Forest fragmentation and bird community dynamics: Inference at regional scales

With increasing fragmentation of natural areas and a dramatic reduction of forest cover in several parts of the world, quantifying the impact of such changes on species richness and community dynamics has been a subject of mu ch concern. Here, we tested whether in more fragmented landscapes there was a lower number of area-sensitive species and higher local extinction and t urnover rates, which could explain higher temporal variability in species r ichness. To investigate such potential landscape effects at a regional scal e, we merged two independent, large-scale monitoring efforts: the North Ame rican Breeding Bird Survey (BBS) and the Land Use and Land Cover Classifica tion data from the U.S. Geological Survey. We used methods that accounted for heterogeneity in the probability of dete cting species to estimate species richness and temporal changes in the bird communities for BBS routes in three mid-Atlantic U.S. states. Forest breed ing bird species were grouped prior to the analyses into area-sensitive and non-area-sensitive species according to previous studies. We tested predic tions relating measures of forest structure at one point in time (1974) to species richness at that time and to parameters of forest bird community ch ange over the following 22-yr-period (1975-1996). We used the mean size of forest patches to characterize landscape structure, as high correlations am ong landscape Variables did not allow us to disentangle the relative roles of habitat fragmentation per se and habitat loss. As predicted, together with lower species richness for area-sensitive speci es on routes surrounded by landscapes with lower mean forest-patch size, we found higher mean year-to-year rates of local extinction. Moreover, the me an year-to-year rates of local turnover (proportion of locally new species) for area-sensitive species were also higher in landscapes with lower mean forest-patch size. These associations were not observed for the non-area-se nsitive species group. These results suggest that landscape structure may influence forest bird co mmunities at regional scales through its effects on the total number of spe cies but also on the temporal rates of change in community composition. Evi dence for higher rates of local extinction and turnover in more fragmented landscapes suggests that bird communities function as metapopulations at a regional scale, and points out the importance of colonizations and recoloni zations from surrounding landscapes to local community dynamics. Further, o ur results illustrate that the methods used to estimate the community param eters can be a powerful statistical tool in addressing questions relative t o the dynamics of communities.