The “secret culling” of 700 koalas by the Victorian government at Cape Otway in late 2013 and early 2014 made front-page news when it was revealed this month. The headlines inevitably provoked outrage.

After all, the koala is one of the world’s most widely recognised and best-loved animals. What is more, it is correctly perceived as being of conservation concern.

How, then, do we explain the government-sanctioned killing of these animals?

Koalas endangered

The koala is listed as vulnerable in Queensland, New South Wales and the ACT, and is declining in much of its range.

Yet in some parts of Victoria and South Australia there are too many koalas. In these situations, koala populations increase to the point where their browsing pressure on preferred tree species (usually manna gum, Eucalyptus viminalis) cannot be sustained. This presents a challenge for management.

In the absence of human intervention, the presence of too many koalas causes widespread tree death, loss of habitat for koalas and other forest-dependent wildlife, and widespread suffering for the koalas as they slowly starve. This is the context in which the events at Cape Otway took place.

The intervention at Cape Otway was not conducted in secret, nor was it a cull. Whereas culls are motivated by a desire to reduce the size of a population, the euthanasia program at Cape Otway was undertaken to reduce the suffering of koalas that were in irreversibly poor condition.

From too few to too many

These recent events are the latest instalment in the long and fascinating history of koalas in southern Australia. Hunting for fur in the 19th and early 20th century saw koalas almost eliminated from Victoria and become locally extinct in South Australia.

Starting in the late 1890s koalas were introduced to French Island in Westernport Bay. In the absence of hunting, disease, predators, or significant bushfires, and with nowhere else for young koalas to disperse to, this population thrived.

It subsequently provided a source of koalas for translocations that re-established koalas throughout southern Australia. The majority of today’s Victorian and South Australian koalas are descendants of these French Island animals.

It was not long before overabundance became a concern at some reintroduction sites. One of the earliest and most high-profile cases occurred on Quail Island in Westernport Bay. Visitors to the rarely-frequented island in 1943 described a tragic scene – defoliated and dying trees and dead and starving koalas everywhere. The koala has always held a strong place in human affections and even 70 years ago, these events were greeted with public dismay around the world.

Moving koalas isn’t the whole answer

For most of the period since the Quail Island event the primary means of dealing with too many koalas was moving them. Literally tens of thousands of koalas have been moved around southern Australia, relieving trees of browsing koalas, and re-establishing koalas where they had disappeared.

In that sense, translocation has been extremely successful. However, in recent years we have come to realise that translocation is not always good for koala welfare.

One study found that translocation to a new site increased koalas’ likelihood of dying in the following 12 months by 40%. Another unpublished study from Parks Victoria suggested that there may be up to 90% mortality in groups of koalas moved to habitat that contains different food trees. Furthermore, moving koalas may be just moving the problem. And there’s a shortage of suitable sites.

Consequently, translocation is no longer favoured as a koala management tool. Alternative approaches include the capture and sterilisation of koalas via surgical or chemical methods.

However, fertility control is a slow-acting method that must be implemented well in advance of desperate situations like that of Cape Otway in 2013.

How does it get this bad?

So, why do koala numbers explode? We don’t know whether these booms occurred before Europeans arrived in Australia, but it is plausible that hunting by aboriginal people and bushfires may have kept koala numbers in check.

The most obvious change brought about by European Australians is land clearance, which has isolated koala populations in islands of habitat with nowhere for young animals to disperse to when things start to get a bit crowded.

Female koalas can produce at most one joey per year. While this may seem low, it actually allows koala populations to double in less than three years. This can result in exponential growth if circumstances are good. With few natural enemies, few adults die, and many young survive.

But this rapid population growth can only happen if there’s enough high-quality food. One thing we do know about koala overpopulation events is that the most dramatic cases almost always occur in coastal manna gum (Eucalyptus viminalis) forest.

Work by one of us (Ben Moore) with local not-for-profit the Conservation Ecology Centre shows that in comparison to most other eucalypts the leaves of this species are especially soft, contain a lot of water and are rich in highly digestible protein. Many other eucalypts contain chemical compounds called tannins which prevent the easy digestion of much or all of the protein, but manna gum contains very little tannin.

And when manna gums lose many or most of their leaves to koalas, they respond by sprouting more, much as they do when recovering from fire. In some eucalypts these young leaves are more toxic and harder to digest. But young manna gum leaves are just as nutritious as older leaves. So koalas have enough nutritious food until the trees exhaust their energy reserves and die.

Could the answer be in koala guts?

As we’ve shown, one problem with moving koalas is they may not be able to eat the eucalypt leaves in their new homes. But research is now focusing on whether microbes in koalas’ guts could help koalas settle in, allowing us to move them from places where there are too many.

Microbes play a vital role in allowing the koala to digest and detoxify eucalypt leaves. The makeup of each koala’s microbial community may be closely matched to the eucalypt species that it feeds upon.

Koalas living in coastal manna gum woodlands usually feed on nothing but this species, but after translocation may be forced to include new, less preferred eucalypt species. This may have catastrophic consequences for the microbial communities in the koalas’ guts, and consequently for the koalas’ health.

In the future we hope to use microbial inoculations to ensure that koalas are equipped with the microbes they need to cope with different diets before they are relocated. This is one way that science is helping improve management options to deal with local koala overpopulation.