Macrophyte succession in Minnesota beaver ponds

Beavers (Castor canadensis Kuhl.) are keystone species that dramatically alter nutrient cycles and food webs in aquatic and terrestrial ecosystems by modifying their hydrology and selectively removing riparian trees. We documented macrophyte succession in 36 beaver ponds ranging from 4 to over 40 years old. We used impounded bogs, because they were isolated from other waterbodies and less prone to washout. Richness and diversity increased linearly 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 explained by propagule dispersal traits. Initial colonists included vagile species, like free-floating macrophytes and desirable foods for waterfowl, like 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 characterized by dense lily pad cover and the other characterized by a rich assemblage of Potamogeton. By developing a predictive model for macrophyte succession in beaver ponds, we provide a basis for studying a variety of ecological processes and organisms that depend on macrophytes in these regionally abundant habitats.