Macrophyte succession in Minnesota beaver ponds

Beavers (Castor canadensis Kuhl.) are keystone species that dramatically al ter nutrient cycles and food webs in aquatic and terrestrial ecosystems by modifying their hydrology and selectively removing riparian trees. We docum ented macrophyte succession in 36 beaver ponds ranging from 4 to over 40 ye ars old. We used impounded bogs, because they were isolated from other wate rbodies and less prone to washout. Richness and diversity increased linearl y in ponds during the first four decades. Pond age and the product of pond size and the number of neighboring ponds in a 0.25-km radius explained 64% (p = 0.001) of the variation in richness. Compositional trends were best ex plained by propagule dispersal traits. Initial colonists included vagile sp ecies, like free-floating macrophytes and desirable foods for waterfowl, li ke the narrow-leaved pondweeds. Ponds of intermediate age (11-40 years) had the highest diversity, with both floating-leaved and submersed life forms represented. Two community types were identified in older ponds: one charac terized by dense lily pad cover and the other characterized by a rich assem blage of Potamogeton. By developing a predictive model for macrophyte succe ssion in beaver ponds, we provide a basis for studying a variety of ecologi cal processes and organisms that depend on macrophytes in these regionally abundant habitats.