Effects of feral horse herds on plant communities across a precipitation gradient

Background/Question/Methods

Feral horse herds in the western United States are managed with the goal of maintaining “a thriving natural ecological balance” with their environment. Because rangeland ecology is complex and grazers can have different effects under different environmental conditions, more data is needed to better inform management decisions. Previous studies have indicated that grazing effects are correlated with productivity, with positive effects of grazing on diversity for more productive sites and negative effects for less productive sites. However, whether grazing by feral horses has similar effects has yet to be determined. The goal of our research is to evaluate the impacts of feral horses on plant communities across a range of potential productivity levels in multiple ecosystems of the western US. In 2014, we assessed plant community composition and diversity on grazed and ungrazed transects at five sites with feral horse populations that spanned a precipitation gradient (~123 mm to 381 mm annual precipitation). We used long-term grazing exclosures and fenceline contrasts to compare areas with and without horse use.

Results/Conclusions

Our preliminary results show a trend supporting the hypothesis that grazing increases plant species diversity at wetter (more productive) sites, and decreases diversity at drier sites. Plant species richness, diversity and evenness were all higher in ungrazed areas at our dry sites, but higher in grazed areas at our intermediate and wetter sites. Across dry sites, grazed areas had lower average richness than ungrazed sites (3.64 vs. 4.99 species per 1-m²). In contrast, across intermediate sites, grazing had little impact on species richness (grazed: 6.90 species per 1-m²; ungrazed: 6.41 species per 1-m²) whereas richness on average increased with grazing for the wettest site (16.27 and 13.28 species per 1-m², respectively). Differences in community composition between treatments were due mostly to shrub species at dry sites and graminoid species at intermediate and wetter sites; however, these compositional differences were not significant. Overall, our results to date suggest that, similar to patterns in other grazing systems, effects of feral horses on plant communities may vary depending on productivity, which has important implications for management of feral horses and their environments.