Given the large changes in a number of physical conditions that occur after a site has been occupied by beaver (Naiman et al. 1994; Johnston et al. 1995), it is difficult to predict a priori whether these sites should have higher or lower species richness than sites that have never been modified by beaver. We found that patches that had and had not been modified by beaver had similar levels of species richness at the patch scale (alpha diversity, sensu Whittaker 1972). Thus, the increase in species richness at the landscape scale contributed by beaver-modified patches is not simply due to the replacement of species-poor forest patches by species-rich meadow patches. Rather, the low overlap in species composition between the two patch types means that the presence of beaver-modified habitats allows a number of species to persist in the riparian zone that otherwise would be excluded.