Wait, some earthworms are BAD?

European earthworms are destroying northern American forests.

Here's the story: Native American earthworms were pushed south by the last ice age, and haven't moved back up into the northern US and Canada. In other words (I never knew this) in the Northern US, there are no native earthworms. It is (or was) a worm-free zone. European worms escaped from those used as fishing bait and home worm composting are moving into these northern forests and changing the environment. Unlike native decomposers which slowly break down leaves, making a thick layer of partially decomposed organic matter called duff, these exotic earthworms gobble it all down in no time, leaving exposed soil which dries out faster, killing native plants, changing soil chemistry and so changing the populations of symbiotic fungi that native trees depend on, and even making the forests more suitable for invasive species like buckthorn and garlic mustard.
While it is too late for wild areas already infested, worms spread very slowly on their own, and humans discarding worms used for composting or fishing are the main way they expand their territory. Spread the word! I don't think enough people know about this.

Read more here and here and if you want to get more technical, here.
Photo from crocknroll on flickr.