Wetland compensation and its impacts on β‐diversity

Abstract

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.

Floristic Quality Assessment: a critique, a defense, and a primer

“…This process, termed biological homogenization, occurs when unique local habitat remnants are lost, and repeatedly replaced with the same few species (in this case, planted high C value species; Price et al 2019). For example, in restoration work there is concern that a specific subset of species are being over-promoted because they have high C values.…”

“…However, achieving target scores by repeatedly planting the same small group of highly conservative species that are cheap and easy to cultivate and establish (commonly referred to as workhorse species) could be lowering overall regional diversity. This process, termed biological homogenization, occurs when unique local habitat remnants are lost, and repeatedly replaced with the same few species (in this case, planted high C value species; Price et al 2019). Discouraging overused or reoccurring species in planted habitats may be a future consideration.…”

Cannot see the diversity for all the species: Evaluating inclusion criteria for local species lists when using abundant citizen science data

“…Patterns of β‐diversity, however, are less obvious to predict as β‐diversity relates to the relative importance of species turnover, nestedness, and species richness differences among sites. Still, a common pattern is that when α‐diversity decreases β‐diversity increases except for when communities get more species poor and homogenized (Clavel, Julliard, & Devictor, 2010; Filgueiras et al., 2016; Price, Spyreas, & Matthews, 2019). Looking closer at the dissimilarity indices among sites, the ratio between total dissimilarity index (Sørensen index) and species turnover (Simpson index) remained unchanged across all criteria applied (Figure S3).…”

The role of organic amendments in wetland restorations

“…Replacing them has proved challenging. Restored wetlands may be lower in plant abundance and species richness (Moreno-Mateos et al 2012) and lack the β-diversity of natural sites (Price et al 2019). It may take decades to develop soil properties, such as bulk density (Db), comparable to natural sites (Ballantine & Schneider 2009).…”