Ecological homogenisation in North American urban yards: vegetation diversity, composition, and structure

Urban ecosystems are widely hypothesised to be more ecologically homogeneous than natural ecosystems. We argue that urban plant communities assemble from a complex mix of horticultural and regional species pools and thus evaluate the homogenisation hypothesis by comparing cultivated and spontaneously occurring urban vegetation to natural area vegetation across seven major US cities. Urban yards were homogenised across cities in terms of their diversity, composition, and structure. First, cultivated and spontaneous yard flora had higher numbers of species than did natural areas but similar phylogenetic diversity, indicating that yard species were drawn from a relatively small number of lineages. Second, yards were compositionally more similar across regions than were natural areas. Finally, vegetation structure, specifically cultivated tree density, was less variable in yards than natural areas across cities. Biodiversity homogenisation likely reflects similar horticultural source pools, homeowner preferences, management practices, and environmental filters across US cities.