This is a must-read for all wildlife-friendly gardeners. Susan Harris of the Sustainable Gardening Blog has done a thorough job of researching all the ins and outs (as in "the worms go in and the worms go out) of composting with worms, including the vexing issue of whether vermicomposting contributes to the destruction of woodland ecosystems.
To summarize the high points: Earthworms change the soil. This is great if you grow vegetables and some conventional garden plants because earthworms produce the kind of soil these plants love. But it's not great if you're trying to protect a woodland ecosystem where earthworm activity has historically been very low. In woodlands, many plants rely on a thick layer of "duff" (organic material) in which to grow; high earthworm activity decomposes this duff too quickly, threatening the survival of many native woodland wildflowers.
As is so often the case, the species that present the greatest threat are not native to North America. Now, this much many of us already knew. But what Harris discovered in her research is that it is beginning to look as though the primary threat is one particular non-native: Lumbricus rubellus, aka the night crawler, the earthworm commonly used as bait. (The popularity of this worm as bait helps to explain why, unfortunately, it has been spread so widely.)
But there's good news for gardeners. According to Harris, the type of earthworm commonly used in vermicomposting does not present a threat to woodland ecosystems. That's because the red wiggler, Eisenia fetida, doesn't survive well in the wild.
Harris advises her readers to make sure they use only red wigglers in their composting efforts, purchasing them from reliable sources. My solution to this problem is simpler and cheaper: I use worms I collect in my own garden. That way I can't add any non-native species to the environment unless they were already there (and if they were already there, my composting efforts won't make any difference to the population). Of course, if you have no red wigglers in your garden, this may not work for you. But why not give it a try? The worst that can happen is that your initial vermicomposting effort will fail, and you will know that you have to purchase red wigglers.
Another good piece of advice Harris passes on in this article is to avoid dumping the contents of a worm bin in a wooded area. As a matter of fact, you should never dump any garden waste in any natural area.
Incidentally, ducks love to eat earthworms--especially those big, juicy nightcrawlers!