PS 33-40
The effect of large ungulates on sagebrush plant communities: Demonstration of a need to combine complex herbivory and abiotic interactions to understand state shifts
Arid rangelands in North America have experienced large changes in grazing pressure in the last 200 years, including the introduction and use by multiple species of wild ungulates and domestic livestock. Herbivory pressure can greatly affect arid plant communities by causing state shifts from one composition to another. Research using single herbivore species models has shown that plant community composition can be shifted differently depending on the herbivore’s feeding guild. However, little is known how overlapping use by multiple species of large ungulates effect plant community composition and alternative state shifts. Using a series of long-term wildlife and livestock exclosures (60+ years), we tested how sagebrush (Artemisia tridentata) plant community composition and structure has responded to three different large ungulate foraging pressures: 1) no ungulate pressure, 2) elk (Cervus canadensis) and mule deer (Odocoileus hemionus) foraging pressure, and 3) elk, mule deer and cattle (Bos primigenius) pressure. We measured plant community composition, seasonal ungulate use, and abiotic factors including soil depth and precipitation. Treatments were compared using MANOVA and ANOVA analysis (significance p<0.01).
Results/Conclusions
Across all sites there were significant treatment effects on different size classes of sagebrush in the plant community. The presence of deer and elk significantly decreased densities of large sagebrush (sizes 50-100cm and 100-200cm). The presence of cattle significantly increased densities of small sagebrush (<15cm), suggesting that cattle may promote sagebrush recruitment. When soil depth was accounted for across sites, the treatment effect (herbivory) was significantly different between deep (>75cm) and shallow (<75cm) soil sites. At deeper soiled sites, sagebrush declined from 22.6% cover in treatments with no large ungulates to less than 2.0% cover with ungulates present. These communities shifted from sagebrush-dominance in the absence of large ungulates to bunchgrass-dominance in the presence of elk and deer. Sites with shallow soils remained sagebrush-dominated regardless of herbivory treatment. Overall, our results show that different combinations of herbivores alter sagebrush plant communities differently. Furthermore, the extent that sagebrush plant community composition and structure is altered by herbivory is controlled by soil depth. Consistent with contemporary community assembly theory, this work highlights that potential plant community states are determined by abiotic factors but the additions of biotic factors (herbivory in this case) ultimately shape the observed plant community.