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Description
Grasslands are one of the most threatened and altered ecosystems in the world. These ecosystems are also severely under protected. Grassland songbirds have suffered substantial population declines in response to the loss and degradation of grassland habitats. Previous urbanization studies investigated changes in avian species composition from rural to urban habitats. The results of these studies do not provide insight into effects of urbanization surrounding intact habitat patches. Grassland songbirds are unable to live directly within urban areas due to nesting requirements, however urbanization adjacent to grassland habitats may still influence grassland birds and ecosystems. Humans have had a tremendous effect on California’s native grasslands, which are now one percent of the original ∼9 million ha, and urbanization is the main threat to remaining fragments in San Diego County. My thesis addressed how grassland songbirds are affected by urbanization at varying scales and intensities. I sampled five grassland sites within San Diego County that differed in the intensity of urbanization. Songbird presence, abundance, and reproductive success were quantified. The relative density of grasshoppers, an important food source, was also measured as well as the vegetation structure and composition. A technique for quantifying the amount of urbanization in the landscape surrounding study sites was developed. This technique was designed to be repeatable in any context and does not use arbitrary nomenclature for the varying types of land use. Overall, the results of this study were complicated, sometimes in unexpected ways. As a result, a more nuanced interpretation of the results was needed. I found that the grassland habitats in San Diego County are idiosyncratic in nature likely due to the spatial patterns of development and land use history. Urbanization in the landscape was found to be correlated with higher quality grassland habitat. There was also a positive correlation among both grassland bird nesting success and abundance and urbanization. These data also show that habitat composition and structure are important for grassland songbirds regardless of urban intensity in the landscape. Finally, it is clear that some avian species will likely avoid urbanized areas even if they contain patches of high quality habitat.
