Patch size determines the strength of edge effects on carabid beetle assemblages in urban remnant forests

Habitat fragmentation due to urbanization is increasing rapidly worldwide. Although patch area and edge effect are both important determinants of species diversity and the number of individuals in fragmented landscapes, studies that tested interaction between two effects were limited. Here we examined the interaction between area and edge effects on species richness and the number of individuals of carabids in highly fragmented forests in Tokyo, central Japan. We surveyed carabids in each of 26 forest patches (1.1–121.6 ha) using pitfall traps set in both edge and interior zones. First, we correlated the edge-to-interior differences of both species richness and the number of individuals with patch area. Second, we examined the interaction between patch area and distance to the edge on species richness and the number of individuals using generalized linear models. We found a significant decrease in carabid species richness and the number of individuals in edge zones. The edge-to-interior differences in both species richness and the number of individuals were positively correlated with patch area. Model selection revealed the evident interaction effects between patch area and distance to the edge: higher number of individuals was predicted in only large interior zones. Our results indicated that carabid beetle assemblages were influenced by the interaction between area and edge effects. Thus, in urban areas where small forest remnants dominate, circularizing the shape of the forest patches to maximize the core areas may be the most feasible and realistic means to preserve biodiversity.