Dispersal-driven homogenization of wetland vegetation revealed from local contributions to β-diversity

Questions: Within a meta-community, what determines how local species composition differs from the regional community? How do local conditions and landscape context affect this differentiation in wetland vegetation?. 

Location: Fleurieu Peninsula, South Australia. 

Methods: We sampled native vegetation across 26 hydrological gradients in a wetland meta-community within a heavily cleared agricultural landscape. We used the local contribution to β-diversity to quantify how species composition at each site differed from the average across all sites. We hypothesized that local contribution to β-diversity would respond to assembly processes (niche, biological interactions, dispersal) through effects on the species turnover and richness difference components of β-diversity. We used beta regression to model local contribution to β-diversity, building a candidate set of 55 models, each incorporating one of the assembly processes. We used standardized regression coefficients to measure effect size, and null models to explore diversity patterns further. 

Results: While variations in among-site niche dimensions were influential, the strongest control on local contribution to β-diversity was a negative association with the number of wetlands within 200 m. Null models showed this was because common species were over-represented in well-connected sites within the meta-community, while rare species were under-represented. 

Conclusions: Our results demonstrate the homogenization of native plant species composition in well-connected wetlands, consistent with theoretical predictions of declining β-diversity when connectivity is high. We recommend comparative analysis of local species composition to regional average diversity to evaluate the role of wetland connectivity in homogenization of composition before conservation or restoration priorities are assigned.