Ryerson and Parmenter report observations of vegetation change in a region of central New Mexico, from 1976-1996, following the removal of all livestock and prairie dogs in 1972-1975, in order to quantify vegetation changes at both landscape and site-specific spatial scales. While each site underwent varying degrees and directions of change, the resulting pattern at the landscape level showed no net change in total perennial plant cover. Grass species increased on nearly every site, while Gutierrezia sarothrae shrubs declined; these different directional changes cancelled each other's contribution at the landscape level, resulting in the appearance of no net change in perennial plant cover. Annual forbs and above-ground accumulation of litter showed significant increases at the landscape level and in all six of the final 1996 plant assemblages. Sites that were generally dissimilar in 1976 had increased their similarities by 1996, so that the number of distinct plant assemblages defined by the similarity analyses declined from 11 to 6. Certain species that were not preferred by livestock or prairie dogs, showed overall declines during drought periods, while other preferred species exhibited widespread increases during wetter periods regardless of herbivore presence. Therefore, the vegetation dynamics cannot be attributed solely to removal of herbivores, and in some cases can be explained by short- and long-term fluctuations in climate. These results emphasize the variety of responses of sites with differences in vegetation to mammalian herbivores under otherwise similar climatic conditions, and illustrate the value of site- and landscape-scale approaches to understanding the impacts of plant-herbivore interactions.
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