SYDNEY: The koala has been touted by conservationists as an iconic threatened species – but is it really? A new study argues for contraceptive implants to control burgeoning populations.

While this cute and cuddly Australian mascot has declined substantially since European settlement, "it remains one of the most widespread of all native mammals, ranging from north Queensland to South Australia (SA), and occupying an area of up to one million km²," said population geneticist, Des Cooper of the University of New South Wales and researcher behind the new study.

"Public is misinformed"

He presented evidence at a recent conservation conference, suggesting that koalas are not as near extinction as conservation groups, such as the Australian Koala Foundation (AKF), based in Brisbane, have argued (see, Demise of the koala?, Cosmos Online). The AKF identifies habitat loss as the major threat to koala extinction, accepting donations from members to help restore Koala habitat and fund conservation.

Though Cooper admitted that scientists don't agree on the status of the marsupial, he believes that the "public is largely misinformed."

He said that translocation and re-introduction programs have restored koalas to much of their former habitat in Victoria and that koalas are now more widespread in SA than they were at the time of European settlement.

"This is arguably one of Australia's greatest conservation success stories," said Cooper. "But, some of these populations now occur at such high densities that they degrade their woodland habitat."

Population explosion

An additional problem is that many of these populations were founded from small numbers of individuals and have endured population bottlenecks rendering them genetically depauperate.

"In Victoria and SA the reintroduction of koalas has seen the widespread establishment of a genetically diminished stock that has a well-demonstrated propensity to undergo exponential growth, resulting in local destruction of habitat" commented Alistair Melzer an ecologist at Central Queensland University in Rockhampton. Koala populations occur at such high densities in these areas that they have been known to deplete eucalypts to the point that they starve themselves, he said.

One example is the population of Kangaroo Island in SA, where 18 koalas were re-introduced from Victoria's French Island in the 1920s. Today the inbred colony has ballooned to 27,000 individuals that are chomping their way through threatened eucalyptus habitat and pushing out endangered birds such as the Glossy Black Cockatoo and the White-naped honeyeater.

"This raises the questions; what is the relative value of these populations in terms of conservation?" said Cooper. "Should we attempt to restore genetic diversity to these populations?"

Controlling such populations through culling seems a logical next step. But there are numerous hurdles said Cooper. The species iconic status makes it hard to convince local governments to adopt this method for a start. As an alternative to culling or sterilisation, Cooper's team has come up with a unique contraceptive implant. The slow-release intra-muscular device lasts two years. It is inserted into a koala's neck and works by slowly delivering gonadotrophin-releasing hormone into the animal's bloodstream.

"Nine smarties on an A4 page"

Though Melzer agreed that Cooper is "in broad and academic terms correct," about the overall status of the koala, he added that habitat destruction and the drought has rendered the species vulnerable in parts of Queensland and possibly NSW.

Deborah Tabbart, CEO of the Australian Koala Foundation, said the scientists are overestimating the damage done by koalas on kangaroo island.

"The koala problem is only one of many problems," she said. "There is damage from other introduced species such as feral pigs and deer which eat young eucalypts, preventing new trees from developing. Other causes of tree death on the island can be attributed to possums, saline soils and diseased trees."

Tabbart also rejects Coopers use of small population isolates as a representative of the national koala situation. "These isolates are like the size of nine smarties on an A4 page" she said.

Cooper's research was detailed at the 2007 Biodiversity Extinction Crisis Conference held at the University of New South Wales in Sydney.