Patterns of plant species diversity and sexual reproduction in extreme urban environments
The flora of cities is largely composed of extreme urban plant life: weedy plants with high tolerances for drought, extreme heat, chemical pollution, and mechanical stress. These urban “extremophytes” are now some of the most widespread terrestrial plants on Earth, but we know surprisingly little about their community and population ecology in urban landscapes. Here we ask, do extreme urban environments support plant species populations? Due to human-mediated propagule transport in urban centers as well as intense disturbance, we hypothesized that extreme urban habitats would harbor high plant species diversity with a low proportion of those species producing flowers and setting seed.
Asphalted parking lots with cracks and curb edges are ideal urban study sites because they are heterogeneous, yet replicated throughout the city, with similar environmental pressure from maintenance regimes (i.e., herbicide application). We surveyed the cracks and curb edges of 15-17 asphalted parking lots on Rutgers University, New Brunswick campus for plant species richness three times in 2014 (late May, early July, and late August). We also randomly selected 25% of the total curb edge area and 10% of the total area of crack habitats to survey in each parking lot for plant species abundance and reproductive status.
One hundred and sixteen vascular plant species (89% herbaceous) representing 93 genera and 39 families were identified across all parking lots and over all three sampling periods. Asteraceae, Poaceae, Fabaceae, and Caryophyllaceae contributed 48.5% of the total species richness (19%, 15.5%, 7%, and 7%, respectively) across all lots in both curbs and crack habitats. In all parking lots combined, nearly 60% of the total species in both curbs and crack habitats - and nearly 40% of total species in cracks alone - had at least one individual flower and/or set seed during the survey season.
The high species diversity and reproduction of numerous species indicate that extreme urban environments do support a diverse flora. The substantial proportion of species reproducing in parking lot habitats suggests that extreme environments may not be sinks for many species, but actually sources, in a broad-scale, urban metacommunity. A greater understanding of the community dynamics and distributions of urban weedy species will be necessary to manage for ecosystem services provided by vegetation in cities. Future work must address mechanisms behind survival and reproduction differences among species as well as dispersal and gene flow within the city to better understand urban plant community dynamics.