Local vs. Landscape Controls on Plant Species Richness in Beaver Meadows

There is considerable interest in determining whether the species richness of communities is determined by forces controlling dispersal into patches that operate at the landscape scale, or forces controlling persistence that act at the local scale. Understanding the relative importance of these two classes of factors in controlling within-patch species richness is particularly important when patches are created via ecosystem engineering. In such cases, factors affecting the population dynamics or behavior of a single species could indirectly affect species richness if richness is controlled primarily by landscape-level factors. We used a combination of experimental mesocosms and field observations to determine whether species richness in beaver wetlands in the Adirondack Mountains (New York) is more strongly controlled by the position of the wetland in the landscape or by within-wetland hydrology. Drainage rate had a significant effect on both richness and composition in mesocosms, with well-drained treatments having significantly higher richness than poorly drained treatments. Seed germinated from the seed bank in sediments collected from different ponds showed relatively small differences in richness or community composition in mesocosms, suggesting a comparatively small effect of dispersal limitation on species richness. Experimental results were mirrored in a survey of 14 meadows over two years, which indicated that variability in water table depth was consistently a significant predictor of species richness, while meadow area and isolation showed little relation to richness. The survey also suggested that the number of years since beaver had abandoned a site was a significant predictor of the number of species found in beaver meadows. The results indicate that species richness in beaver meadows is strongly controlled by local factors, but that the population dynamics of beaver could also potentially affect species richness by altering the age distribution of meadows across the landscape.