Urbanisation can cause local and sometimes global extinction of plant species. Using a plant functional trait approach may help to distinguish species that persist in urban areas from those that have become extinct. We analyzed historical and recent survey data from 11 cities from across the globe (Adelaide, Auckland, Chicago, Hong Kong, Los Angeles, Melbourne, New York, San Diego, San Francisco, Singapore, Worcester) using hierarchical logistic regression models. Data were obtained for habitat and the following plant traits: growth form, clonal spread, dispersal mode, nutrient uptake strategy, spinescence, pollination system, photosynthetic pathway, plant height and seed mass. We then combined the results using meta-analysis to look for common and differing responses to urbanization.
Results/Conclusions
The proportion of native species that became locally extinct varied substantially among the 11 urban areas, ranging from less than 1% in San Diego to nearly 28% in Singapore. Five urban areas had extinction rates of less than 0.05% species per year, which made identifying clear correlates of extinction in noisy data difficult. For these cities there appear to be no consistent determinants of extinction outcomes and chance and idiosyncratic factors may overwhelm any trait-extinction patterns. Six cities (Auckland, Chicago, Melbourne, New York, Singapore and Worcester) had extinction rates that exceeded 0.1% species per year which provided more power to detect patterns. In these cities both seed mass and height were strongly associated with extinction with small seeded, short plants consistently more likely to become extinct. Given that plants in urban environments face a similar suite of filters that would be expected to select for certain traits the lack of strong and consistent patterns across cities suggest other factors such as initial abundance or preferential habitat loss may be more important drivers of urban plant extinction than plant traits.
