The majority of humanity now lives in cities or towns, with this proportion expected to continue increasing for the foreseeable future. Cities are novel ecosystems that provide natural laboratories for examining processes of colonization, primary succession, community assembly, and natural selection. Evaluating such processes in cities is critical as much of ecological knowledge and theory has been derived from pristine or non-urban systems which may function quite differently. Cities are also unusual in the degree to which human facilitation has a strong influence on what species make up local communities and are ideal for studying contemporary ecological and evolutionary patterns and processes. Cities share common features such as fragmented natural habitats and a high proportion of impervious surfaces. Consequently, cities may be expected to harbor assemblages of plant species that share common traits or life history characteristics that increase their fitness in urban areas. One important question in urban ecology is: how do cities influence global, regional, and local patterns of taxonomic and trait diversity? Understanding how humans influence community structure and processes will aid in the management, design, and planning of our cities to best support biodiversity.
Globally, cities support significant plant diversity, conservative estimates show that ~20% of the world’s plants are found in cities. Within cities, patterns of plant diversity are driven by land use and human decision making. Regional patterns of plant diversity have been linked to urbanization and have shown the importance of remnant natural habitats in preserving plant diversity within and across cities. Particular plant traits have been linked to urbanization, although due to limitations on data collection, comparable methods, and the heterogeneity of urban habitats, only a few clear patterns have emerged. Traits associated with plants growing in urban areas include non-native origin, annual life span, C-strategists (competitors), tolerance of fertile, dry and alkaline soils, wind-pollination, flowering in mid-summer, reproducing by seed and vegetatively, heavier seed mass, dispersing by wind or birds, greater plant height, and having a high demand for light and nutrients. Natural history and basic ecology of urban species is needed to identify important shifts in plant traits across urban gradients. Additionally, a better knowledge of plant functional traits and their underlying adaptedness or adaptive potential will improve our understanding on how species and communities respond to urbanization.