Potential Effects of Sudden Oak Death on the Oak Woodland Bird Community of Coastal California

Oak-dependent birds are expected to suffer severe population declines as a result of sudden oak death (SOD). We investigated how the disappearance of two highly SOD-sensitive tree species, tanoak (Lithocarpus densiflorus) and coast live oak (Quercus agrifolia), may in turn affect levels of bird species richness, diversity, and equitability in coastal oak habitats of California. Combining avian census data from Audubon Christmas Bird Counts and North American Breeding Bird Surveys with oak distributional data from the California Gap Analysis, we use the statistical relationships between bird community indices and oak species diversity and areal extent to quantify expected bird responses to SOD while assuming complete loss of Q. agrifolia or L. densiflorus and complete, partial, or no loss of oak habitat. Additionally, we combine existing Phytophthora ramorum locality data with estimates of temperature, precipitation, and seasonality to model the potential distribution of SOD. Results suggest that species richness, diversity, and equitability of oak-dependent birds will decline by 5 to 15 percent in areas where SOD eliminates either tanoak or coast live oak. According to the spatial disease model, these declines are expected over 10,130 km, or 24 percent of the combined California ranges of Q. agrifolia and L. densiflorus. Although the overall influence of SOD on the statewide avian community is likely to be modest, bird losses are predicted to be relatively great for central regions already harboring the disease and, following a slight southward expansion of P. ramorum into San Luis Obispo and Santa Barbara counties, within neighboring oak habitats that will potentially suffer SOD mortality in the near future.