Earthworms, including the nightcrawler, have thinned forest leaf litter in northern areas. The reduction of this resource has changed the forest understory.

Some rare plants are endangered because of understory changes caused by earthworms. For example, one third of the populations of Minnesota’s endangered goblin fern have been extirpated because of nightcrawlers.

Nightcrawlers change the environment, allowing exotic plants like garlic mustard (Alliaria petiolata) to establish.

Scientists believe that earthworms can carry foot-and- mouth disease, a devastating livestock disease.