Biotic homogenization is the process of erosion of beta diversity. This occurs, for example, when several specialist species are replaced by a few widespread generalists at multiple sites, increasing taxonomic similarity among sites. This process threatens biodiversity by diminishing beta diversity (compositional turnover) across all spatial scales. Biodiversity offsetting practices such as compensatory wetland mitigation, which rely on the restoration or creation of replacement ecosystems to compensate for the loss of naturally occurring ecosystems, may contribute to biotic homogenization if: (1) restoration activities favor a subset of species from the regional species pool, (2) standardized restoration techniques result in homogenous abiotic conditions among offset sites, and/or (3) different offset sites are planted with uniform species assemblages. We assessed whether mitigation wetlands, in comparison to naturally occurring wetlands, possessed lower beta diversity at local (within site) and regional scales, thus contributing to biotic homogenization. We compared plant communities at 26 compensation wetlands to those at 107 naturally occurring wetlands in Illinois, USA. Comparisons of the spatial structure of beta diversity were assessed using species turnover across plots at the local scale, and rates of distance decay in community similarity at the regional scale.
Results/Conclusions
Contrary to expectations, compensation wetlands were not more homogenous than natural wetlands at the local scale. At the regional scale, compensation wetlands were more homogenous than natural wetlands when analysis was based on species occurrence, but not when based on species abundance. There was a high degree of compositional overlap between compensation sites and natural wetlands that had already homogenized. Repeated sampling from natural wetlands indicates significant homogenization during a period spanning 1997 to 2015 due to the increased presence and abundance of a widespread invasive species, Phalaris arundinacea, and the simultaneous decline of several other species. Therefore, the practice of compensating for wetland losses as it is practiced currently may not overcome the continued degradation of beta diversity that is occurring regionally.