Urban environment leads to functionally diverse but vulnerable avian assemblages

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Yu Tsai-Chen and Mao-Ning Tuanmu, Biodiversity Research Center, Academia Sinica, Taipei, Taiwan, Chia Hsieh, Department of BioSciences, Rice University, Houston, TX

Background/Question/Methods
Urbanization has long been recognized as a major threat to biodiversity by homogenizing functional structure. However, the general mechanisms of how urbanization reshapes biological assemblages are still unclear because most studies only focus on a single or a few cities and are highly biased to the cities in developed countries. In this study, we explored the potential mechanisms by examining the patterns of functional richness and redundancy among avian assemblages in 90 cities, including 55 cities in developing countries, across six continents. By integrating the species occurrence records from citizen scientists, expert species range maps, and information on five traits for 4,466 bird species, we examined (1) whether the observed functional richness of the avian assemblages is higher (i.e., functionally overdispersed) or lower (i.e., functionally clustered) than expectation, measured by randomly sampled assemblages from expert range maps, (2) whether the observed functional redundancy is higher or lower than expectation, and (3) whether the patterns are different among different traits.
Results/Conclusions
Results showed that the urban bird assemblages generally had higher-than-expected functional richness but lower-than-expected functional redundancy, indicating strong niche differentiation among species. However, the patterns varied among the traits. The diet, foraging stratum and nest attachment approaches showed the same patterns with the general one, suggesting that the strong niche differentiation is probably due to competition for food resources, and foraging and nesting habitats among species. In contrast, nest structure showed an opposite pattern, indicating that strong niche filtering restricted the types of nest structure in urban bird assemblages. In addition, we found that higher-than-expected proportion of functional generalists in the urban bird assemblages increased functional richness and redundancy. Contrary to general thoughts, this study shows that the functional richness of urban bird assemblages is higher than expectation. However, the high functional diversity is likely to result from strong interspecific competition for limited resources in the urban environment, which increases the vulnerability of the assemblages to environmental changes. While the replacement of specialists with generalists in an urban assemblage may increase the assemblage’s functional diversity, it may lead to biotic homogenization among assemblages. By revealing the general patterns and underlying mechanisms of functional diversity among urban bird assemblages worldwide, this study not only improves our understanding of the impacts of global urbanization on biodiversity, but also provides critical information for creating sustainable cities in the Anthropocene.