Contrasting effects of mosaic structure on alpha and beta diversity of bird assemblages in a human-modified landscape

Habitat loss and fragmentation are key processes causing biodiversity loss in human-modified landscapes. Knowledge of these processes has largely been derived from measuring biodiversity at the scale of ‘within- habitat’ fragmentswith the surrounding landscape considered as matrix. Yet, the loss of variation in species assemblages ‘among’ habitat fragments(landscape-scale) may be as important a driver of biodiversity loss as the loss of diversity ‘within’ habitat fragments(local-scale). We tested the hypothesis that heterogeneity in vegetation cover is important for maintaining alpha and beta diversityin human-modified landscapes. We surveyed bird assemblages in eighty 300-m-long transectsnested within twenty 1-km vegetation ‘ mosaics’, with mosaicsassigned to four categories defined by the cover extent and configuration of native eucalypt forest and exotic pine plantation. We examined bird assemblages at two spatial scales: 1) within and among transects, and 2) within and among mosaics. Alpha diversitywas the mean species diversity within- transectsor within- mosaicsand beta diversityquantified the effective number of compositionally distinct transectsor mosaics. We found that within- transect alpha diversitywas highest in vegetation mosaicsdefined by continuous eucalypt forest, lowest in mosaicsof continuous pine plantation, and at intermediate levels in mosaicscontaining eucalypt patches in a pine matrix. We found that eucalypt mosaicshad lower beta diversitythan other mosaictypes when ignoring relative abundances, but had similar or higher beta diversitywhen weighting with species abundances. Mosaicscontaining both pine and eucalypt forest differed in their bird compositional variation among transects, despite sharing a similar suite of species. This configuration effect at the mosaicscale reflected differences in vegetation composition among transects. Maintaining heterogeneity in vegetation cover could help to maintain variation among bird assemblages across landscapes, thus partially offsetting local-scale diversity losses due to fragmentation. Critical to this is the retention of remnant native vegetation.