Koalas in parts of Australia have been sterilised and given long-term contraceptives to control overpopulation – and it seems to have worked.

David Ramsey at the Arthur Rylah Institute for Environmental Research in Melbourne, Australia, and his colleagues have analysed the effect of two fertility control programmes on koala populations in areas where the animals have bred too successfully, risking starvation.

The researchers studied a programme implemented between 2004 and 2013 in Budj Bim National Park, Victoria, in which female koalas were captured and treated with an implant of levonorgestrelhormone, a contraceptive that usually lasts for 10 to 12 years. They also looked at the effectiveness of a sterilisation programme on Kangaroo Island, South Australia, between 1997 and 2013.

In certain Australian states such as Queensland and New South Wales, koalas are listed as a vulnerable species. But in parts of Victoria and South Australia, their populations have increased to such high densities that the trees they feed on are at risk.

Koalas prefer to eat the foliage of only a few species of eucalyptus trees – the manna gum (Eucalyptus viminalis) in particular, which is only found in south-east Australia. The overbrowsing of manna gums – consumption of all leaves on a specimen – kills the trees.

Overpopulation has previously led to koala starvation and drastic drops in the animal’s numbers in certain areas – with more than 70 per cent of koalas dying in one case.

Moving koalas to other less populated areas is also used as a solution, but is expensive and not always possible. The team found that the koala fertility programmes led to the recovery of manna gum trees with light or moderate defoliation, and significantly reduced tree deaths in Budj Bim National Park.

The researchers suggest that koalas should live at a density of less than one per hectare to avoid overbrowsing.

Journal reference: Biological Conservation, DOI: 10.1016/j.biocon.2020.108870