Why have birds in the woodlands of Southern Australia declined?

Abstract

This paper reviews the reasons why so many species of birds have declined in the eucalypt woodlands and associated habitats across the agricultural zone of southern Australia. The extent of habitat lost, over 90% in some regions, has led to the local extinction of some bird species, simply through random sampling effects. Habitat specialists and those that move sequentially among several habitats, are especially at risk, as some habitats have been lost disproportionally. Fragmentation introduces additional problems by subdividing populations into small, isolated sub-populations. Whereas some of the remaining species of birds appear able to move through highly fragmented landscapes, it is possible that they suffer high mortality while doing so. Some species that have been lost regionally may have had difficulty dispersing, but there have been few detailed studies of the demography of Australian birds in fragmented landscapes. Such studies are necessary before we can assess the value of corridors, or other means, to assist dispersal of birds. Fragmentation also leads to edge effects, which, when compounded by habitat degradation, may alter the intensity of a number of ecological processes. There is circumstantial evidence suggesting that loss of nest sites and increased predation on nests and free-living birds have contributed to the decline of woodland birds. Increased interspecific competition, for instance with noisy miners Manorina melanocephala, may also have a major impact on smaller insectivores and honeyeaters. Effects of parasites and disease have barely been studied in Australia, though brood parasitism could account for local losses. Dieback of eucalypts and loss of understorey are common in fragmented and degraded landscapes and are associated with a greatly reduced diversity of birds. The effect of fragmentation and degradation on food has received minimal attention. We propose further research that tests the importance of some of these ecological processes in causing the decline and loss of bird species in agricultural woodlands. Although management should proceed immediately, including a cessation of any further clearing of native vegetation, it should be conducted in conjunction with research. We suggest how the findings of research can inform managers, which will make management more effective in achieving conservation of regional avifaunas.