Some eucalyptus species may have high protein content, but anti-nutrients such as tannins bind the protein so it cannot be digested by koalas. Hume estimated that current levels of global carbon dioxide emissions would result in a noticeable reduction in Australia's koala population in 50 years due to a lack of palatable leaves.

Out of more than 600 eucalyptus species in Australia, koalas will only eat the leaves of about 25, Hume said. Changing the toxicity levels in the trees could further reduce the varieties that koalas 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," Hume said.

Hugh Tyndale-Biscoe, a marsupial physiologist, described Hume's predictions of declining koala numbers as speculative but credible.

Eucalyptus leaves already have little nutritional value, he said, and koalas have adapted to their poor diet by sleeping to conserve energy.

"It's a very precarious existence," 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.

Tyndale-Biscoe said koalas had already disappeared from parts of Australia but remained plentiful in others and were unlikely to be wiped out by climate change. They already have been displaced from the most nutritious trees on the most fertile land by the spread of farms and suburbs, he said.