Large herbivores in sagebrush steppe ecosystems: livestock and wild ungulates influence structure and function

Improving understanding of the connections between vegetation, herbivory, and ecosystem function offers a fundamental challenge in contemporary terrestrial ecology. Using exclosures constructed during the late 1950s, we examined effects of grazing by wild and domestic herbivores on plant community structure, aboveground herbaceous primary production, and nutrient cycling at six sites in semi-arid, sagebrush rangelands during 2001-2002 in Colorado, USA. Enclosures provided three treatments: no grazing, grazing by wild ungulates only, and grazing by wild and domestic ungulates. Excluding all grazing caused an increase in shrub cover (F = 4.97, P = 0.033) and decrease in bare ground (F = 4.74, P = 0.037), but also a decrease in plant species richness (F = 6.19, P = 0.018) and plant diversity (F = 7.93, P = 0.008). Effects of wild ungulate grazing on plant cover and diversity were intermediate to the effects of combined domestic and wild grazing. Aboveground net primary production was higher in both grazed treatments than in the ungrazed one (F(wild + domestic) = 2.98, P = 0.0936 and F(wild only) = 3.55, P = 0.0684). We were unable to detect significant effects of grazing on other ecosystem states and processes including C:N ratios of standing crops, N mineralization potential, or nitrification potential. Best approximating models revealed positive correlation between N availability and herbaceous cover and a negative correlation between herbaceous primary production and the ratio of shrub-herb cover and plant diversity. We conclude that ungulate herbivory, including both wild and domestic ungulates, had significant effects on plant community structure and ecosystem function during this 42-year span. Responses to the wild ungulate treatment were consistently intermediate to responses to the no grazing and wild + domestic grazing treatments. However, we were unable to detect statistical difference between effects of wild ungulates alone and wild ungulates in combination with livestock.