Last year, on a night walk in Inwood Park, our guide said that earthworms were slowly transforming, indeed, destroying, our northern hardwood forests. Whoa! I’d never heard that before and wanted to look into it. After all, earthworms are the gardener’s and the composter’s friend, right? Hasn’t that been drummed into our heads for years? (Actually, a surprisingly number of things we’ve always been told turn out to be bunk.) Well, I never did look into it, and time, which sweeps by with terrible speed, hurried on. But thanks to a recent tweet from the Flatbush Gardener, the issue hit my consciousness again. Here’s the article he sent ‘round. Here are a couple more sources: University of Minnesota’s Great Lake’s Worm Watch and Tree Hugger.

Worms are good for gardens and they do make great compost. But they are really, really bad for the hardwood forests. Why? The last glacial advance of the Pleistocene scraped away all the soil and virtually all lifeforms, including native worms. (I type this on the Harbor Hill Moraine, which stretches east to Cape Cod and, in the other direction, into the Midwest; it’s made up of all that plowed rubble the last pulse of the Wisconsin ice sheet dumped.) The northern forests that developed in the wake of the ice did so without earthworms. Further south, where the ice didn’t reach, there are native worms. Our earthworms are aliens, brought here by the Europeans. New species continue to arrive, no thanks to fishermen dumping their excess bait.

What these non-natives do is chow down on the “duff,” the ground-cover spongy layer of fallen leaves so vital to protecting seedlings and feeding the understory. In a wormless forest, bacteria and fungi break this stuff down slowly, releasing nutrients for others. That’s great for trees, understory plants, and, further up the web of life, all the animals that depend on these. But worms plow through the stuff, almost, one might hazard, like a glacier, leaving little for the future of the forest, except other invasive species.

You know what the pictures of before earthworms and after on the Minnesota site remind me of ? When I first moved to Brooklyn in the 1993, the woods in Prospect Park were in pretty bad shape. There were trees, yes, and some were spectacular specimens, but there was virtually no understory. There were, in short, no future trees. The soil was compacted from abuse and neglect and idiotic management practices. It was ugly and mean looking, barren. They spent a decade and some ten million dollars getting it back in shape. That’s why the fences are in the Ravines now, although plenty of dickheads aren’t detoured by those.

But the fences don’t stop worms. What do you want to bet the nightcrawler population of the park is astronomical? Hell, I get them in the Back 40, where they have to crawl yards of concrete to find some soil.

UPDATE 2/29/12: Just reading this article about a study in the northern Midwest, where the Ovenbird decline is linked to these invasive earthworms. As ground-nesters and ground-level eaters, the Ovenbirds depend on leaf litter, too.

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