Threshold effect of eucalypt density on birds in buloke woodlands: the importance of species choice in restoration projects

Abstract

The buloke woodlands of south eastern Australia are of high value for bird conservation but are nationally endangered and highly depleted. Buloke is a slow-growing species and current projects aiming to restore buloke woodland usually incorporate a high density of faster-growing eucalypts in plantings. I aimed to determine whether the habitat value of buloke woodland to woodland bird assemblages is altered when eucalypts are present, and in particular, whether a particular density of eucalypts facilitates invasion by agressive noisy miners - a species absent from pure buloke woodland. Transects in 31 buloke woodland sites containing eucalypt densities between .0-16/ha were surveyed three times each over a period of one year. I found that the probability of noisy miner presence in buloke woodland increased markedly where eucalypts were present at a density of approximately five per hectare. This is the likely cause of a substantial difference in bird assemblage structure and composition between sites with >5 eucalypts/ha and those with fewer eucalypts. Low-eucalypt density sites had more hooded robins, dusky woodswallows, varied sittellas and yellow thornbills, all smaller-bodied insectivorous species which are experiencing population declines in southern Australia. Higher eucalypt density sites were characterised by Australian magpies, white-plumed honeyeaters, crested pigeons and eastern rosellas, all common open-country or aggressive species. These findings have implications for revegetation and restoration practices in buloke woodlands, and potentially also in other non-eucalypt woodlands within the range of the noisy miner.