| Urbanization has profound effects on biodiversity, but few studies have addressed the consequences of multiple stressors along urbanization gradients on both populations and communities. I conducted three studies of urbanization effects on landbird demography and community structure, sampling landbird abundance and diversity at 375 sample points and monitoring 494 nests of 10 focal species at 97 points along a development gradient in the Lake Tahoe basin, California and Nevada. First, I examined effects of nesting strategy and body size on some key demographic parameters: abundance, nest success, and finite growth rate. Abundance of large-bodied species and nest success of cavity nesters stayed constant or increased as urban development increased, whereas abundance of most small-bodied species and nest success of open nesters declined. For some species, the entire basin was a population sink, and for others, points along the development gradient may be ecological traps. Identifying sinks and traps along gradients rather than assuming discrete subpopulations is a superior approach for many landscapes and can highlight opportunities for habitat restoration. Second, I used model selection to determine the relative importance of development, human activity, local and landscape vegetation, topography, and location in explaining richness, abundance, and dominance. Urbanization stressors were consistently among the principal forces structuring the community, and disturbance from human activity was often the most important factor, surpassing even habitat loss from development. My results suggest likely consequences for urban ecosystem function that call for local- and landscape-scale management, focused research, and long-term monitoring to retain biodiversity in urban areas. Third, I used metacommunity theory to guide a variation-partitioning analysis of the spatial and environmental structuring of the landbird community. Environmental variation generally exceeded spatial variation, but all groups displayed a significant component of spatial variation, demonstrating a likely mix of dispersal limitation with niche-based and source-sink processes. Structuring of urban avoiders, adapters, and exploiters differed in interesting ways. Substantial roles of space, habitat, and landscape configuration illustrated the value of retaining native vegetation, the need to consider connectivity in managing biodiversity in urbanizing landscapes, and the value of a metacommunity approach for addressing urbanization's multifaceted effects on biodiversity. |
