When farmers’ friend turns forests’ foe

If you have a garden, you know how welcome earthworms are: they nibble on all the decaying litter and break them up into nutrient-rich chunks that make it easier for plants to absorb and grow. They also help aerate the soil as they dig around, thus helping the roots penetrate further down. The result? A bonanza of flowers, vegetables and fruits in the garden.

But earthworms aren’t harmless when they are in places they aren’t supposed to be. Of the 6,000 plus species of earthworms in the world, over 120 are widely distributed and have become invasive species in parts of the world, particularly in North America. Since the 1600s, European settlers who came to this part of the world brought along many invasive species, including the ‘helpful’ earthworms, most of them belonging to the genus Lumbricus. The lack of local earthworms may have also played a role.

More than 10,000 years ago, the glaciers from the Pleistocene Age (also called the Ice Age) wiped off many native species of earthworms in North America. The remaining few went underground deep into the layers of the soil as invading species began establishing their colonies in recent centuries. Now, most earthworms on the continent have become a problem for the forests that house some of the oldest trees on the planet.

As earthworms chew up leaf litter on the forest floor, they enrich the soil with nutrients, thus disturbing the soil composition that native plants have adapted to grow in. As a result, many non-native plants thrive on the enriched soil, stifling other native plants. Over the years, scientists say earthworms have resulted in many native orchids and birds being endangered. As younger forests are full of leaf litter, they have more earthworms than older forests.

The story of the farmer-friendly earthworms shows the concerns with invasive species: they may be super helpful on their native land but when they are introduced to other places, they often always have unexpected consequences, sometimes affecting the whole ecosystem.