Contrasting effects of wild and domestic ungulates on sagebrush plant communities

Authors: VEBLEN, KARI - Utah State University; NEHRING, KYLE - Utah State University; MCGLONE, CHRISTOPHER - Former ARS Employee; RITCHIE, MARK - Syracuse University

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/14/2015
Publication Date: N/A
Citation: N/A

Interpretive Summary: Plant consumption by cattle and large wild herbivores, such as elk and deer, is presumed to have profoundly influenced ecosystems in many parts of the world. In spite of this fact, there is very little experimental evidence supporting or refuting this presumption. Additionally, there are few scientific data that help explain whether grazing by cattle has a different impact on local vegetation than plant consumption by large wild animals. In a sagebrush community on a large, private cattle ranch in northern Utah, we erected fences to keep different groups of herbivores out of portions of three different pastures. Some areas excluded only cattle, others excluded cattle and large wild herbivores, and some areas allowed access to all herbivores. After 22 years, we returned to the ranch to measure changes in the plant community and important soil properties that can be indicators of ecosystem health. Different groups of herbivores created diverse responses in the plant community. Cattle grazing had a negative influence on the native grass species bottlebrush squirreltail. This species has been shown to be important in suppressing the spread of nonnative annual plants, such as cheatgrass. We did observe greater levels of nonnative species in areas grazed by cattle. Conversely, cattle grazing had a positive influence on Sandberg’s bluegrass, an important early season forage species. Either group of plant consumers, cattle or large wild herbivores, caused increased bare soil and reduced soil biotic crusts, an important stabilizer of soils in arid regions. While no group of herbivore had a severe impact on the local plant community after 22 years, there were significant differences caused by the inclusion of cattle to the pool of local grazers. In a dry region, such as the Interior West, where plant community changes can occur very slowly, our results suggest that there will be increased differences in these plant communities in the presence of different groups of large herbivore. Long-term management strategies should take into account the unique impacts that cattle have on an ecosystem.

Technical Abstract: Herbivory by both domestic livestock and native wild ungulates shapes the structure and functioning of terrestrial ecosystems worldwide, and both types of herbivory have been implicated in major ecosystem state changes. Despite the ecological consequences of differences in diets and feeding habits among herbivore guilds, studies that experimentally distinguish effects of domestic livestock from co-occurring wild ungulates are extremely rare. Here we use a set of long-term exclosures in northern Utah, USA, to determine how livestock vs. wild ungulates affect sagebrush-dominated plant communities which historically covered ~62 million ha in North America. We sampled plant community properties and found that after twenty-two years wild and domestic herbivores elicited perceptible changes in overall plant community composition and distinct responses by individual plant species. The perennial bunchgrass, Elymus elymoides, responded positively to release from cattle grazing, and in turn appeared to competitively suppress another more grazing tolerant perennial grass, Poa secunda. Livestock activity also was associated with increased non-native species biomass. In the woody plant community, release from wild herbivore browsing (but not cattle) increased densities of larger sagebrush at the expense of small sagebrush, while disturbance associated with either wildlife or cattle alone was sufficient to increase bare ground and reduce cover of biological soil crusts, an important soil stabilizer. These results illustrate that although different herbivory regimes have not caused sagebrush plant communities to undergo dramatic state shifts, clear changes are evident only twenty-two years after herbivore exclusion. In a dry, perennial-dominated system where plant community changes can occur very slowly, our results provide insights into long-term trajectories of these plant communities under different large herbivore regimes and can be used to guide long-term management strategies.