The rapid conversion of natural lands to cement-dominated urban centers is causing great losses in biodiversity. Yet, comparative studies of urban biodiversity leading to robust numbers on the extent and information on the drivers of biodiversity loss in cities at the global scale are lacking. Previous urban biodiversity research has looked the local impacts of urbanization and did not consider overall impacts on global biodiversity.
Contrary to conventional wisdom that cities are a wasteland for biodiversity, a recent study involving 147 cities worldwide found that while a few species - such as pigeons (Columba livia) and annual meadow grass (Poa annua) - are shared across cities, overall the mix of species in cities reflects the unique biotic heritage of their geographic location. Surprisingly high numbers of plant and animal species persist and even flourish in urban environments:
We found that the majority of urban bird and plant species are native in the world's cities. Few plants and birds are cosmopolitan, the most common being Columba livia and Poa annua. The density of bird and plant species (the number of species per km2) has declined substantially: only 8% of native bird and 25% of native plant species are currently present compared with estimates of non-urban density of species. The current density of species in cities and the loss in density of species was best explained by anthropogenic features (landcover, city age) rather than by non-anthropogenic factors (geography, climate, topography). As urbanization continues to expand, efforts directed towards the conservation of intact vegetation within urban landscapes could support higher concentrations of both bird and plant species. Despite declines in the density of species, cities still retain endemic native species, thus providing opportunities for regional and global biodiversity conservation, restoration and education.
Don't get this wrong - overall, cities supported far fewer species (about 92 percent less for birds and 75 percent less for native plants) than expected for similar areas of undeveloped land. The process of urbanization has profound effects on biodiversity; cities worldwide contain substantially lower densities of species. However, cities are capable of retaining a unique regional flavor. That uniqueness is something that needs to be sustained. Conserving green spaces, restoring native plant species and adding biodiversity-friendly habitats within urban landscapes could, in turn, support more bird and plant species. If we act now and rethink the design of our urban landscapes, cities can play a major role in conserving the remaining native plant and animal species and help bring back more of them.