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Mercury's silent toll on the world's wildlife

By Rebecca Kessler - posted Monday, 25 February 2013


Frederick discovered a remarkable example in white ibises from the Everglades. There, mercury levels are low but constant, and ibises seem to nest less and abandon their nests more often than elsewhere. To see if chronic mercury exposure was responsible, Frederick captured 160 ibis nestlings and fed them food with mercury levels similar to their wild fish prey. He and his team observed the birds for three years to see if it affected their breeding behavior.

As expected, the dosed birds produced far fewer offspring than undosed controls. There were the usual reasons: eggs didn't hatch and chicks died under lousy parenting. But Frederick was wholly surprised to see widespread homosexual pairing among the dosed males and to find this caused much of the reproductive deficit. Avian homosexuality usually occurs with stark sex imbalances - which wasn't the case here, Frederick says.

No one had ever reported homosexuality as an effect of mercury, or any other contaminant for that matter, Frederick says. Moreover, the effects appeared in ibises he'd fed as little as 0.05 ppm of mercury in their food - one-tenth of what Heinz fed his mallards. Further work indicated that hormonal changes wrought by mercury's effects on the ibises' endocrine systems were at work. In a 2011 paper, Frederick and a colleague estimated that out in the Everglades, mercury could cut the number of ibis fledglings by half - easily enough to curtail the population.

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No one has checked wild ibises for poor parental behavior or homosexuality, which might lay the blame more squarely on mercury, he says. (Different species react to mercury differently, and Frederick stresses that for many reasons his results in no way suggest that mercury might play a role in human homosexuality.) Nevertheless, the broader implications for chronically exposed wildlife are chilling. "We can be essentially neutering populations by cutting off reproduction through the endocrine system," he says. "This could easily be going on in the wild with many kinds of contaminants. Mercury is not the only endocrine disruptor."

Like Frederick's study, much of the research on mercury's sublethal effects has been conducted on captive animals. In nature, it's very difficult to get the large sample sizes and control groups needed to identify subtle differences statistically, says Erick Greene, a conservation biologist at the University of Montana.

Studying ospreys living near Montana's polluted Clark Fork River, Greene and two colleagues found that about half the eggs laid by high-mercury birds fail to hatch. But they've been puzzled as to whether the surviving chicks are affected. In humans, blood levels around .005 ppm can cause cognitive deficits, Greene says. But his osprey chicks commonly have levels 100 - and even 1,000 - times higher. The chicks seem to do fine in the nest, he says. "They may look all right, but I don't know if I would recognize a mentally impaired osprey chick."

Once they're fledged they soon migrate south, out of sight. Greene suspects they may have trouble making the demanding migration to Central or South America (where mercury flows freely in small gold mining operations), or just figuring out how to survive on their own. His team has begun outfitting fledglings with satellite transmitters to determine how far mercury-loaded birds get compared to their normal peers, and how long they live.

It's one thing to show that wild animals are exposed to harmful levels of mercury, but solid evidence that whole populations are harmed is harder to come by, experts say. A notable exception is loons. Evers and more than a dozen colleagues amassed an impressive 18-year data set of nearly 5,500 mercury measurements from loons on 700 lakes across 17 U.S. states and Canadian provinces. They showed that when mercury in loon blood hits 3 ppm, the number of young fledged drops by 41 percent - and that enough loons are affected to set back some New Hampshire and Maine populations.

In a forthcoming paper, Hopkins and another researcher go a step further with a population model they developed based on four years of field data on American toads. Toads readily move between small populations scattered throughout the landscape. Mercury exposure can kill eggs and tadpoles, and survivors are often small and slow to mature. The model revealed that not only can mercury kill enough tadpoles to wipe out small populations, but that nearby uncontaminated populations can also drop or go extinct because there are too few toads around to replenish them if their numbers happen to dip for other reasons. Hopkins says he thinks the paper will change biologists' understanding of contaminants. "Contaminant effects in one population can actually affect adjacent populations that aren't being exposed to that contaminant," he says.

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Whatever its weaknesses, the new treaty represents a "great step forward," says Evers, and the good news is that once local sources are controlled, mercury in nearby wildlife can drop quickly. The bad news is that mercury from coal burning can travel great distances - for instance, from China to North America - before settling.

Overall, Evers says the forecast for wildlife is cloudy. When it comes to mercury, "the more we look the more we find, and the more we find the lower that toxicity level is going," he says. "Right now at a global level, mercury is just being released more and more in the system. Those trend lines are going in the wrong directions."

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This article was first published on Yale Environment 360.



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About the Author

Rebecca Kessler is a freelance science and environmental journalist based in Providence, Rhode Island.

Creative Commons LicenseThis work is licensed under a Creative Commons License.

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