Interspecific interactions, population variation, and environmental forcing in the context of the community

We evaluated the contribution of interspecific interactions, intraspecific processes, and environmental forcing to variation in species' abundance in a habitat undergoing rapid successional change. We applied a Bayesian hierarchical approach to a 29-yr time series of territory density of seven landbird species at a site in coastal California where secondary plant succession has occurred. We found that interspecific interactions were the least important driver in our system, explaining between 0% and 5% of variation. The combined effects of vegetation and rainfall variation explained 6% to 30% of variation in species trends. Intraspecific processes explained between 0% and 39% of variation. Between 27% and 90% of variation was attributed to unexplained variation. Our results demonstrate that in the system studied, interspecific interactions among landbirds are relatively unimportant. These results suggest that in some cases it may be valid to model projections of individual populations to predict community responses to future conditions; however, this conclusion should be interpreted with caution because interspecific interactions in our community did not include novel interactions that could result from distributional shifts in species ranges.