Wednesday, September 21, 2016

Mercury contamination in fire ants

In a new paper, just out in Insectes Sociaux, my collaborator Brent Tweedy and I did something no one had ever thought to do—we looked for mercury in ants.

Mercury contamination is a serious environmental and health concern in the US and other countries that rely on coal-fired power plants.  Government agencies routinely check for mercury in drinking water and in other situations that might lead to its ingestion by humans.   When looking for environmental mercury contamination, it might seem that ants—one of the most conspicuous animals on the planet— would be an ideal place to start.

But it turns out that mercury contamination is largely an aquatic issue.  Mercury comes in many forms, some of which are relatively harmless.  The dangerous version of mercury—the one that can kill or debilitate humans and wildlife—is an organic form known as methylmercury.  And methylmercury is produced by microorganisms that live in water.  Once produced, methylmercury moves around in water bodies and builds up in the species that inhabit them, but doesn’t easily make the jump onto dry land.

So while we routinely monitor mercury in wild fish to determine whether they can be safely eaten by humans, we usually ignore it in land animals.  Ants, regardless of how abundant and pervasive they are, are blithely overlooked where mercury is concerned.

The error in this approach—water is potentially toxic, but land environments are safe—is of course that land animals eat aquatic animals.  The living world is stitched together by food webs, and when one animal eats another it inherits the methylmercury of its prey.  In this way mercury can move into terrestrial food webs when aquatic insects get eaten by predators like spiders and birds.

Ants that live near water bodies may likewise prey on aquatic insects, or scavenge dead ones that wash ashore.  Colonies have long lifespans and forage over large territories, so they could potentially accumulate a lot of mercury.  To test this idea, we measured the mercury contents of four ant species eaten by Purple Martins in southern Oklahoma.  To ensure we studied individuals that actually ended up as prey, we took flying queens and males directly from the mouths of hunting birds.

Most species had only low levels of mercury typical of land animals.  But one—the invasive Red Imported Fire Ant—had mercury contents similar to aquatic insects living in contaminated reservoirs.  The males had especially high concentrations and contained 50% more mercury than queens.

Fire ants (Solenopsis invicta), especially the males, contain more mercury than other ant species eaten by Purple Martins.  Species with different letters above them have different mercury contents.

We don’t know why fire ants contain more mercury than other species.  It may be because they tend to nest in wet areas like lakeshores, marshes, and watered lawns, and are thus more likely to feed on aquatic prey.  The results are unsettling regardless.

Fire ant queens and males can fly several kilometers away from their birth colonies and are major prey for flying predators.  Purple Martins alone eat billions of fire ants a year.  Fire ants may thus be major vehicles for the transfer of mercury among food webs.  Colonies concentrate mercury from throughout their territories and pack it into winged queens and males that disperse into the atmosphere.  There they are eaten by predators, passing their mercury burdens onto other species.

More work is needed to determine whether the mercury concentrations are high enough to harm birds or other predators, and to figure out exactly how the mercury gets into fire ant colonies.

For now, the results provide just another example of how much we still have to learn about ants and what they do in the skies above our heads.

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