Australia’s woodland birds are continuing to “pay the debt” imposed on them through large-scale land clearing since the nineteenth century.
That “debt” is labelled “the extinction debt” in the literature of ornithology because its “payment” involves the local extinction – over a period of many decades – of some animal and bird species in any isolated patches of vegetation that remain after clearing.
Delegates to the fifth biennial Australasian Ornithological Conference in Armidale earlier this month were able to compare the fortunes of woodland birds in their own study areas with those on the Northern Tablelands, where scientists from the University of New England have been documenting declining populations of some woodland bird species for many years. The conference, which ran from the 29th of November to the 4th of December, included a day-long symposium on woodland birds.
“At the time of clearing, the remnant vegetation has more species than can survive at equilibrium,” explained UNE’s Professor Hugh Ford, who contributed to the symposium, “so there is a ‘relaxation’ in species diversity. The region is said to contain an ‘extinction debt’ that needs to be paid, and a number of species will be lost during the payment of the debt, which may take many decades.
“It’s a depressing story, but a really encouraging thing to come out of the conference was the consensus that, as a result of an increased level of research activity, we now have a better understanding of the ecological processes involved – even though much is still unknown.”
Professor Ford has conducted – and supervised – fieldwork on woodland birds in the Armidale area over the past 30 years. He is the principal author of a paper published this month in the international journal Biological Conservation (Vol 142, pp 3182-3190) that focuses on the progressive loss in the Armidale region of two ground-foraging woodland birds: the brown treecreeper (Climacteris picumnus) and the hooded robin (Melanodryas cucullata – pictured here). The other researchers who contributed to the paper are Stephen Debus from UNE, Jeff Walters from Virginia State University (USA), Caren Cooper from Cornell University (USA), and Veronica Doerr from CSIRO in Canberra.
The fact that most of the land clearing in the Armidale area occurred over 100 years ago indicates the involvement of an extinction debt, their paper says.
Titled “Extinction debt or habitat change? – Ongoing losses of woodland birds in north-eastern New South Wales, Australia”, the paper reveals that the population decline in each of the two species is caused by a different mechanism. In brown treecreeper populations, females seem to be unable (or unwilling) to disperse among pockets of remnant vegetation. The resulting isolated groups of males then die out.
In contrast, hooded robins suffer high levels of nest predation in fragmented landscapes, leaving too few developing chicks to replace losses caused by adult mortality.
The authors report that brown treecreepers have disappeared from 14 out of 21 study sites (all of them within a radius of 40 km of Armidale) that had populations of the species 30 years ago, and that hooded robins remain in only three of their study sites – compared to 11 sites in 1992.
“There’s a reasonable chance that both of these species will continue to contract westwards, out of the Northern Tablelands,” Professor Ford said. They would thus follow the crested bellbird and the grey-crowned babbler into regional extinction.
While an obvious measure in attempting to prevent these losses would be extensive tree planting, a less-obvious measure, Professor Ford said, would be changes in patterns of stock grazing. Both the brown treecreeper and hooded robin are ground-foraging birds, and the reintroduction of periodic grazing in areas such as disused travelling stock routes could make it easier for birds to feed there. “Managers may need to introduce grazing and/or fire at appropriate levels to conserve some kinds of ground-foraging birds,” the paper concludes.
Finally, in widening their perspective, the authors say: “It is likely that many other eucalypt woodland birds are following similar trajectories towards regional extinction, and that this pattern is being repeated in woodlands and forests around the world.”