Think of earthworms and a few things come to mind: they make great bait for fishing, they aerate the soil, and they’re an excellent addition to a compost pile. But, what a lot of people don’t know is many earthworms are actually invasive species. Earthworms may be small but when they take over a forest, the impact is dramatic. They cause the rapid incorporation of organic material into the soil, changing its structure, chemistry and nutrient dynamics. What's known as the duff layer is suddenly removed, and this duff, or decaying organic material on the forest floor, is habitat for several species of insects, spiders, small vertebrates, bacteria and fungi. It is also the primary rooting zone for most plants.