Edge effects for songbirds vary with forest productivity

Abstract

We hypothesized that edge density more strongly influences species abundances in more productive environments. To test this hypothesis we collected songbird point count data across broad biophysical gradients and gradients in forest patch edge density in the west and east slopes of the Cascade Mountains of Oregon and Washington, U.S.A., which differ in ecological productivity. We then analyzed bird response (75 species) at both the species and community level to gradients in edge density (m/ha) of open and closed-canopy forest within 1-km radius landscapes. We found that (1) differences in vegetation and structural conditions between open and closed-canopy stands were significantly greater at a highly productive landscape than a landscape with intermediate levels of productivity; (2) more bird species responded to changes in edge density in more productive west-slope Cascade forests than less productive east-side Cascade forests; (3) pooled abundance data from both sites showed that 25 of the 60 most abundant bird species responded significantly to the interaction between forest productivity and changes in landscape-level edge density; and, (4) at the community level, ordinations showed that bird community similarity in the productive west-slope Cascade forests differed across low and high levels of edge density whereas no such differentiation occurred in harsh, east-side Cascade forests. These results provide some of the first evidence supporting the hypothesis that edge effects are more pronounced in productive west-side forests where higher levels of edge density benefit generalist and open-canopy species while negatively influencing closed-canopy species. Consequently, forest management aimed at supporting species diversity will be most effective if tailored to ecosystem productivity.