Impacts of urbanization and agricultural legacy on taxonomic and functional diversity in isolated wetlands.
Land use changes pose major threats to biodiversity. In this study, we analysed the impacts of urbanization and agricultural legacy on plant communities of isolated wetlands (Laval, Québec, Canada). We sampled 35 marshes and 32 swamps. We used multi-factor analysis of variance to compare taxonomic richness and functional diversity between wetland types, urbanization levels and former agricultural use. We calculated beta diversity as between-site similarities in composition, and differences were evaluated using tests for homogeneity in multivariate dispersions. We also analysed the effect of exotic species on beta diversity. We found that urbanization and former agricultural use had no effect on taxonomic richness, functional diversity or beta diversity. However, exotics tended to be more abundant in urban and previously cultivated wetlands than in more natural ones. Furthermore, despite the absence of difference in beta diversity in response to present and past human disturbances when analysing the total species pool, we found that exotic species tend to favor homogenization. We also found that species composition differed in response to urbanization in marshes, while in swamps it differed according to former agricultural uses. This contrasting result was likely due to the fact that marshes are dominated by herbaceous species and swamps by woody species (short vs. long regeneration time). Our study highlights the importance of looking at different types of habitats within the same urban context and considering land use legacy to understand the response of ecosystems to urbanization, especially due to the time lag between disturbances and vegetation responses.