Given the significant differences in resource availability, we hypothesized that beaver-modified riparian patches are sufficiently different from the forested riparian zone to support a distinct assemblage of plant species. If so, we would expect the presence of beaver-modified patches to increase plant species richness of the riparian zone compared to a landscape consisting entirely of forested riparian zone. To test the hypothesis that ecosystem engineering by beaver increases species richness at the landscape scale, we sampled the plant community of beaver-modified riparian sites and riparian sites with no history of beaver modification to determine the degree of similarity in species composition. We then quantified the magnitude of the engineering effect on species richness at the landscape scale by using resampling techniques. Such estimates of the strength of the effect of an engineer on species richness provide a valuable tool for comparing the relative importance of different species of engineers in structuring ecosystems.