Evaluating three decades of change in San Francisco Bay's waterbirds: An assessment of population abundance and community composition

Background/Question/Methods

The San Francisco Bay estuary provides critical habitat for over one million waterbirds annually. Although the landscape has been altered for well over a century by increasing levels of urbanization, and by the historic establishment of evaporator ponds for salt production, it remains heavily used by waterbirds.  The area also hosts the west coast’s largest tidal wetlands restoration project; the South Bay Salt Pond Restoration Project is implementing a plan to convert thousands of acres of salt ponds into tidal and managed wetland habitat. While the restoration to tidal marsh will increase habitat for many species, it also will reduce the overall habitat available for waterbirds.  Through adaptive management, the Project is committed to maintaining historic levels of waterbirds in this landscape.  To inform these efforts, the USFWS, San Francisco Bay Bird Observatory, USGS and UC Davis partnered to assess changes in bird population abundance and community composition for nine waterbird guilds.  Data collected during the early 1980s by air were compared with recent ground count data using a conversion factor to identify significant changes that have occurred over the past 30 years. 

Results/Conclusions

We found that overall, total waterbird abundance has changed little in the San Francisco Bay estuary over the past three decades.  However, we identified substantial increases in population abundance for some waterbird guilds, such as gulls, and clear declines at all or some locations for other guilds such as divers, terns and grebes.  Our findings indicate that while significant changes in habitat availability, condition, and water management have occurred in this urban estuary over the past 30 years, historic levels of waterbird abundance have been maintained.  Strong evidence of change in community composition, however, indicates that there are clear “losers” and “winners” in this ever-changing landscape.  Further work is needed to identify the likely causes of observed declines in some species/guilds and rapid rates of increase in others.  In addition to these results documenting long-term waterbird community changes in an urban estuary, our study aids ecologists by providing an example of how to develop and implement a conversion factor that facilitates comparison of abundances identified using different survey methods, a challenge that many ecologists face when working with long-term data.