Using a species-centered approach to predict bird community responses to habitat fragmentation

Abstract

The relative importance of habitat fragmentation versus loss on species richness has been much debated. However, recent findings that fragmentation effects are relatively weak may be an artifact of using human-classified vegetation rather than adopting a species-eye view to measure landscape structure. We present the first example of a species-centered approach for examining fragmentation effects on ecological communities. We tested hypotheses relating to the relative influence of habitat amount, configuration, and focal patch size on southwest Oregon bird communities. We used boosted regression trees based on unclassified Landsat TM to create ‘stacked’ species distribution models (S-SDMs) for a large pool of avian species and nested subset of habitat specialists. We tested the relative importance of S-SDM-derived habitat amount, patch number, mean patch size, and focal patch size in explaining species richness. We compared this approach to metrics based on generic land-cover classifications. Species-centered models had greater statistical support than land-cover models. In species-centered models, species richness increased as a function of focal patch size and decreased with patch number, supporting the hypothesis of negative effects of fragmentation per se. Land-cover based models indicated inconsistent support for habitat amount but a positive effect of fragmentation. The species-centered approach identified habitat configuration relationships obscured by land-cover based approaches. While positive land-cover based fragmentation effects were consistent with recent synthesis work, the species-centered approach consistently revealed strong negative effects of fragmentation matching traditional theoretical expectations. S-SDMs may offer promise for generalizing ecological theory to real species distributions.