The phrase “canary in a coal mine” is a shorthand expression that indicates an early warning of an environmental hazard.
For instance, recently, I read that frogs in wetlands are analogous to “canaries in coal mines.” That’s because their thin skins absorb toxins that might not affect other species as quickly. When we start seeing declines or diseases in frog populations, we should be concerned about what is causing the problem and whom it will affect next.
The origin of the expression is literal. Canaries were the original carbon monoxide sensors. After a fire or an explosion, a canary in a small cage would be brought into a mine. If it showed any signs of distress, the mine was considered unsafe for humans.
In these examples, frogs and canaries are known as a sentinel species. They are sensitive to toxins and other hazards and therefore provide a harbinger of trouble ahead. For instance mussels and other bivalves are sentinel species because, as filter feeders, they can accumulate heavy metals from sediments in their tissues.
A long-running scientific effort called Mussel Watch measures contaminants in the tissue of mussels collected from sites on the Great Lakes, Alaska, Hawaii, and Puerto Rico. In this way, scientists can monitor the health of vast bodies of water.
It’s not practical to monitor deep oceans or remote forests for every potential hazard. But the absence or decline of a single species can alert us to a problem that deserves closer attention. When we identify sentinel species in an ecosystem, we have a great way of keeping a finger on the pulse of the ecosystem’s health.
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Photo, taken on June 10, 2012, courtesy of Paukrus via Flickr.
Earth Wise is a production of WAMC Northeast Public Radio. Support for Earth Wise comes from the Cary Institute of Ecosystem Studies in Millbrook, NY.