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Large herbivores influence the composition and diversity of shrub-steppe communities in the Rocky Mountains, USA


It is widely believed that wild and domestic herbivores have modified the structure and composition of arid and semi-arid plant communities of western North America, but these beliefs have rarely been tested in long-term, well-replicated studies. We examined the effects of removing large herbivores from semi-arid shrublands for 40–50 years using 17 fenced exclosures in western Colorado, USA. Shrub cover was greater (F=5.87, P=0.0020) and cover (F=3.01, P=0.0601) and frequency (F=3.89, P=0.0211) of forbs was less inside the exclosures (protected) relative to grazed plots. However, we found no significant effects (minimum P=0.18) of protection from grazing on cover or frequency of grasses, biotic crusts, or bare soil. Although mean species richness and diversity were similar between treatments, protected areas had much higher dominance by fewer species, primarily sagebrush. Exclusion of herbivores changed the relationship between species richness and evenness. Consistent with theoretical expectations, species evenness was positively correlated with richness in protected plots (r 2=0.54). However, contrary to theory, evenness and richness were inversely related in grazed plots (r 2 adjacent=0.72, r 2 distant=0.84). We suggest that these differences resulted because grazing acts as a stressor promoting facilitative relationships between plant species that might compete for resources in the absence of grazing. We conclude that exclusion of grazing in the sites we studied caused minor changes in cover and diversity of herbaceous plants, but caused a clear increase in the cover of shrubs. Importantly, the exclusion of ungulates changed the relationship between evenness and richness.

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We are grateful to the staff at the Natural Resource Ecology Laboratory, Colorado State University, for their help in this research. We are particularly indebted to the diligent field crews who assisted in the data collection: M. Astele, B. Chemel, S. Farrell, J.P. Jensen, J. Levick, I. Roblee-Hertzmark, D. Rosie, S. Street, and A.Young. We appreciate the cooperation and assistance of the U.S. Forest Service staff for the Gunnison, Grand Mesa, and Uncompahgre National Forests and field office staff of the Bureau of Land Management in Craig, Grand Junction, Gunnison, Kremmling, and Montrose. This research was funded by the Colorado Division of Wildlife.

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Correspondence to Daniel J. Manier.

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Communicated by Jim Ehleringer

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Manier, D.J., Thompson Hobbs, N. Large herbivores influence the composition and diversity of shrub-steppe communities in the Rocky Mountains, USA. Oecologia 146, 641–651 (2006).

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  • Succession
  • Herbivory
  • Ungulates
  • Semi-arid
  • Sagebrush