Diversity of residential plant communities along an urbanization gradient

Background/Question/Methods

Urban areas generally have higher plant species richness than surrounding natural areas, but phylogenetic diversity may not follow the same pattern. We examined species richness and phylogenetic diversity of the spontaneous flora of residential yards along an urban-to-rural gradient in Boston, Massachusetts. We compared species richness, total community branch length (Faith's PD), mean pairwise distance, and mean nearest taxon distance against three urbanization metrics: lot area, house age, and local road density, and compared residential sites to more natural reference sites.

Results/Conclusions

Species richness and PD were very tightly correlated and were positively influenced by lot area alone. Mean pairwise distance showed no relationship with urbanization, but density had a weak negative influence on mean nearest taxon distance, indicating that increasingly urban yards have fewer closely related species. Conversely, the proportion of introduced species increased with decreasing yard area and introduced species were much more closely related to each other than native species. Forest sites had significantly higher mean pairwise and nearest taxon distance than pasture and residential sites, which did not differ significantly from one another in this respect. Regardless of urban intensity, residential development filters the regional species pool, making yards more phylogenetically clustered and therefore more similar to grasslands than to the forests that once dominated the landscape.