Testing the effects of four urbanization filters on forest plant taxonomic, functional, and phylogenetic diversity.

Ongoing urban development has significant effects on ecosystems, including changes to land cover, environmental conditions, and species' distributions. These various impacts may have opposing or interacting effects on plant communities, making it difficult to predict how plant biodiversity will respond to urban development. A frequently cited conceptual framework predicts how urban development influences plant taxonomic, functional, and phylogenetic diversity by simplifying multiple coincident effects of urbanization into four primary filters of biodiversity: habitat transformation, fragmentation, the urban environment, and human preferences. Each filter prevents some plant species from persisting in urban areas while promoting others, but species introductions according to human preferences are expected to cause a net increase in biodiversity while the other filters limit diversity. In this study, we used structural equation modeling to test these predictions and examine the relative importance of each filter on the taxonomic, functional, and phylogenetic diversity of riparian forest plant species sampled along an urban-to-rural gradient in the Research Triangle area of North Carolina. Most diversity measures declined with urbanization, but some (e.g., functional Rao's Q) increased with urbanization. We found support for some of the predicted relationships between urbanization filters and biodiversity, as well as some unexpected relationships, including positive effects of urban environments. Overall, urban environments and human preferences were stronger predictors than habitat transformation and fragmentation. Our approach could be used to test a general framework predicting the effects of urbanization on plant diversity across multiple cities and contribute to a more synthetic understanding of urban biodiversity.