No one would argue that earthworms are cute. But to most people, they are benign and helpful creatures — fertilizers of the garden, aerators of the soil, indispensable fishing companions. "Earthworms are truly nature's little farmers," goes one common view, this one on the Web site of Agriculture and Agri-Food Canada, "plowing the soil and fertilizing at the same time!" But for a growing body of researchers, the traditional view of earthworms is giving way to a much more ominous one. Most earthworms common in the United States are exotic intruders from Europe, Asia or South America, these scientists point...