Dec. 21, 1998 Like victims of a flood, some of Australia’s rarest marsupials have retreated to the tops of the highest mountains in the wet tropics in a bid to avoid the fatal consequences of long-term climate change.
Seven species of leaf-eating marsupials - two tree kangaroos and five kinds of possum -- are now stranded on the cool peaks, the last representatives of an extraordinary rich fauna that flourished in northern Australia over the past 20 million years.
The issue that John Kanowski is working against the clock to resolve is whether two degrees more of global warming will dramatically reduce the numbers of these unique animals.
The research is being carried out at the Rainforest Cooperative Research Centre (Rainforest CRC) as part of Mr Kanowski’s doctoral studies at CSIRO and James Cook University.
“These animals are relicts of the time when rainforests covered much of Australia. They are the last remaining representatives of an ancient and diverse fauna,” he explains.
“The possums and tree-kangaroos are essentially temperate animals that have become isolated in the high misty mountains of the tropics. They are tree-dwelling, cool-adapted and very territorial in their behaviour.
“They may be highly susceptible to global warming. A two degree warming in global climate which took place between 5000 and 3500 years ago is thought to have caused a major extinction event, and led the remaining marsupials to retreat to the mountain tops.”
Mr Kanowski’s concern is that changes in climate and the composition of the earth’s atmosphere over the coming 50-100 years could have an equally dire impact on the last surviving possums and tree kangaroos in the Australian tropics.
“For one thing, the atmosphere is becoming richer in carbon dioxide, and this makes the tree leaves on which these animals subsist, tougher and less digestible,” he explains.
Because humans have cleared the rainforest off most of the richer soils of northern Australia, what is left now mostly grows on thin, poor, granite soils about as nutritious as sand. This means the trees themselves are low in the nutrients needed by tree kangaroos and possums to sustain themselves.
“Because the leaves are poor in nutrients, the animals need to eat more to survive - and that means their intake of natural plant toxins is higher too, so they may be gradually poisoned by their own diet.”
Leaves of the rainforest tree contain natural substances which protect the plant against over-browsing by animals and insects - poisons such as phenolics, alkaloids and cyanide-forming compounds.
“In effect, land clearing, climate change and their own behaviour have already penned these animals into poor upland areas above 700 metres, where they are increasingly vulnerable to further changes in their diet or temperature.”
According to Mr Kanowski, understanding the physiology of these animals is the key to predicting their response to climate and habitat change.
“We think the animals need cool conditions, not only to keep their body temperatures down, but also to provide the dew they drink on misty mountain tops,” he says.
“If the cool wet forest retreats, the animals have no choice but to go with it.”
Mr Kanowski is also concerned at the potential impact on the rainforest itself which the extinction of tree-dwelling animals might have.
“These animals have been part of the rainforest for millions of years. Along with insects, they perform an important task in ‘mowing’ the forest.
“Take away the animals and you may also start to lose the diversity of the trees,” he warns.
More information: Julian. Cribb@nap.csiro.au
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