Plant diversity and community composition in six major USA cities

Background/Question/Methods

As urban areas continue to expand in the coming decades, so too will the size of urban plant habitat. The urban environment imposes specific constraints on plant survival; we thus hypothesize that urban plant communities resemble other cities more closely than their rural surroundings. Increasingly, ecologists are looking to quantify urban community structure by examining the functional traits and phylogenetic diversity of urban plants. There is a hope that a better understanding of how urban plant communities are structured will allow us to design cities that promote biodiversity and the survival of native plant species. We present results from systematic surveys of the urban flora in household yards from six major cities across the continental US: Baltimore, Boston, Los Angeles, Miami, Phoenix, and St Paul. We contrast the diversity of these cities with those of their surrounding natural and agricultural areas, using detailed surveys from nearly 200 sites, a novel phylogeny, and plant trait data collected from the sites.

Results/Conclusions

Species composition across sites and cities was variable, but urban yards were generally higher in diversity than their rural surroundings. However, a high fraction of this diversity reflects human-cultivated, non-native species; while the mean species richness of naturally regenerating plants was greater in urban St Paul than surrounding rural sites (urban - 49, rural - 19), more than 20% of these species were non-native. Phylogenetic and trait beta-diversity analyses suggest urban plant communities are more similar to each other than to their semi-natural surroundings. Plants in urban areas have distinct functional characteristics, and tend to be more closely related to one another phylogenetically than rural communities. Combined, these lines of evidence suggest that urban plant communities are distinct from those of their surrounding natural habitat, and we discuss the implications of this for city planning, and the management of both urban and semi-natural plant habitats.