The Effects of Edge Permeability and Habitat Geometry on Emigration from Patches of Habitat

Abstract

Using computer simulations, we investigated two factors that may affect emigration from insular patches of habitat, where emigration is the proportion of dispersing individuals that leave the habitat patch. The first factor, edge permeability, $\phi$, reflects the tendency of a disperser reaching the edge of a habitat patch to cross the boundary and emigrate; edge permeability is positively related to emigration for any given habitat patch. The second factor is the edge-to-size ratio (ESR), the proportion of home ranges at the edge of a habitat patch; for any degree of edge permeability greater than zero, the ESR is positively related to emigration. When habitats have relatively hard edges (e.g., 0 $\leq \phi \leq$ 0.1), edge permeability is a more important determinant of emigration than is the ESR. Conversely, when habitats have relatively soft edges ($\phi$ > 0.1), the ESR is the more important determinant of emigration. These interactions between the effects of edge permeability and patch size and shape have important implications for field biology. In particular, investigators should be cautious when comparing hard- and soft-edged habitats, since patch size and shape may have important ecological effects in soft-edged, but not in hard-edged, insular patches of habitat. Field studies of phytophagous insects and of vertebrates in fragmented or insular habitats tend to support the predictions of these models. Hence, these models may be relevant to both conservation and control policies.