Landscape-level Patterns of Avian Diversity in the Oregon Coast Range

We used a comparative mensurative landscape-level experiment to quantify the relative importance of mature forest area and fragmentation and differences among watersheds in influencing avian community diversity in the Oregon Coast Range, USA. Our study design included three large hydrological basins, two levels of fragmentation, and six levels of mature forest area. We recorded 82 species of birds in a total of 1046 plots in 30 landscapes. Our four response variables were species richness, species density, species evenness, and total bird density. We used a combination of factorial ANOVA and partial canonical variates analysis to quantify the relative importance of differences in mature forest area, fragmentation, and basin in influencing each response variable and community diversity overall. Bird community diversity was influenced by both the extent and fragmentation of mature forest at the landscape level. Species richness and density responded more strongly to mature forest area than to fragmentation and were significantly lower in landscapes that were completely dominated by mature forest than in landscapes with a mixture of seral stages. This reflected a selective loss of a number of early-seral species when mature forest completely blanketed the landscape. Species evenness was more strongly related to fragmentation than to area of mature forest at the landscape level. In fragmented landscapes, the most dominant species decreased in abundance, whereas moderately abundant species increased in relative abundance. This resulted in bird communities that were more even in fragmented landscapes. Unlike several other studies, we did not find that the relative strength of fragmentation effects increased as habitat area decreased. Total bird density, in contrast, was not related to either mature forest area or fragmentation, but it varied significantly among basins. In summary, although both area and fragmentation of mature forest at the landscape level were strongly related to bird community diversity, they had qualitatively different effects. These results must be interpreted within the scope of this study, which took place in a forest-dominated landscape. Our results provide an interesting view of the factors that are related to bird diversity in a spatially complex seral mosaic but may not reflect the patterns that would exist in a high-contrast forest–nonforest landscape.