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Ecosystem engineers

Lowly earthworms keep house, sequester seeds

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3:13pm, June 27, 2008
Magazine issue: Vol. 174 #2, July 19, 2008
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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 U.S. northern 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 then arrived with settlers. The newcomers are slowly changing northern 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. The physiology, the chemistry — they have a profound effect on nutrient cycling.”

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.

“They might be priming the pump for successful germination,” she adds.

Led by weed ecologist Emilie Regnier of OhioStateUniversity in Columbus, researchers 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 powers 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.

“The burrow is an environment that the worm is actively maintaining — that’s its universe,” comments soil and ecosystem ecologist Patrick Bohlen, director of the MacArthurAgro-ecologyResearchCenter in Lake Placid, Fla. “Maybe it’s sweeping its front porch. We don’t really know. There isn’t a lot of evidence that they are eating the seeds, but clearly it’s creating an architecture.”

“You might think of earthworms just burrowing around — the intestines of the earth,” he adds. “But the worm is living there 365 days a year.”

Experiments by Regnier’s team revealed that the night crawlers buried ragweed seeds as deep as 22 cm. There were six times as many seeds in the worms’ burrows as in the surrounding soil. After one season, there was an average of 127 seeds per burrow.

“We were astonished by how quickly the seeds were removed,” Regnier says. Seeds that were too large for the worms to pull underground were dragged to the worm’s midden, the little pile of debris that marks the burrow’s front door. Researchers aren’t sure what these middens are for. They are usually made of worm castings, shreds of leaves and grass, but the worms will also add nonedibles, such as stones or old shards of tile.

The work enhances our understanding of plant-animal interactions, Regnier says. “We think of ants and mice and squirrels as being very important in dispersing seeds,” she says. “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.

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