Wetland degradation leads to homogenization of the biota at local and landscape scales

Summary 1. We examined whether the anthropogenic degradation of wetlands leads to homogenization of the biota at local and/or landscape scales and, if so, what specific factors account for such an effect. We compared 16 isolated wetlands (Michigan, U.S.A.) that varied in surrounding land use: half had developed, and half undeveloped, riparian zones. Samples of macrophytes, epiphytic diatoms, zooplankton, macroinvertebrates and water chemistry were collected along three transects in each wetland. 2. Developed wetlands were more nutrient‐rich with higher Cl concentrations. The plant community at developed sites was dominated by Lemnaceae (duckweed), while undeveloped wetlands were dominated by rooted, floating‐leaved vegetation and sensitive plant species. Undeveloped wetlands contained heterogeneous and species‐rich plant communities, greater species richness of zooplankton and diatoms, and heterogeneous zooplankton distributions as compared to developed sites. 3. A comparison among wetlands showed that diatom and zooplankton assemblages in developed wetlands were nested subsets of richer biota found in less developed wetlands. Conversely, plant communities were more heterogeneously distributed among developed wetlands at the landscape level. This may be attributable to patchy invasions by exotic species, which were a feature of the degraded wetlands within developed landscapes. 4. Our results indicate that several taxonomic groups showed similar, probably inter‐dependent, responses to wetland degradation and habitat homogenization at both the local and landscape scales. This change in community structure from a species‐rich and heterogeneous community dominated by floating‐leaved plants in undeveloped wetlands, to nutrient‐rich wetlands dominated by duckweed may represent a shift to an alternate stable state.