Grasslands provide numerous ecosystem services, including habitat, carbon sequestration and conservation of biodiversity, and are often used as rangelands to support livestock. Primary consumers play a critical role in ecosystem structure and function, but human-related activities in rangelands have greatly altered top-down control by large-herbivores, which may result in consequences for biodiversity and other ecosystem services. Grasslands in British Columbia have experienced dramatic change over the past hundred years as a result of invasion by exotic plants, forest encroachment, and climate change but little is known how large-herbivores influence these changes.
To better understand the impacts of herbivory on ecosystem structure and function in semiarid grasslands, forty-five large-herbivore exclosures were established over the past 100 years in the southern interior of British Columbia. Vegetation surveys, biomass harvests, and soil samples were taken at each site both within and outside of the exclosures. We explored the impacts of exclosure, elevation (450-1300m), and age of exclosure (2-75 years) on productivity, richness, diversity, and plant community dynamics. We expected that large-herbivores would have greater negative impacts on productivity at higher elevations, which tend to be more productive compared to lower elevations. We expected that greater increases in productivity would increase competition for resources like light and space and decrease plant diversity
Results/Conclusions
Herbivory decreased productivity at all elevations, but the differences in productivity between grazed and ungrazed sites increased along an elevational gradient. Overall richness and diversity (1-D) decreased on ungrazed sites regardless of elevation or age of exclosure. While richness was not impacted by elevation, diversity increased with increasing elevation. There was an interactive effect of elevation and exclosure on grass and legume diversity (1-D). At lower elevations (<400m), grass diversity was greater under grazing pressure, but at higher elevations, grass diversity was greater when grazing pressure was removed. Legume diversity generally decreased with elevation, but grazing pressure decreased legume diversity at lower elevations (<800m) and increased legume diversity at higher elevations. Forb diversity was not impacted by elevation but was higher with grazing pressure. These results suggest that elevation plays an important role in determining the response of plant communities to large-herbivore grazing pressure.