Earthworms--an environmental problem?

I REALLY did not want to know that earthworms are endangering forest ecosystems in Pennsylvania, New York, and Maine. All my life, when I thought about earthworms at all, I thought of them as helpful little beings, busily aerating the soil, converting organic matter into worm castings, and fertilizing plants so I don't have to. They do those things; it just turns out that that's not all they do.

I did know (vaguely) that most earthworms are not native to North America, having been frozen out by the glaciers fifteen thousand or so years ago, save a few found in the South. Reading Charles C. Mann's 1493: Uncovering the World Columbus Created, this afternoon, I learned that some ecological historians credit (or blame, depending on your perspective) John Rolfe with introducing earthworms in quantity to our continent. Yes, the same John Rolfe who married Pocahontas and took her to England, where she promptly fell ill and died (as his first wife did when he took her to Virginia--the man did not have good luck with women). It seems that he was the first Englishman to have success with tobacco as a cash crop (yes, we can thank him for that, too) and used soil as ballast in the barrels that transported Virginia tobacco to England. When the ships landed in Virginia, they dumped English soil and English earthworms.

So why is this a problem? Most of eastern North America was forested, as much of it still is, and forests create lots of leaf litter. (Think about all the leaves raked in the fall. Pre-European settlement, those leaves tended to stay on the ground for years.) According to Dennis Burton, Director of Land Restoration at The Schuylkill Center for Environmental Education, healthy forests need their leaf litter to stay put for a while, cooling root zones, providing a safe haven for overwintering insect eggs and pupae, and decomposing slowly, gradually returning nutrients to the soil. Earthworms disrupt this cycle, churning the soil and breaking the litter down much faster than our native detritivores (and isn't that a great word?) would. Weeds sprout, and plants that need a stable forest floor (like some of our favorite spring ephemerals) decline. Without the protective cover of leaf litter, some insect species decline, leading to a decline in the number of birds. All because of feral EARTHWORMS, for goodness' sake!

Just what I needed--something else to worry about.