Discussion
Based on our estimates, beaver-modified patches may contribute as much as 25% of the total herbaceous plant species richness of the riparian zone. This estimate of the effect of beaver-modified habitat on the species richness of the riparian zone is likely to be an underestimate as it was based on the relative proportion of stream length flowing through beaver-modified habitat. Beaver tend to occupy sites where the stream gradient is low and the riparian zone is relatively wide (Howard and Larson 1985), thus the proportion of the riparian zone that has been modified by beaver is likely to be much higher on a per unit area basis than on a stream length basis. Furthermore, this study looked at only the effects of beaver modification on the vascular plant community. Pollock et al. (1998) found that mosses were quite diverse in beaver meadows in Alaska, and contributed significantly to overall diversity. If beaver have a similar effect on moss diversity in the Adirondacks to that in southern Alaska, their effect on total plant species richness of the riparian zone might be even greater than their effect on vascular plant diversity.