Riparian systems are key landscape elements for bird populations in arid regions. They support among the highest levels of avian diversity of any habitat in the western United States. These habitats are highly sensitive to human development and have been severely degraded historically. Channelization of stream corridors and clearing of riparian habitat for human development has contributed to the decline of riparian habitats and bird communities. The loss of riparian habitat may be one of the most important factors in the decline of western United States landbird populations. Even in undisturbed riparian habitats, however, changes in adjoining land-use or land-cover often adversely harm the avian community. Changes in land-use on a watershed scale often affect the diversity and composition of riparian bird communities. My study examines the relative influence of watershed-level human development over time on riparian avian diversity in a protected area. I analyzed the changes over thirty-five years in a riparian bird community inside a biological preserve. I examined the habitat-level changes in vegetation, adjoining changes in land-cover, watershed-scale human development, and changes in climate.

Results/Conclusions

Avian diversity declined significantly over thirty-five years, especially among Neotropical migrants and human-sensitive species. Structural complexity of the vegetation and tree species diversity increased over time, but despite these changes, the habitat remained in an early seral state. Vegetation changes had little of the predicted effects on the bird community. Adjoining riparian habitat extent increased over time while chaparral habitat decreased, and avian habitat guilds reflected these trends. Slight changes in average minimum January temperature did occur, but land-use change on local and watershed scales explained most of the observed changes in diversity and abundance. In general, the species assemblage and abundances shifted to a more human-tolerant set of bird species. Many human-sensitive species decreased in abundance or disappeared entirely, while human-tolerant species increased in abundance. This temporal pattern of bird changes in the preserve occurred despite trends like increased riparian extent that should lead to greater diversity, indicating the importance of watershed scale changes on riparian bird communities. My study suggests an increased need for riparian habitat protection, judicious planning of human development along sensitive stream corridors, and particularly buffer areas around riparian and reserve habitats.