The relative role of the patch attributes and the nature of the surrounding matrix in structuring avian communities in oak woodlands in Oregon, USA

Background/Question/Methods

The extent of oak woodlands and savannahs of the Pacific Northwestern United States has been dramatically reduced by conifer encroachment and land-use change over the last 150 years. Less than 1% of their historical distribution in the Willamette Valley, Oregon remains. The remaining woodlands provide habitat for a number of at-risk species as well as a diverse avian community. These changes are happening rapidly and at several spatial scales. 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. We focused on two key questions—are bird communities influenced more by landscape patterns or stand composition and structure, and in what contexts are those scales most important. We conducted point counts in each of the 44 oak woodland sites to determine avian abundance, richness, and evenness. We then used Canonical Correspondence Analysis (CCA), partial CCA, and generalized linear models to analyze overall community patterns, functional group diversity, synanthropic effects and the abundance of individual species of concern.

Results/Conclusions

As predicted, the factors influencing avian community structure and diversity depend on context. The relative importance of local, matrix and patch variables for oak habitats of the Willamette Valley of Oregon depends on the type of response evaluated as well as the bird communities targeted. Local habitat variables and the surrounding matrix both play a significant role in structuring overall avian community composition and determining avian diversity for most avian communities studied. For overall avian community structure and diversity, patch area alone was not a good predictor of overall diversity. In fact, patch scale predictors were insignificant for overall avian diversity and community structure. At finer contextual scales, however, patch area plays an important role. Area was significant in determining synanthropic species abundance, the abundance of many functional groups as well as the evenness of functional groups and the abundance of one species of concern (Chipping Sparrow). Given the small amount of historic oak habitat that remains, the results of the study may provide important information for targeted protection, management, and restoration of oak habitat in the Pacific Northwest.