Independent effects of forest cover and fragmentation on the distribution of forest breeding birds

The aims of this study were (1) to determine the relative importance of the independent effects of forest cover and fragmentation on the distribution of forest breeding birds, and (2) to test the hypothesis that the negative effect of forest fragmentation on species distribution increases with decreasing forest cover, i.e., the negative interaction effect of forest cover and fragmentation on distribution. The independent effects of forest cover and forest fragmentation on the distribution of forest breeding birds were studied in 94 landscapes, 10 X 10 km each, ranging in forest cover from 2.5% to 55.8%. For each landscape, percent forest cover was measured, and a fragmentation index (independent of forest cover) was generated using PCA from the measures of mean forest patch size, number of forest patches, and total forest edge. Presence of 31 forest breeding bird species in each landscape was determined using Breeding Bird Atlas data. The effects of forest cover and forest fragmentation on species presence were analyzed using multiple logistic regression. All responses of individual species to forest cover were positive. Responses to forest fragmentation were weak and variable. There were only two species for which the interaction between cover and fragmentation was significant (one positive, one negative). We found no evidence for the hypothesized negative interaction effect between forest cover and forest fragmentation on species distribution. We conclude that(1) forest cover at the 10 X 10 km (Universal Transverse Mercator [UTM]) scale has a positive effect on the distribution of forest breeding birds, (2) forest fragmentation does not have a consistent negative effect on the distribution of forest breeding birds, (3) the effect of forest cover is greater than that of forest fragmentation, (4) responses to forest fragmentation are generally weak and highly variable among species, and (5) the effect of forest fragmentation on species distribution does not increase with decreasing forest cover. These results suggest that conservationists' primary focus should be on preventing a decrease in forest cover. They should not be misled by recent discussions of "fragmentation effects" to think that the negative effects of forest loss can somehow be countered by careful consideration of the spatial patten of remaining forest. Our results indicate that they cannot.