The Way of the Possum

What One Animal Can Teach Us About Global Warming

The white possum forages high in the forest canopy.© Mike trenerry/Wet TropicS Management Authority

For the denizens of a tropical cloud forest, the perils of life soon become apparent high up on the escarpment of Mount Lewis, an ancient granite massif that soars 4,000 feet above mangrove swamps and the Coral Sea. Near the summit, gnarled trees rake through swirling mists. Antedeluvian rainforests like this once sprawled across some two-thirds of the Australian continent. Today, these biodiversity hotspots are mere relicts.

Mount Lewis—where 20-foot-long pythons lie in ambush for tree-kangaroos—has begun to change abruptly. One creature there—the white possum (technically, the northern lemuroid ringtail)—is on the verge of extinction. Its looming fate warns of an environmental crisis just over the horizon.

In an era of record-breaking temperatures—12 of the 14 hottest years in recorded history have occurred since 1995—that a species might disappear from global warming is not surprising. What is unexpected is the nature of this beast: It is an inhabitant of a balmy rainforest, an ecosystem that, unlike the frozen poles or frigid boreal regions, hardly seems vulnerable to rising temperatures.

Among field biologists, the white possum is famously easy to detect. Strikingly pallid in color, the animals forage high in the forest canopy, freeze in the spotlight and have a stunningly bright eyeshine. While the southern possum—the white possum’s close cousin—ventures down some 2,000 feet, the northern variety is confined to the coolest, wettest forests near the summit of Mount Lewis. As a consequence, the animal seems exceptionally vulnerable to the vicissitudes of climate.

Why are tropical species so acutely sensitive to temperature? The prevailing explanation, first proposed in 1967 by biologist Daniel Janzen, is deceptively simple. Imagine a polar bear living in the Arctic. In winter, it faces conditions so glacially cold it must hibernate to survive, whereas in summer the thermometer might climb past 50 degrees. Tropical species, however, experience just a tiny fraction of this thermal variability (typically, they encounter a greater temperature difference between night and day than among any month during the year). Hence, they often become remarkably fine-tuned to their thermal environment.

This is why elevation is so important in the tropics: Lowland species often become specialized for the hottest conditions, mid-elevation species for balmy weather and montane species for the coolest climes. In the tropics, high-elevation specialists are effectively trapped on sky-islands.

As the world gets hotter—the Intergovernmental Panel on Climate Change expects a rise of between 2.5 and 10.4 degrees Fahrenheit by the end of this century—tropical-latitude species, especially those requiring cool weather, will increasingly suffer. Among the leading scholars of this crisis-in-the-making is Stephen Williams, director of the Centre for Biodiversity and Climate Change Research at James Cook University.

Combining years of field data with his computer models, Williams is attempting to predict the fate of 65 species of locally endemic mammals, birds, reptiles and frogs in the north Queensland rainforests. The end-product is a graph that shows the number of species expected to vanish as a function of rising temperature. Beyond about four degrees Fahrenheit, his extinction curve shoots abruptly upward, indicating that many species are expected to disappear. After eight or nine degrees of warming the curve flattens out again, because most of the endemic species will already have vanished.

If Williams" predictions are right, the white possum—whose final demise would likely come as a result of dehydration, as the cloud base providing the animal’s water source vanishes—is a portent of far more catastrophic biological impoverishment to come. Except the white possum hasn’t disappeared—not yet. Williams and his colleagues found three of the animals living in a remote pocket of Mount Lewis in March 2009. But those three—the only known living members—cling to survival.

If the white possum truly vanishes, what will we have lost? Australian biologists have revealed large genetic differences—clearly meriting species status—in other possums, birds, lizards and frogs living in isolated upland distributions in tropical Queensland. And this pattern—in which many species are confined to higher elevations, sometimes to a single mountaintop—is repeated across the tropics. In a color-coded map in which reds and yellows denote hotbeds of endemic species, the world’s tropical mountains appear to be on fire.

It is not the coldest places on Earth that are most vulnerable to rising temperatures. Rather, it is these epicenters of biodiversity that will suffer the most. Further, it is not the progressive rise in the thermometer that one needs to worry about. It’s the peaks and valleys—the extreme temperatures, droughts, deluges and storms—that will create the greatest perils.

WILLIAM LAURANCE, Ph.D. is a senior scientist at the Smithsonian Tropical Research Institute in Panama and the author of several books, including Emerging Threats to Tropical Forests (University of Chicago Press).