Influence of Pocket Gophers on Meadow Voles in a Tallgrass Prairie

Creation of mounds by plains pocket gophers (Geomys bursarius) may affect the distribution of meadow voles (Microtus pennsylvanicus) and subsequent patterns of herbivory. An association between abundance of meadow voles and density of neighborhood mounds might be expected if voles use mounds as feeding sites (positive association), or if vole avoid mounds due to lack of cover (negative association). Mounds also may indirectly affect vegetative patterns, if mounds affect the distribution of herbivores. During two consecutive summers, we livetrapped voles and mapped mounds on three 80 m X 80 m plots. Regressions of vole captures at each trap station against the number of mounds within a range of radii of each trap were consistently negative across most spatial scales, but strongest correlation was found to be at 10 m. In all cases, bivariate scattergrams revealed a "factor-ceiling" distribution, with data points widely scattered below an upper limit. We also conducted an experiment to determine if seedlings had a greater or lesser mortality due to small-mammal herbivory when growing on mounds versus off mounds. A two-tailed paired t-test on the proportion of seedlings eaten after 13 days failed to show differences between on-mound and off-mound rates of herbivory (P = 0.066); however, prior analyses showing voles avoiding areas with high rates of disturbance give weight to the hypothesis that seedlings growing off mounds experience higher rates of herbivory. Thus, through a second-order indirect effect, the presence of gopher mounds could lead to distinct changes in the plant community.