Corey Thomas Callaghan, Alistair G. B. Poore, Richard E. Major, William K Cornwell, John Wilshire, Mitchell Lyons

Cities are novel environments compared with the evolutionary history of the species that reside within them. Collectively, cities and their fauna can be thought of as ecosystems, recognized as playing a critical role in supporting global biodiversity, but they are fundamentally a combination of old species surviving or thriving in a new environment, and the mechanisms and underlying processes which support biodiversity within cities have not been investigated at broad macroecological scales. We aimed to understand — at a broad macroecological scale — how biodiversity responds both among and within cities. We integrated > 5 million eBird citizen science observations with remotely sensed landcover products throughout 1,581 cities within the continental United States. We first investigated the species-area relationship as it pertains to cities and compared the slope of this relationship to randomly sampled polygons. Second, we investigated how biodiversity responds to an urbanization gradient at the level of localized bird observations. We found strong support for the longstanding species-area relationship theory: geographically larger cities had greater species richness. Surprisingly, the species-area relationship was greater in cities when compared to the overall relationship for randomly sampled polygons in the study region (continental United States), which included many different land use and land cover types. Our finding suggests that diverse and heterogeneous cities play a significant role in supporting biodiversity. We also found that there is a consistent threshold where the level of urbanization begins to profoundly and negatively affect biodiversity. Critically, urban planning at the city-scale and at a local-scale (e.g., neighborhood) should focus on preserving attributes of water and tree-cover for increased biodiversity to keep as much of the city as possible above this threshold value.