Wetland compensation and its impacts on β-diversity.

The anthropogenic degradation of natural ecological communities can cause biodiversity loss in the form of biotic homogenization (i.e., reduced β-diversity). Biodiversity offsetting practices, such as compensatory wetland mitigation, may inadvertently cause biotic homogenization if they produce locally homogenous or regionally recurring communities. The fact that compensation wetlands often resemble degraded wetlands suggests that potential impacts to β-diversity are likely. Yet, it is unknown how high-quality, low-quality (degraded), and compensation wetlands compare in terms of β-diversity. We compared the β-diversity of high-quality, low-quality, and compensation wetlands at local and regional scales. β-diversity was quantified as the average distance to group centroids in multivariate space based on pairwise comparisons of community composition. The local spatial structure of β-diversity was assessed using species turnover across plots. Indicator species analysis was used to describe compositional differences potentially contributing to differences in β-diversity. Overall, the β-diversity of compensation sites did not differ from high-quality or low-quality natural wetlands. However, compensation wetlands had a high degree of internal turnover along the hydrological gradient, which culminated in homogenous zones in the wettest areas. Compared to high-quality wetlands, low-quality wetlands had significantly lower β-diversity at local scales, but significantly greater β-diversity at regional scales. Indicator species results showed that compensation wetlands were distinguished by low conservation value species typically found in old fields and waste areas. This analysis also indicated that the invasive grass Phalaris arundinacea was indicative of low-quality and compensation wetlands. This species is likely contributing to differing patterns of β-diversity between high-quality and low-quality wetlands. These results indicate that conclusions regarding β-diversity depend on scale and scope of analysis. Particularly, the unique architecture of compensation wetlands makes conclusions regarding within-site β-diversity dependent on the observer's position along the hydrological gradient. Additionally, while we conclude that compensation wetlands are not contributing to biotic homogenization at the regional scale, these wetlands are distinct from both high-quality and low-quality wetlands in their composition and structure. Therefore, assessments of the overall success of wetland mitigation programs should acknowledge the reality of these differences.