comunidades de plantas. Nuestros resultados demuestran la importancia de la variacion climatica en la determinacion de los efectos ecologicos de las practicas de pastoreo, y recomendamos el mejoramiento de los esfuerzos de conservacion en llanuras aridas mediante el desarrollo de planes de gestion que anticipen esta variacion. Author Affiliation: (*)Center for Environmental Sciences and Education, Northern Arizona University, Box 5694, Flagstaff, AZ 86011-5694, U.S.A. ([dagger])Environmental Studies Program, Prescott College, 220 Grove Avenue, Prescott, AZ 86301, U.S.A. Article History: Paper submitted August 16, 2005; revised manuscript accepted May 15, 2006. Article note: ([double dagger]) Current address: Biology Department, Yakima Valley Community College, P.O. Box 22520, Yakima, WA 98907-2520, U.S.A., email mloeser@ yvcc.edu

Under rapid landscape change, there is a significant need to expand and connect protected areas (PAs) to prevent further loss of biodiversity and preserve ecological functions across broad geographies. We used a model of landscape resistance and electronic circuit theory to estimate patterns of ecological flow among existing PAs in the western United States. We applied these results to areas previously identified as having high conservation value to distinguish those best positioned to maintain and enhance ecological connectivity and integrity. We found that current flow centrality was highest and effective resistance lowest in areas that spanned the border between southern Oregon and Idaho, and in northern Arizona and central Utah. Compared to other federal jurisdictions, Bureau of Land Management lands contributed most to ecological connectivity, forming 'connective tissue' among existing PAs. Our models and maps can inform new conservation strategies and critical land allocation decisions, within or among jurisdictions. [ABSTRACT FROM AUTHOR]