The effects of grazing on the endangered autumn buttercup
Autumn buttercup (Ranunculus acriformis var. aestivalis) is a federally listed endangered plant. Overgrazing by cattle and horses was thought to be the primary threat to the species, so The Nature Conservancy acquired and fenced a 44 acre parcel of land and grazing was discontinued. Since the removal of large herbivores from the site the autumn buttercup has become extirpated and it was hypothesized that an increase in small herbivores, like voles, was a major cause. June 2013 an experimentally driven reintroduction of the autumn buttercup was undertaken to determine if small herbivores were a threat to the plant, and if managed grazing by large herbivores was an effective strategy for reducing the density of small herbivores. Prior to the reintroduction, half of the site was grazed by cattle, and during planting, half of the plants were caged to protect them from small mammalian herbivores. Differences in survival and herbivory between the plants in the 4 treatment groups (grazed and uncaged, grazed and caged, ungrazed and uncaged, and ungrazed and caged) were measured monthly during the 2013 and 2014 growing season. Effects of grazing on small mammal densities was also measured using mark recapture methods.
Grazing had beneficial effects on the survival of autumn buttercup. By the end of the 2014 growing season 50% of the caged plants on the grazed side remained while only 18% of the caged plants on the ungrazed side remained. Survival rates were less than 10% for the uncaged plants throughout the preserve (Wilcoxon Χ2=84.14 p<0.001). The uncaged plants on the grazed side experienced significantly lower rates of herbivory than the uncaged plants on the ungrazed side (19% vs 29% respectively Χ2=6.1 p=0.013). None of the caged plants showed signs of herbivory. A total of 270 small mammals were trapped over 15 trap nights with deer mice (Peromyscus maniculatus n=202) and long-tailed field voles (Microtus longicaudus n=65) being the most numerous species trapped. The density of deer mice was two times higher on the ungrazed side compared to the grazed side in 2013 (p<0.05) and the density of voles was higher on the ungrazed side in July and August of 2014 (p<0.05). Small herbivores represent a major threat to the survival of the endangered autumn buttercup. Managed grazing by large herbivores is an effective management strategy to reduce the presence of small herbivores.