People who introduce worms to their gardens or compost piles should be careful to obtain native species. Where I live in northern Minnesota, we have no native species of worms. If they existed here before the ice ages, they were scraped away with the glaciation. Exotic worms are common, as is the devastation that occurs as they invade the forest. They are introduced here through the importation of planting stock and fish bait. As they move through the woods, they consume the complete litter layer (fallen leaves and twigs), exposing roots and killing young trees and shrubs. The vegetative diversity is drastically reduced. For more about worms, check out http://www.nrri.umn.edu/worms/
