Biotic homogenization of regional wetland plant communities within short time-scales in the presence of an aggressive invader

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.