Are tropical species intensely vulnerable to global warming?

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From William Laurance at www.alert-conservation.org

On 12 January 2002, Cambridge University doctoral student Justin Welbergen was studying the behavior of a large colony of flying foxes in subtropical eastern Australia.  What he witnessed that day shocked him.

Doesn't like the heat… a grey-headed flying fox.

Doesn’t like the heat… a grey-headed flying fox.

It was a hot afternoon, and as the thermostat climbed above 40 degrees Centigrade, the giant bats became obviously distressed.  They began fighting over shady spots in the canopy.  Then they began licking their wrists and flapping their wings in a desperate effort to cool themselves.

Finally, as the temperature hit 42 degrees C, they began to die — in the thousands.  On that day at least 3,500 bats died, in nine different nearby colonies.  Females and juveniles were especially vulnerable.

What Welbergen observed was a phenomenon that has now been seen elsewhere — from mass disappearances of lizards in Mexico to the dramatic population collapse of the white lemuroid possum in north Queensland rainforests.

There are two striking conclusions from these observations.  First, to the surprise of many, tropical species may be the most vulnerable of all organisms on the planet to global warming.  Second, it isn’t a steadily rising thermostat that endangers most species, but short, intense pulses of unusually warm conditions — heat waves.

Why are tropical species so vulnerable?  In short, many are thermal specialists.  Think, for instance, about a polar bear — our traditional icon for global warming.  It has to deal with temperatures ranging from, say, minus 50 degrees C in winter to plus 35 degrees C in summer — a huge range of temperatures.

But tropical species are different.  Lowland tropical species, for instance, might see temperatures ranging from just 25 to 35 degrees during the course of a normal year — a far narrower range.  As a result, they can become much more thermally specialized.

Where temperatures vary the most in the tropics is as a function of elevation.  On average, for every thousand meters that one goes up in elevation, the temperature drops by 6 degrees C.

What that means is that tropical species are not just thermal specialists, they also tend to be elevational specialists.  Species tend to be adapted for the very warm lowlands, or for the cooler mid-elevations, or for the wet, cloudy high elevations, where conditions are almost chilly.

And it’s the high-elevation specialists — such as the white lemuroid possum — that a lot of scientists are really worried about.

Many tend to be locally endemic species, because their populations are genetically isolated from other populations on different mountaintops.  Hence, they have small geographic ranges and, often, small population sizes.

And they may be intensely vulnerable to global warming.  As temperatures rise, the geographic ranges of many high-elevation species in the tropics are predicted to shrink and fragment — potentially disappearing altogether.  For instance, in the Australian wet tropics, most upland-endemic species of mammals, birds, reptiles, and amphibians are predicted to disappear entirely if temperatures should rise by more than 4-6 degrees C.

It’s a frightening prospect, and it suggests that global warming could have far wider-reaching impacts than many might suspect — especially in the tropics, the world’s biologically richest real estate.

ALERT member Pierre-Michel Forget has just given a wonderful 30-minute interview on this topic.  Forget is a highly authoritative scientist — a former president of the Association for Tropical Biology and Conservation and now Vice-President of the Society for Tropical Ecology.  He asks, what would just a 1 degree C increase in temperature do to tropical forests and their species?

It’s definitely worth a half-hour to hear this enlightening lecture — and to share it with your colleagues and students.

The bottom line is this: Given that tropical ecosystems are so rich in species and thermal specialists, the best icon for global warming might not be a polar bear — but a tropical white possum or flying fox.