Taxonomic homogenization of plant communities in Illinois wetlands and the role of wetland compensation
Biotic homogenization, a process by which β-diversity erodes, represents a severe threat to biodiversity. Wetlands are especially susceptible to biotic homogenization; however, this process has rarely been documented and represents just one of many possible outcomes of compositional change. Additionally, it has been suggested that the practice of compensatory wetland mitigation, which seeks maintain the integrity of wetlands at the national level by offsetting the destruction of wetlands with the restoration of wetlands elsewhere, may inadvertently result in homogenization. This thesis consists of two studies that address these issues. In the first, I investigated how β-diversity has changed through 15 years in 59 Illinois wetlands (48 herbaceous emergent wetlands and 11 floodplain forests). This study focused on both within-site and regional scales, and assessed whether taxonomic homogenization has occurred in these wetlands. In the second study, I used 38 compensation wetlands (26 herbaceous emergent and 12 floodplain forest wetlands) and 146 natural wetlands (107 herbaceous emergent wetlands and 39 floodplain forests) to investigate whether plant communities in compensation wetlands were more homogenous than those of natural wetlands in Illinois at within-site and regional scales. In both studies, pairwise occurrence- and abundance-based dissimilarity metrics were used to quantify β-diversity. In the first study, these metrics were used to track changes in β-diversity over four sampling periods over 15 years. Nonmetric multidimensional scaling was used 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 indicate that herbaceous emergent wetlands homogenized in Illinois over the period spanning 1997 to 2015 due to the increased presence and abundance of Phalaris arundinacea L. and the decline of several other species. The canopy layer of floodplain forests has also homogenized as Celtis occidentalis L. has increased in importance. While wetlands did display patterns of changing dissimilarity that would be consistent with internal, within-site homogenization, these patterns were not significant. Using both presence-absence and abundance data to investigate taxonomic homogenization helped avoid an underestimation of the impact of taxonomic homogenization on beta diversity, and points to different dynamics of beta diversity at these two resolutions of community organization. Still, this study highlights some limitations associated with investigating taxonomic homogenization, such as the problems of documenting the process in a region where homogenization is not occurring in all sites, or where sites are converging towards more than one homogenized state. Incorporating concepts like alternative stable states may helpful in resolving these issues. In the second study, pairwise occurrence- and abundance-based dissimilarity metrics were calculated for 146 natural wetlands and 38 compensation wetlands to investigate whether compensation wetlands are themselves more homogenous than natural wetlands at within-site and regional scales. Differences in the within-site and regional spatial structure of β-diversity between wetland types were assessed by comparing values of pairwise dissimilarity to hydrological and distance gradients, and compositional differences that may account for differences in β-diversity were assessed using indicator species analysis. Results indicate that internally, compensation wetlands were more differentiated with respect to taxonomic identity (i.e. had higher internal beta diversity) than natural wetlands. However, differences in the structure of turnover along the hydrological gradient, which in compensation wetlands showed consistently declining beta diversity amongst nearest neighbors along the hydrological gradient, suggest that whether compensation wetlands are considered more or less homogenous may depend on where along the hydrological gradient the assessment is based. Regionally, compensation wetlands were not more homogenous than natural wetlands. Rather, greater differentiation was found in compensation forest herbaceous layers, which may reflect differences in successional stage between natural and compensation wetlands. Although compensation herbaceous emergent wetlands do not, by themselves, represent a more homogenous set of communities at the regional level, there is a high degree of compositional overlap between compensation sites and natural sites that have already homogenized. Therefore, the practice of compensating for wetland losses in herbaceous emergent wetlands as it is practiced currently may not overcome the continued degradation of β-diversity that is occurring regionally.
Biotic homogenization, wetland mitigation, wetland restoration, Phalaris arundinacea, compositional similarity, beta diversity
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign