Effects of urbanization on avian diversity across North America

Background/Question/Methods

The United States and Canada are experiencing widespread urbanization with potential consequences for ecological communities and biodiversity. On the one hand, urbanization results in altered landscapes and modified habitats. On the other hand, urban centers, with their high densities of human population, provide resource subsidies that are stable through time, even in places with high seasonality. Studies along local urban gradients have generally found urbanization to coincide with decreased biodiversity. Here we use spatial geographic data on urban extent and Christmas Bird Count surveys for 2200 sites from across North America to investigate the influence of urbanization on alpha diversity of avian communities. We use residual values from linear regressions between winter bird diversity with latitude and environmental productivity in nonurban sites to test the null hypothesis that urbanization has no effect on avian diversity. Deviations from these regressions allow us to test the impact of urbanization on avian diversity, independent of latitude and productivity. If urbanization negatively impacts avian diversity, we expect to see a decrease in avian diversity with increasing urbanization. However, if energy subsidies provided by cities benefit avian diversity, we expect increased avian diversity to coincide with urbanization above and beyond what is predicted by latitude and environmental productivity.

Results/Conclusions                        

Latitude and productivity are strong predictors of winter alpha diversity in birds across North America. However, and contrary to expectations, residual variation in alpha diversity vs latitude and productivity are not explained by urban land cover. Thus, when controlling for latitude and productivity, alpha diversity is generally invariant across the urban gradient. This is surprising since gamma diversity at the regional to continental scale is declining overtime and with increased urbanization. A possible explanation for these patterns that remains to be tested is that species turnover (i.e. beta diversity) is lower between urban than non-urban sites, allowing for similar alpha diversity at local scales but declining gamma diversity at regional scales.