Bird Assemblages in Patchy Woodlands: Modeling the Effects of Edge and Matrix Habitats

As habitats become more fragmented, understanding landscape-level effects on habitat quality becomes increasingly important. These effects include factors intrinsic to the habitat fragments, such as vegetation cover and structure, and extrinsic factors, such as the modifying influences of surrounding (matrix) habitats. We develop a spatial model, the Effective Area Model (EAM), that predicts the effects of matrix habitats on species abundances in habitat patches. Model predictions are based on two sets of parameters: measures of species abundances at various distances from habitat edges (‘‘edge responses’’) and measures of the size and shape of the habitat fragments. We test model predictions for bird assemblages occupying fragmented habitats in central coastal California. Predictions of the relative abundances of birds, based on results from previous studies at nearby sites, are made for six small (,3 ha) patches of oak woodland habitat, three surrounded by grassland and three surrounded by chaparral. Results from field studies of these patches show significant differences between the two groups, indicating that the type of habitat surrounding a patch influences the composition and structure of the bird assemblage it supports. The rank order of species abundances correlated more closely with predictions of the EAM than with those of a null model that did not account for edge and matrix effects. The EAM is an improvement over models that do not account for the influence of surrounding habitats on the distribution and abundance of animals in small habitat patches, and over those that assume a consistent response, for a given species or taxon, at all types of edges. This approach may prove useful in attempts to understand and predict the effects of habitat fragmentation and restoration on the organization of animal assemblages.