Homogenizing effects of urban land cover on North American winter bird diversity

Background/Question/Methods

The United States and Canada have experienced widespread urbanization. The potential consequences for ecological communities and biodiversity are unclear. Urbanization results in a high density of altered landscapes that may appear more similar to other urban areas than to adjacent natural landscapes. Urban centers also feature high densities of humans. Their activities may provide resource subsidies that are stable through time, even in seasonal environments. In this study, we used geographic data on urban extent and Christmas Bird Count surveys to investigate the influence of urbanization on winter bird diversity over a continental scale. We compared 42 urban bird communities with nearby wild sites across North America. 

Results/Conclusions

When controlling for the effects of latitude, we found that alpha diversity (species richness) was statistically indistinguishable between urban and wild sites. Community similarity decreased more rapidly over distance in wild compared to urban sites. This shows that urbanization has a homogenizing effect spatially, resulting in lower beta diversity. Rank-frequency of occurrence plots reveal that twenty-three exotic or native generalists occur across all urban sites, whereas no species occur in all wild sites. This is likely due to habitat similarity across cities. Mean-variance scaling shows that urban bird populations are less variable over space compared to wild sites, possibly due to resource subsidies. Both of these processes contribute to the widespread homogenizing effect of urban landscapes on winter bird communities.