Urbanization contributes to the loss of the world's biodiversity and the homogenization of its biota. However, comparative studies of urban biodiversity leading to robust generalities of the status and drivers of biodiversity in cities at the global scale are lacking. Here, we compiled the largest global dataset to date of two diverse taxa in cities: birds (54 cities) and plants (110 cities). 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 km ² ) 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.

Around the world the development and growth of cities and towns are having a significant impact on local and global biodiversity. There is growing interest in the adaptation of nonhuman organisms to urban environments, and we distinguish between the concepts of adaptation and adaptedness. Most of these studies have focused on animals, especially birds. Commonly recorded responses to urban environments include regulatory and acclimatory responses involving changes in behavior, communication, and physiology. Developmental responses tend to be morphological in nature but can also involve cultural learning. There is growing evidence of microevolutionary changes associated with adaptive responses to urban environments. This review also highlights the urgent need to refine the terminology currently used to describe the adaptation of organisms to urban environments in order to improve scientific understanding and more effectively identify and communicate the actions required to create biodiversity- and adaptation-friendly cities and towns for the future.