Effects of local vegetation and landscape patterns on avian biodiversity in the threatened oak habitat of the Willamette Valley, Oregon

Abstract

Both fine scale patterns of vegetation and coarser scale landscape patterns affect bird community composition and structure. However, the relative importance of the drivers at these two spatial scales continues to be debated. Here, we show how the factors that drive avian diversity and community structure depend on context, including the particular environment studied, the response variables analyzed, and the groups of species examined. We explored the relative roles of landscape pattern and stand structure and composition in defining bird communities in 44 remnant oak stands in the Willamette Valley, Oregon. We focused on two key questions--are bird communities influenced more by landscape patterns (at the matrix and patch levels) or stand composition and structure, and in what contexts are each of these spatial scales more important. We conducted point counts to determine avian abundance, richness, and evenness and categorized birds into functional groups based on diet and foraging tactics. We then used canonical correspondence analysis and generalized linear models to analyze overall community patterns, functional group diversity, synanthropic and non-synanthropic species diversity, and the abundance of individual species of concern. Both local and landscape factors significantly influenced each group of avian species for every measure of diversity we tested, but their relative importance varied markedly. Local factors explained four times more variance than landscape factors for overall species diversity and double the variance for two functional groups. For the other functional groups, landscape factors were up to ten times more important. We found the same high variability for individual species, depending on the species evaluated. When we evaluated factors more specifically at the landscape level, we found that the surrounding matrix was much more important than patch variables for each group of birds we tested. However, we also found that patch size influenced some groups and individual species much more than others, and some not at all. Understanding the degree to which species respond to local environmental conditions and landscape patterns is an essential part of optimizing scarce conservation resources and our results indicate that such an understanding will need to be put into very specific context.