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Mercury's silent toll on the world's wildlife

By Rebecca Kessler - posted Monday, 25 February 2013


This month, delegates from over 140 countries gathered in Geneva and finalized the first international treaty to reduce emissions of mercury. The treaty - four years in the works and scheduled for signing in October - aims to protect human health from this very serious neurotoxin.

But barely considered during the long deliberations, according to those involved in the treaty process, was the harm that mercury inflicts on wildlife. While mercury doesn't kill many animals outright, it can put a deep dent in reproduction, says David Evers, chief scientist at the Biodiversity Research Institute (BRI), who serves on a scientific committee informing the process. "It is a bit of a silent threat, where you have to kind of add up what was lost through studies and demographic models."

Harmful levels of mercury have turned up in all sorts of animals, from fish and birds living around the world to pythons invading the Florida Everglades and polar bears roaming far from any sources of pollution. In recent years, biologists have been tracking mercury's footprints in unexpected habitats and species. Their research is illuminating the subtle effects of chronic exposure and is showing that ever-lower levels cause harm.

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Coal burning, gold mining, and other human activities release mercury into water bodies or the atmosphere, where it can travel great distances before settling back to Earth. Mercury contamination is ubiquitous and hotspots are common around the world, with fish and human hair collected in 14 countries regularly exceeding U.S. Environmental Protection Agency (EPA) standards, according to a BRI report released just before the Geneva negotiations. And while mercury emissions are declining in North America and Europe they are rising quickly in the developing world, according to the United Nations Environment Programme, the treaty coordinator.

The new global treaty bans the production, import, and export of certain mercury-containing products, requires governments to create plans to reduce mercury in small gold mining operations, and puts some controls on industrial facilities - but some environmental groups warn that it is too weak. The U.S. is going further. On January 1, an export ban on elemental mercury took effect, and the EPA is finalizing new limits on coal plant emissions.

"In the end the treaty will reduce mercury that's being released into the environment. And I think the question will be, as we move along, 'Is it enough?' - especially for areas that are sensitive to mercury input. And then 'Is it enough for wildlife conservation purposes?' which really wasn't addressed all that well," Evers says.

Exposed animals have trouble ridding their bodies of mercury, and it accumulates in tissue with every link in the food chain. Long-lived predators tend to carry the heaviest loads. Research and public attention have largely focused on contaminated fish, the main route of human exposure. In water, mercury converts quickly to methylmercury, its most toxic and bioavailable form, so for many years wildlife biologists trained their sights on aquatic, fish-eating birds and mammals, says Bill Hopkins, a Virginia Tech physiological ecologist.

Lately, though, Hopkins and others have uncovered mercury in reptiles, amphibians, insects, spiders, terrestrial songbirds, and a wider variety of mammals than expected. "All these different groups can be exposed to mercury and pass it on to their babies," says Hopkins.

Mercury is also turning up in strange places, he says. Invertebrate-eating songbirds living in the floodplain bordering a contaminated Virginia riverhad as much mercury in their blood as the river's fish-eating birds, and sometimes more, showing that mercury pollution doesn't stay put in aquatic habitats. Scientistshave found mercury-laden food chains in mountainous forests, and shown that methylmercury forms in the woods, as well as in water. BRI scientists and collaborators discovered high levels in many invertebrate-eating songbird and bat species living in varied habitats across the U.S. Northeast and Mid-Atlantic states, including remote uplands. The pollutant has also emerged as a serious problem in the Arctic.

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Mercury plays havoc on vertebrates' development and their neurological and hormonal systems, and doses too low to kill can cause problems that aren't always obvious in the wild, experts say. "Methylmercury is one of most toxic environmental pollutants we've ever come upon," says Gary Heinz, a recently retired federal wildlife biologist who studied it over four decades.

In the earliest studies of these sublethal effects in the 1970s, Heinz reported that captive mallards fed mercury-laced food laid fewer eggs than control ducks and laid them outside the nest. Also, their ducklings didn't respond well to their calls. Numerous examples have accumulated since. Fish form loose, sloppy schools and are slow to respond to a simulated predator. Several bird species sing different songs. Loons lay smaller eggs, and they incubate their nests, forage, and feed their chicks less. Salamanders are sluggish and less responsive to prey, Hopkins and colleagues found. Egret chicks are similarly lethargic and unmotivated to hunt.

Changes like these could be grave for wild animals, says Peter Frederick, a University of Florida ecologist who was part of the egret study. "Getting lunch or a mate depends on milliseconds and millimeters. You have to perform that courtship dance just right. You have to make the calls just right. You have to stab your prey to within a millimeter. If you're off by a microsecond, it's gone," he says.

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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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