Nestedness in fragmented landscapes: birds of the box-ironbark forests of south-eastern Australia

Nestedness in biota as a function of species richness - biota of depauperate assemblages being non-random subsets of richer biotas - has been widely documented in recent years (see Wright et al. 1998, Oecologia 113: 1-20). Ordering sites by richness maximizes nestedness indices; however, ordering by other criteria such as area or isolation may be more ecologically interpretable. We surveyed birds in true fragments (35 in all), and in reference areas in large extant forest blocks (30 locations), of the same range of areas (10, 20, 40, 80 ha). The avifauna was divided into bush birds - species dependent on forest and woodland, and open country species. We looked at nestedness in four data sets: bush birds in fragments and reference areas, and all birds in fragments and in reference areas. All data sets were significantly nested. Ordering by area in all cases was not significantly less nested than ordering by richness. Ordering by area in fragments was significantly greater than in reference areas, but the differences in standardized nestedness indices were small (< 15%). We identified those birds that had distributions among fragments that conformed strongly with area, those that were more randomly distributed and some species that were more likely to occupy the smallest fragments. Among the latter was a hyperaggressive, invasive, colonial native species (noisy miner Manorina melanocephala). A suite of small, insectivorous birds was more likely to strongly conform with expected distributions in relation to area, which was consistent with observations of their vulnerability to the effects of the noisy miner in smaller fragments.