Musings of an amateur observer

I don’t know much about koalas, but some decades ago I noticed the gathering concern about koala conservation. Since hunting koalas for their furs ended earlier in the 20th century, I thought it strange that in many places they seemed to be rare and sickly while in others they were abundant and apparently healthy. I heard about efforts focused on identifying and protecting prime koala habitat, research identifying chlamydia and other afflictions affecting some populations, and relocation programs from places of abundance to those of scarcity. It seemed curious that in some places koalas were so abundant that they were killing the gum trees they depended on for food. A few anecdotes and personal observations added to my sketchy knowledge about koalas.

My partner Su Dennett recalls going to Phillip Island in the 1950s with her father for the motorbike races and seeing the local koalas gathered to watch the races from the trees fringing the track. According to the locals, they began gathering days before any race! Whatever the case it seems they were not put off by the noise and dust.

Locally around Melliodora, koalas were occasional sightings in the bush and we did occasionally see one in one of the few large manna gums that grew in our local creek dominated by blackberries and deciduous trees. In the early 90s we wondered if our track making along the creek was facilitating koala travel between their fodder trees.

Around the same time on a trip to Kangaroo Island, I became aware of the thriving koala population that had grown from those introduced early in the 20th century.

A couple of decades later camping at the Aire River East campground there were so many koalas that the manna gums were overgrazed. The substantial accumulation of koala droppings added to the fertility from years of human camping (and maybe centuries of indigenous occupation). This supported a bright green growth of kikuyu grass that was nearly a metre deep around each tree where the mowers, wood ducks, rabbits and wallabies hadn’t been able to keep it in check. It didn’t seem like koalas needed a pristine native ecosystem to thrive.

The nutritional knife edge hypothesis

I noted that while some species of eucalypts seemed to be favourites, koalas consumed a variety of species in different places. Everyone knows how the koala digestive system has to deal with toxic eucalyptus oil, but few are aware of the poor nutritional status of gum leaves. The abundance of eucalypt leaves in most Australian ecosystems is the food supply for a great diversity of insect herbivores but few mammals consume eucalypt leaves at all. While the much simpler metabolism and tiny brain structure of insects allows them to thrive on such tucker, mammalian herbivores can struggle to get adequate mineral nutrition even from far more nutritious plant fibre to support their large brains. Adaptations like the four stomachs of ruminant animals reflect this struggle for mineral nutrition.

It seemed to me that by eating only eucalypt leaves, koalas must be on a nutritional knife edge, constantly in danger of not getting enough carbohydrate, protein and, most importantly, minerals, or alternatively, becoming over stuffed with hard to digest fibre and toxic eucalyptus oil that must be broken down or excreted.

This big picture systems view of the koala led me to think that maybe the quality of tucker could explain the plight of the koala. Manna gums are well recognised as a favourite of koalas. They grow across a wide range of country but in central Victoria they are particularly characteristic of the volcanic landscapes that were extensively cleared for farming during the gold era. I also noted that Phillip Island is also volcanic with an abundance of manna gums. The manna gums at the Aire River campground were growing on the edge between calcareous sands and the peat soils fringing wetlands.

I wondered if it was the sugar gums growing on the better calcareous soils of Kangaroo Island that were favoured by the koalas over the mallee vegetation on the poorer sands that predominated at the western end of the island. I knew sugar gums were more favoured by livestock than most eucalypts.

I figured that before settlers cleared the volcanic and other minerally richer soils for cropping, koalas might have received adequate nutrition from browsing eucalypts growing on these richer soils, all managed carefully by indigenous folk who were similarly concentrated on these better soils.

While there are more than enough eucalypts in most regions to theoretically support abundant koalas, most of those trees are growing on more infertile soils and so might be inferior in their nutritional value. As a general principle, plants can thrive with a much greater range of mineral deficiencies than herbivores that consume those plants.

Further exacerbating this naturally poorer tucker, the degradation of many bush soils following overgrazing by livestock in the 19th century, combined with the impact of gold mining, bushfires and over-dense eucalypt regrowth has further depleted the precious but thin A1 horizon topsoil across vast areas of forest including national parks and reserves. The gum trees and the insects might be coping ok but I suspect tucker for possums and other arboreal browsers is poor at best. For koalas, our bushlands might be a nutritional desert (even worse than the human nutritional deserts in the outer suburbs of our capital cities).

Blue gum feedlots

In more recent decades, as harvesting of the widely planted blue gum plantations in southern Australia was underway, I was amused to hear about the contractors having to hire koala spotters ahead of the harvesting machines to identify and protect the koalas that had given their vote of approval to these bleak short rotation industrial monocultures. I figured since most of the plantations were planted on naturally better farmland soils, the foliage was a good deal more balanced than trees on adjacent poorer bushland soil. I also wondered if decades of fertilised and improved pastures grazed by sheep and cattle was also a factor in making the blue gums more palatable and nutritious for the koalas, especially in the first rotation of growth after conversion from pasture.

At the recent Off Grid Living Festival, I had interesting discussions with many people at our book stall, but one stood out, which triggered this essay. I was in discussion with a wildlife ecologist managing 500 hectares of blue gum plantation that had survived the bushfires of December 2019, which devastated much of Kangaroo Island including most of the Blue Gum plantations. The failure to build an export port had already dealt a blow to the prospects for the plantations and maybe the bushfires were the final straw. I heard how prior to the fires the plantations (on ex-pasture farmland) supported 10 times the density of koalas that lived on the bushland parts of the island. Even before the blue gums, Kangaroo Island had been good country for koalas. Being an “introduced species” from the Australian mainland, the conservation authorities had for years been exporting Kangaroo Island koalas to country all up the east coast where koalas had been in decline. With plans to return the burnt plantations to pasture, the koala refugees now concentrated in the 500 ha will apparently be resettled in native revegetation areas once mature enough to handle the predation. It seems the opinion of our national icons voting with their feet and mouths doesn’t count for much.

My discussion with the Flinders University trained wildlife ecologist ranged around how the blue gum plantations could have been the first step in creating magnificent mixed forest producing honey, livestock shelter and fodder, fine timber, dispatchable biomass energy, biochar and happy koalas. In the process I took the opportunity to outline my koala malnutrition hypothesis.

Putting it down on (digital) paper

My interlocutor was wide eyed at the implications and was probably in a better position to shoot down my idea than anyone else I had spruiked the idea to over the decades. Maybe in the early days of research on koalas, funded by taxpayers and donors, someone did consider the idea and it was set aside for lack of evidence. But just maybe the reductionist myopia that I see afflicting science everywhere combined with established interests pushing barrows is also at work in koala care and conservation. Anyway my interlocutor suggested she would be in touch and maybe a joint paper published somewhere might help open up fruitful avenues for koala conservation.

I didn’t get her name or contact details, but our conversation inspired me to pen this piece while the subject was on my mind. I have resisted the temptation to surf the net to correct, moderate or top up my case so this recording captures the idea as it has been sitting with me for nearly three decades.

David Holmgren
Melliodora
April 2024