The Scientific Activist (Archives)


Jan 13, 2006

Amphibian Disease Heats Up

What do global warming and epidemic diseases have in common? Apparently they have a lot, at least when it comes to amphibians.

Microorganisms have a knack for showing up in unexpected places. In the 1980s, two scientists discovered a bacterium called Helicobacter pylori that causes over 80% of stomach ulcers, once thought to be primarily caused by stress. This turned medical dogma upside-down and earned them the 2005 Nobel Prize in Physiology or Medicine . Even microorganisms aren’t safe from other microorganisms, with bacteria, for example under constant barrage from viruses called bacteriophages.

A group of scientists led by J. Alan Pounds reported yesterday in Nature that recent extinctions of tropical toad species in Central and South America are caused by a fungal infection and, more importantly, the increased outbreaks of the disease are caused by global warming. These findings emphasize an important consequence of climate change: widespread extinction of animal species.

On purely scientific grounds, the study is impressive. It starts with an important question, overcomes a seeming paradox, presents extensive statistically analyzed data, and presents a compelling and far-reaching conclusion. The study examines the recent extinctions of species of Atelopus, also know as the harlequin frogs (even though they apparently belong to the toad family), which live in the American tropics. Amphibians in general lend themselves to this type of study because they are becoming extinct so quickly that habit loss, another major cause of extinction, cannot alone explain the phenomenon.

The study estimates that 67% of these toads have become extinct, and notes that 80% of these extinctions have taken place after exceptionally warm years. Based on previously published reports, the authors began with the hypothesis that the chytrid fungus (Batrachochytrium dendrobatidis), which grows on amphibian skin in a condition called chytridiomycosis, is causing the extinctions. This presented the researchers with a paradox:
[Chytrid] is associated with host mortality in highlands or during winter, and, according to theory, becomes more pathogenic at lower temperatures. Hence, the idea that it causes declines in warm years is paradoxical. Moreover, the fungus is apparently more lethal under moist conditions, yet, at many affected sites, warm years are comparatively dry.

In the end, though, this turned out to be the key to solving the problem. The authors statistically linked the extinctions to both increased temperatures, but they also demonstrated that the greatest concentration of extinctions took place at intermediate to high altitudes, where the effects of global warming are slightly contradictory. Think of Roland Emmerich’s The Day After Tomorrow—where global warming causes the majority of the northern hemisphere to become frozen and inhabitable because of changes to the Gulf Stream—but without the Hollywood sensationalism.

As the temperature at sea level increases due to global warming, increased moisture is carried up to the highlands, meeting one of the conditions for chytrid growth there. This increased moisture leads to greater cloud cover, which has a moderating effect on the temperature, increasing the nighttime low but decreasing the daytime high—in effect homing in on the optimal temperature for chytrid fungus growth. Under these conditions, the toads are no match for their microbial invaders.

This study isn’t just about amphibians though. The authors conclude their study with a warning:
We establish that global climate change is already causing the extinction of species. Taking our results and recent findings that tie the same losses to disease, we conclude that climate-driven epidemics are an immediate threat to biodiversity. Our study sheds light on the amphibian-decline mystery by showing that large-scale warming is a key factor. It also points to a chain of events whereby this warming may accelerate disease development by translating into local or microscale temperature shifts—increases and decreases—favourable to Batrachochytrium. The case illustrates how greenhouse warming and the resultant intensification of the hydrological cycle, together with aerosol pollution, may affect life on Earth. Influencing patterns of cloud formation, these agents alter the thermal, light and moisture environments of many organisms, changing ecological interactions and threatening species survival.

The take-home lesson is that global warming is real, and it has real effects—tearing down ecosystems, brick by brick, that took millions of years to evolve. More than that, though, it is insidious and dangerously subtle. Global warming isn’t going to make us drown in rising waters, die of heat exhaustion, or freeze to death, at least not anytime soon. In the meantime, it slowly exerts its effects, often under the radar and often in unexpected ways, much like the microbial invaders riding in its coattails. Luckily for them, as human industry continues to produce greenhouse gases, global warming doesn't show any signs of stopping.

These effects are only just now coming to light, and by the time the damage has become so unavoidable to spur large scale action and change, we can only hope that it’s not too late.


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