ONE of Australia's most iconic creatures is under threat because its food is being poisoned by growing levels of carbon dioxide in the atmosphere, research has shown.
The koala could be the latest species to fall victim to increasing levels of greenhouse gas emissions, according to Ian Hume, emeritus professor of biology at Sydney University.
His studies have revealed that carbon dioxide saps nutrients from euc
alyptus leaves, which are the animals' only source of food.
Professor Hume estimates that at current pollution levels, a reduction in Australia's koala population will be evident in 50 years due to a lack of leaves edible to the creatures.
Prof Hume, who has written books on the future of marsupials, discovered the leaves became more toxic as the level of carbon dioxide inside a greenhouse was increased.
This was because carbon dioxide in eucalyptus leaves affects the balance of nutrients and "anti-nutrients" – substances such as tannins which are either toxic or interfere with the digestion of nutrients.
As carbon dioxide increased, the eucalyptus trees started producing more anti-nutrients than nutrients, meaning the leaves became poisonous to koalas.
The animals are already fussy over which eucalyptus they will eat. There are more than 600 species of the tree in Australia, but koalas will only eat the leaves of about 25 of them, according to Prof Hume.
He thinks the new research suggests that if toxicity levels increase, even more varieties could become inedible to koalas.
He said: "If there is a significant rise in concentration in the atmosphere, which we're already seeing, that's going to push the ratio of nutrients to anti-nutrients even lower by increasing the concentration of these carbon-based anti-nutrients.
"What currently may be good koala habitat may well become, over a period of not so many years at the rate that concentrations are rising, very marginal habitat.
"I'm sure we'll see koalas disappearing from their current range even though we don't see any change in tree species or structure of the forests."
A smaller food supply could affect population growth, according to Prof Hume.
"Koalas produce one young each year under optimal conditions, but if you drop the nutritional value of the leaves, it might become one young every three or four years," he said.
He presented his research on the effects of carbon on eucalyptus leaves to the Australian Academy of Science this week.
Hugh Tyndale-Biscoe, a marsupial physiologist in Australia, described Prof Hume's predictions of falling koala numbers as speculative, but credible.
"It's a very precarious existence and the distribution of koalas tends to shift," he said.
"They basically sleep for 20 hours a day and then they've got four hours to do everything else – occasionally eat a leaf and maybe once a year go after another koala to mate," he added.
Dr Tyndale-Biscoe said koalas had already disappeared from some parts of Australia, but remained plentiful in others and he thinks they are unlikely to be wiped out by climate change.
They had already been displaced from the most nutritious trees on the most fertile land by the spread of farms and suburbs, he said.
BEAR NECESSITIES• IT IS believed koalas were not always so fussy in their eating habits. Fossils of the marsupials have been found in north Australia dating to 20 million years ago, when the region was covered in rain forest.
• At that time it is thought koalas munched through other types of leaves, and it was only when the climate cooled and eucalypt forests grew in place of rain forests that they developed their modern tastes.
• Eucalyptus was perhaps not the best choice of food, as the leaves are low in protein, high in indigestible substances and contain compounds that are toxic to most species.
• It is believed that as koalas adapted to a diet low in energy, their brains shrank. Those of their ancestors filled the whole cranial cavity, but today they are much smaller.
• Although koalas are often called bears, their closest relative is actually the wombat.
• Their five fingers are arranged with opposable thumbs, for better gripping ability.
• The koala is also one of the few mammals, other than primates, that has fingerprints.
• They are generally silent, but males can let out a very loud call during the mating season that can be heard from almost a kilometre away. If healthy, females can produce one baby a year for about 12 years. Occasionally they have twins.
