Homogenization of plant diversity in six major USA cites: Integrating socio-economic, environmental, and phylogenetic information
As urban areas continue to expand both regionally and globally in the coming decades, so too will the area of urban plant habitat. While cities are often relatively species-rich, it is widely considered that urban plant assemblages are novel ecosystems that do not resemble the natural species pool. This may, in part, result from the unusually intimate association between human preferences and the composition of urban plant assemblages, as well as the unusual abiotic conditions within cities. We present results from systematic surveys of the urban flora (representing more than 3000 different plant species) in household yards from six major cities across the continental US: Baltimore, Boston, Los Angeles, Miami, Phoenix, and Minneapolis-St Paul. These plant surveys are combined with detailed interviews and questionnaires about landowners' preferences and socio-economic background, alongside a novel species phylogeny, species' functional traits, and environmental data. We use these data to determine the major drivers of plant assembly in our study sites.
While each city contains unique species, the phylogenetic and functional trait diversity of each city is similar and highly overlapping. This suggests that while cities may contain a distinct and diverse set of species, their diversity represents an evolutionarily and functionally biased subset of the total regional species pool that is consistently promoted and selected by humans in the urban environment. This bias is particularly apparent with regard to the 'neat lawn' aesthetic, where landowners value a classic green, grassy space over other aspects of biodiversity. Where there is a mismatch between homeowners' intentions for their yards and the species composition of the yards themselves, we link this to the influence of environmental conditions and the wider biogeographical context of cities.