A CENTURY OF AVIAN COMMUNITY TURNOVER IN AN URBAN GREEN SPACE IN NORTHERN CALIFORNIA

Over long time periods, urbanization is expected to have a negative effect on species diversity. Predicted effects generally follow one of three competing paradigms: diversity decay, homogenization, or community-composition turnover. However, it has been hypothesized that urban green spaces may provide a means by which urban areas can maintain or increase their species diversity over time. We used surveys conducted in 1913-18, 1938-39, and 2006-07 on the campus of the University of California, Berkeley, to evaluate how an avian community has changed over time in the context of urban growth. In each of the three periods the community differed greatly, yet we found no evidence for a decline in species or functional diversity. Despite the birds of the 1913-18 community having a greater affinity for native habitats than birds of later periods, we found no further evidence that specialists were being replaced by generalists. Of the three paradigms, our results strongly supported community-composition turnover. Parsimoniously, the habitat preferences of groups of species that changed over time were concordant with known changes in landscaping. While urbanization often does result in decreased biodiversity, our results provide an example of how an urban green space can mitigate and potentially reverse this trend within the context of dynamic community change. Our results are concordant with the view that urban green spaces can maintain original bird communities and disturbance-sensitive species can reestablish themselves given appropriate conditions.