Birds of different biogeographic origins respond in contrasting ways to urbanization

Impacts of urbanization on ecological systems are expected to increase during the 21st century, and identifying which species may not survive under urban constraints is of practical importance to conservation biology. I study whether a species’ biogeographic origin might be a useful predictor of vulnerability to urban pressures, and if this depends on the geographic scale of the study. By means of multivariate ordination analyses and computer intensive, Monte Carlo simulation techniques, I study the patterns of zoogeographical affinity (a proxy for biogeographic origin) of passerine birds living in 14 non-urban (NU) and 11 urban (U) sites throughout Mexico, a biodiversity rich and urbanizing country between the Nearctic and Neotropical Regions; at a regional scale, I also study differences between NU and U sites in the state of Puebla, in the ‘Temperate Sierras’ ecoregion. Differences in zoogeographical affinities between NU and U sites were statistically significant, and were mainly caused by changes in the number of neotropical and nearctic species. In terms of the ‘equivalent number of zoogeographical classes’, a useful measure of zoogeographical diversity, bird assemblages in urban settings were less diverse than those simulated by random drawings from the corresponding species pools. In fact, urbanization seems to be provoking the selective removal of neotropical and endemic species from urban assemblages, whereas at the same time retaining more-than-expected nearctic (temperate) species; birds with nearctic affinities could be successful candidates to ‘urban exploiters’ in the study case. At a landscape scale, results suggest that urbanization is creating holes in the geographic range of both neotropical passerines and birds endemic to Mexico, which are selectively removed by the urban filter, and can be identified as those more heavily threatened by urbanization if proactive strategies to conserve biodiversity in urban ecosystems are not implemented. Among other reasons for the observed patterns, the tropical niche conservatism is invoked as an explanation.