CANBERRA — Koalas are threatened by the rising level of carbon dioxide pollution in the atmosphere because it saps nutrients from the eucalyptus leaves that the animals feed on, a researcher said Wednesday.
Ian Hume, emeritus professor of biology at Sydney University, said he and his researchers found that the level of toxicity in the leaves of eucalyptus saplings rose when the level of carbon dioxide within a greenhouse was increased.
Prof. Hume presented his research on the effects of carbon dioxide on eucalyptus leaves to the Australian Academy of Science in Canberra on Wednesday.
The researchers found that carbon dioxide in eucalyptus leaves affects the balance of nutrients and “anti-nutrients” – substances that are either toxic or interfere with the digestion of nutrients.
An increase in carbon dioxide favours the trees' production of carbon-based anti-nutrients over nutrients so that the leaves become toxic to koalas, he said.
Some eucalyptus species may have high protein content but also anti-nutrients such as tannins that bind the protein so it cannot be digested by koalas.
Prof. Hume said his team had not conducted modelling on how predicted future global greenhouse gas emission levels would affect the koalas' habitat, but he estimated that at current pollution levels a reduction in Australia's koala population would be noticeable in 50 years for lack of palatable leaves.
Out of more than 600 species of eucalyptus in Australia, koalas will eat the leaves of only about 25, Prof. Hume said. Changing the toxicity levels in the trees could further reduce the leaf varieties that the marsupials find palatable, he said.
“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."
They may be forced to travel in search of more nutritious leaves, increasing their risk of being hit by cars and attacked by dogs as they lumber slowly on the ground, he said.
Hugh Tyndale-Biscoe, a marsupial physiologist, described Prof. Hume's predictions of falling koala numbers as speculative but credible.
Eucalyptus leaves already have little nutritional value, he said, and koalas had adapted to their poor diet by sleeping 20 hours a day to conserve energy.
“It's a very precarious existence and the distribution of koalas tends to shift,” Dr. Tyndale-Biscoe 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 have already disappeared from some parts of Australia but remained plentiful in others and are unlikely to be wiped out by climate change. They have already been displaced from the most nutritious trees on the most fertile land by the spread of farms and suburbs, he said.
