Relative influences of patch, landscape and historical factors on birds in an Australian fragmented landscape

Aim

Animal assemblages in fragmented landscapes are likely to be determined by contemporary (e.g. patch size, biotic interactions) and historical (e.g. change in patch area) characteristics of patches and their positioning in the landscape (e.g. connectivity). We considered the influence of habitat structure, landscape context, history and an aggressive, native bird species [the Noisy Miner Manorina melanocephala (Latham 1801)] on three characteristics of woodland-dependent bird assemblages in a fragmented, eucalypt-forest landscape. The response variables were: (1) species richness (SR), (2) sums-of-densities (i.e. total number of individual birds), and (3) occurrence of rare species [through a rare-species index (RSI)].


Location

Box-ironbark, eucalypt forests of central Victoria, Australia (broadly bounded by: 36–37° S, 142–146° E).


Methods

There were replicates of four size-classes of fragments (10, 20, 40, 80 ha) and replicates of the same size-classes set within large blocks of extant forest. This design allowed us to distinguish between area-specific and fragmentation effects by comparing same-sized fragments and reference areas to establish fragmentation-specific impacts.


Results

Species richness was less than expected in smaller fragments and this was apparently because of current fragment area, density of the Noisy Miner and habitat quality. The RSI exhibited similar dependencies, while sums-of-densities appeared to be related to the number and quality of wooded linkages to fragments. The historical variable was forested–area change of each fragment between 1963 and 1996, and was found to have no relationship to any of the response variables.


Main conclusions

The lack of historical influence suggested that the avifaunas of the fragments are unlikely to be `relaxing' from isolation effects through local extinctions, but are more likely to be dominated by dynamic recolonization and abandonment both seasonally and among years. The dependence of sums-of-densities on the linkage variable appeared to be largely because of the responses of two species, the Red Wattlebird Anthochaera curunculata (Shaw, 1790) and the Yellow-tufted Honeyeater Lichenostomus melanops (Latham, 1801). These species are among the three most common in the large forest blocks, but are much rarer in fragments. The connectedness effect suggests that these species may depend upon linkages to occupy fragments that are only moderately distant from large forested areas within this region.