Earthworms generally tend to be viewed as a friendly find in gardens and farm fields. The squirmy creatures can be hugely beneficial in their native habitats, helping to aerate soil, breakdown organic matter, and recycle nutrients. Non-native species, however, can wreak havoc on ecosystems as some parts of North America are unfortunately starting to discover.
A new study finds that 70 alien earthworm species have colonized the North American continent. In fact, these invaders account for 12 of the 13 most widespread earthworm species and now have larger geographical ranges than native species. The report authors say there is about one alien earthworm for every two native species across most of the lower 48 U.S. states and Mexico. In Canada, the proportion of alien earthworms was found to be three times greater than that of native species.
The problem is that some of these invasive earthworms have novel ecological functions, which can stress native plants, trees, and wildlife by altering soil properties and encouraging the spread of invasive plant species. According to Jerome Mathieu, an ecologist at the Sorbonne University and one of the paper’s authors, many native earthworms are “soil feeders,” meaning they mostly burrow and feed on organic matter underground. Many of the introduced species, on the other hand, are “litter feeders,” meaning they eat through the layer of fallen leaves and other material that gathers on the surface.
Litter feeder species can rapidly remove the forest floor when it invades and is associated with big changes in the forest soils and plant communities. Some of these worms can alter soil properties such as nutrients, pH, and texture, leading to poorer quality crops, among other impacts.
One particularly destructive species, the “Asian Jumping Earthworm,” has been spreading rapidly across the US. They can be found in the Southeast, along the Eastern Seaboard, and in the mid-Atlantic, Midwest, and some Northwestern states. Asian jumping worms devour organic matter more rapidly than their native counterparts, stripping the forest of the layer critical for seedlings and wildflowers. They also can severely damage the roots of plants. Jumping worms also grow twice as fast, reproduce more quickly, and can infest soils at high densities.
The biggest impact of non-native earthworms has been documented in more northern ecosystems. That’s because of the last Ice Age some 20,000 years ago that covered the continent from New York City through Montana northward in a blanket of ice. Over the centuries, ecosystems south of the so-called “glaciation boundary” all adapted to include earthworms, but ecosystems north of that line haven’t historically had them. In 2017, researchers at Michigan Technological University linked the spread of non-native earthworms with crown dieback in sugar maples in the Upper Midwest.
The primary mode of distribution for alien earthworms is through human activities such as trading them as fish bait or for vermicomposting. According to the study, Canada alone annually exports over 500 million alien earthworms to various countries, particularly the US, contributing to the spread of these invasive species.
Mathieu warns that the current situation is likely just the beginning. “These ratios are likely to increase because human activities facilitate the development of alien species that threaten native earthworm species, a phenomenon still largely overlooked,” he said.
The researchers note that not all alien earthworms pose a threat to native ecosystems. However, their large distribution and unknown impact on a range of native ecosystems, such as grasslands and conifer forests, make them worth further research. Among other solutions, they suggest policymakers focus on prevention, such as encouraging the use of native worms for composting and fishing bait, and early detection through regular monitoring and citizen science. (Sources: Nature, Ecological Society of America, Stanford University)
