Cities are novel ecosystems characterized by fragmented and disturbed environments, high densities of fabricated structures and impervious surfaces, and elevated levels of certain resources. Globally, these commonalities, in combination with human mediated biotic interchange, may result in the homogenization of species composition among cities. In addition, cities are often located in species rich regions where native species are threatened by habitat loss and species introductions. Given the World’s urban population is growing 1% per-annum on average, a better understanding of the global patterns and drivers of urban biodiversity is necessary for sustainable planning and conservation. We compiled city-wide bird species lists for 54 cities and city-wide floras of spontaneously established vascular plants for 110 cities. The lists encompass 36 countries on six continents and represents the largest compilation of urban biodiversity data to date. We examined patterns of species diversity within and among cities and their environmental and anthropogenic correlates.
Bird and plant species richness (median: birds = 112.5, plants = 762) and the number of non-native species (median: birds = 3.5, plants = 213) differed broadly among cities with cities containing, on average, more native species. The number of species in each city was predicted primarily by the physical features of the city related to population size and land-cover, with the species-area relationship playing a dominant role. The proportion of non-native plants in each city was predicted by multiple factors suggesting their prevalence was not determined exclusively by human activities. Globally, the compositions of plant and bird species were highly dissimilar among cities. Within regions, the composition of non-native plants and birds were more similar among cities, particularly within regions outside of Europe. Non-native plants identified as European archaeophytes (introduced into Europe prior to ad 1500) and species of plants and birds identified as global invaders were shared more broadly among cities, particularly within regions outside of Europe. Younger cities further from the equator with close climatic and cultural associations to Europe tended to have more similar compositions of plant species. For birds, compositional patterns did not differ along similar gradients, likely due to the presence of a limited number of identical non-native bird species. Despite worldwide biodiversity loss, cities currently retain their regional biological identities, with Europe playing a historical role as a source of urban adapted species. Urban biodiversity can play an important role supporting conservation, restoration, and education initiatives in a rapidly urbanizing world.