Wander through many woodlands in southern Australia these days and they are much quieter than they were 50 years ago. The birds are disappearing. One of Australia’s leading bird ecologists, Charles Sturt University’s (CSU) Associate Professor David Watson, believes he has some answers, which may have implications for Australian agriculture.
The numbers of woodland birds in southern Australia have been in decline for some years, but finding sound reasons for these declines and devising management solutions to arrest them have defied scientists, says Professor Watson, a lead researcher with CSU’s Institute for Land, Water and Society.
“Australia’s northern forests still abound with most of the birds that were first seen by Europeans over one hundred years ago.
“But in some southern forests, the silence can be deafening …”
Red Capped Robin, Stud Creek, Sturt National Park, NSW. Photo: Tom RambautScientists such as Professor Watson have long been concerned about declines in previously widespread birds in the woodlands of southern Australia, “but working out why has defied explanation”, he says.
“Land clearing, exotic predators such as feral cats and extensive degradation of remaining habitat have all contributed to the declines of most animals that depend on these woodlands. Yet not all woodland habitats have been equally affected; not all regions are involved; not all woodland bird species have declined,” says Professor Watson.
“This lack of uniformity has confounded scientists and refuted many otherwise sound explanations. Twenty-six bird species have rapidly declined over the last 50 years, disappearing from many parts of south-eastern Australia. However, these species persist elsewhere in inland and northern areas.”
Decline rooted in the past
By drawing on past research describing how water and nutrient availability has changed across southern Australia over the past century, Professor Watson has isolated two important factors that have changed the woodland environment: the selective clearing of the most productive land, and nutrient overload in the remaining habitats..
Firstly, he notes that land clearance for agriculture and other land use was not random, but limited mainly to habitats growing on the most fertile soils.
“Fertile land was selectively cleared, so that now there is a tiny fraction remaining of the most productive woodland that can support high numbers of birds,” Professor Watson says.
“Most remaining woodlands these days are on rocky ridges. In good seasons, these would have been valuable habitat for some species, but otherwise they simply don’t have the resources needed to support many animals—there’s just not enough food available.
“The areas that now grow wool and wheat, beef and canola used to grow lots of robins and babblers, treecreepers and honeyeaters.
“Rather than all woodland being equally valuable, we need to appreciate that most of our remaining woodlands were originally marginal habitat for woodland species — they grow on parts of the landscape that are incapable of supporting these birds for long periods and can’t provide the resources the birds need to successfully breed and maintain their numbers.”
Poor soils limit numbers
Overgrazing by native animals such as kangaroos as well as sheep and cattle have profoundly changed the delicate soil under the remaining woodlands.
“Australian soils are usually poor, but when you add herds of hoofed animals, they become so compacted that rainwater no longer infiltrates through the surface and plants can no longer survive the seasonal drought associated with our long hot summers.”
If water is no longer stored in the soil, nutrients become concentrated around dams and shade trees, and weeds begin to dominate under the trees, out-competing native plants that have evolved under very different conditions.
“This also changes the microbes in the soil, leading to far fewer insects and other soil-borne creatures — and these are the food sources for 25 of the 26 declining bird species in Australia’s southern woodlands!”
Not just birds
Professor Watson believes his ‘productivity-based explanation’ applies to all woodland animals, explaining why nectar availability has decreased, why tiny marsupial dunnarts, goannas and other large lizards and insectivorous bats are no longer found in many parts of southern Australia.
Rather than setting up a conflict between biodiversity conservation and agriculture, he suggests this understanding helps to prioritize current conservation efforts across the south.
“Not all woodlands are the same — those few remaining woodlands on deep fertile soils are very precious, as these have the highest productivity where restoration efforts will have the greatest success.
“By finding these places, targeting our efforts to enhance their value and expanding the bush corridors connecting them, the last remaining individuals of native animals living in those areas will have the greatest chances of finding each other, breeding, and begin to increase their populations again.”
Professor Watson believes his research is relevant to similar declines in birds found in many other parts of the world where commercial agriculture has severely affected biodiversity.