Biotic homogenization (BH), a process by which β‐diversity erodes, represents a severe threat to biodiversity. Wetland plant communities may be especially susceptible to BH; however, this process has rarely been quantified and represents just one of many possible outcomes of compositional change. Likewise, few studies have used more than two timepoints to investigate BH as a dynamic temporal process. To address these issues, we quantified changes in β‐diversity amongst 48 herbaceous emergent wetlands across a landscape over four timepoints from 1997 to 2015. Pairwise occurrence‐ and abundance‐based dissimilarity metrics were used to quantify β‐diversity. Nonmetric multidimensional scaling was used to map these changes in compositional similarity through time, and repeated measures ANOVA via randomization was used to test for a significant effect of time on β‐diversity. Results indicated that herbaceous emergent wetlands homogenized in Illinois over 15 years due to the increased presence and abundance of Phalaris arundinacea and the decline of several other species. Temporal dynamics of β‐diversity differed between occurrence‐ and abundance‐based analyses. Synthesis. Using both presence–absence and abundance data to investigate BH helped avoid underestimating the impact of BH on β‐diversity, and drew attention to different temporal dynamics in β‐diversity at these two resolutions of community structure. This study highlights challenges associated with investigating BH, such as problems documenting the process where homogenization is not occurring in every site in a region, or where sites are converging towards more than one homogenized state. Incorporating concepts like alternative stable states may be helpful in resolving these issues, and present a more realistic picture of ecological change.

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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.