Earthworms keep house: these ecosystem engineers collect and plant ragweed

Science News, July 19, 2008 by Rachel Ehrenberg

Unlike Richard Scarry's Lowly Worm, real worms don't drive cars or go to school. But the wriggly creatures appear to live a more purposeful life than previously thought. Earthworms deliberately gather and bury ragweed seeds from around their burrows, reports a new study in the Journal of Applied Ecology.

The findings fit with recent work documenting how nonnative earthworms are changing northern U.S. forests. Though native worms were wiped out from the northern United States in the last glaciation--only persisting south of the ice sheet and permafrost--European worms arrived with settlers. The worms are slowly changing deciduous forests by eating through the leaf litter and "duff" that native plants need to thrive.

"Worms do a great job in gardens, it's true," comments Cindy Hale of the University of Minnesota Duluth. "But take the same organism and put it in a native hardwood forest that's evolved over 10,000 years earthworm-free, and the worms change everything about the ecosystem."

In the study, seeds that the worms buried grew into the healthiest plants, suggesting that the crawlers' activity could help not only ragweed thrive, but perhaps also help invasive plants gain a foothold in new territory, Hale says.

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Weed ecologist Emilie Regnier of Ohio State University in Columbus and her colleagues conducted field experiments to determine how exotic European night crawlers, Lumbricus terrestris, affected the survival of the seeds of Ambrosia trifida, giant ragweed.

In addition to its prowess as an allergen, ragweed is a major weed of soybean fields and cornfields in the Midwest, Regnier says. This fact has puzzled scientists because ragweed seeds are usually quickly eaten by birds, rodents and beetles.

Worms collected and buried more than 90 percent of ragweed seeds from the surface of the soil around their burrows, the team reports. There were six times as many seeds in the worms' burrows as in the surrounding soil, and after one season, there was an average of 127 seeds per burrow. These night crawlers buried some seeds as deep as 22 centimeters.

The work enhances understanding of plant-animal interactions, Regnier says.

"We were astonished by how quickly the seeds were removed," she says. "We think of ants and mice and squirrels as being very important in dispersing seeds. Here's a new mechanism--they are burying them quite deliberately."

On their own, nonnative worms probably spread only 10 meters a year, but they move faster with human help. Leftover fishing bait should be thrown in the trash, Hale says, not dumped in the dirt. It's likely that the worms will keep moving west--not in a car with Lowly Worm--but with humans, the same way they arrived.