Alien invasion: non-native earthworms threaten ecosystems

An alien invasion capable of triggering catastrophic changes is underway across North America. At least 70 species of imported earthworms have colonized the continent and pose a largely overlooked threat to native ecosystems, according to a new study by researchers at Stanford University, Sorbonne University and others institutions.

The analysis, published February 8 in Ecology and evolution of natureprovides the largest database ever on these earthworms and warns of the need to better understand and manage the invaders among us.

“Earthworms tell the story of the Anthropocene, the era we live in,” said study lead author Elizabeth Hadly, the Paul S. and Billie Achilles Professor of Environmental Biology at Stanford. School of Humanities and Sciences. “It is the story of a global homogenization of biodiversity by humans, which often leads to the decline of unique local species and the disruption of native ecosystem processes.”

Friend or enemy?

Mostly invisible and largely unrecognized, earthworms are worth their weight in gold to farmers and gardeners because their movements create tunnels that allow air, water and nutrients to penetrate, while their waste serves as a rich fertilizer.

They also play a central role in many processes that impact aerial communities and the atmosphere. For example, although the mechanical movement of earthworms in the soil may initially release carbon dioxide, the long-term impacts of digestion of organic matter result in a net increase in sequestered carbon where earthworms are present.

Since the late 1800s, people seeking to profit from these services have brought earthworms to North America from Asia, Europe, South America, and Africa. In some places, these non-native introductions have succeeded in strengthening the agricultural economy. However, in other cases they have been detrimental. These transplants are more likely to consume aerial leaf litter than native earthworms, thereby altering habitat quality in ways that can harm native plants, amphibians, and insects.

In hardwood forests of the northern United States and Canada, the impact of exotic earthworms on the soil places stress on trees such as sugar maples by altering the microhabitat of their soils. This, in turn, triggers a series of impacts on the food web that promote the spread of invasive plants. Ironically, for a creature synonymous with soil improvement, some alien earthworms can alter soil properties such as nutrients, pH and texture, leading in particular to lower quality crops.

Exotic earthworms have a distinct advantage. Unlike the majority of our native species, many female species of exotic earthworms can produce offspring without fertilization by a male. Additionally, climate change is opening new niches for their colonization in northern regions of the continent, where permafrost is melting and which are devoid of native earthworms.

Understanding the Impacts of Extraterrestrial Earthworms

Despite all this, only a limited number of studies have documented the spread of exotic earthworms, and none have covered colonization dynamics on large spatial scales or across large numbers of species.

For their study, the researchers drew on thousands of records from 1891 to 2021 to create a database of native and alien earthworms, then combined it with a second database documenting earthworm interceptions from extraterrestrials on the US border between 1945 and 1975. With the help of machine learning, the team used the combined databases to reconstruct the supposed introduction and spread pathways of the alien earthworm species.

They found exotic earthworm species in 97% of soils surveyed in North America, with alien occupancy higher in the northern part of the continent and lower in the south and west. Overall, aliens make up 23% of the continent’s 308 earthworm species and 12 of the 13 most widespread earthworm species. For comparison, in the United States, only 8% of fish species, 6% of mammal species, and 2% of insects and arachnids are exotic.

In Canada, the proportion of exotic earthworms is three times that of native earthworms. In most of the 48 U.S. states and Mexico, there is about one exotic earthworm for every two native species.

“These ratios are likely to increase because human activities facilitate the development of exotic species that threaten native earthworm species, a phenomenon still largely neglected,” said the study’s lead author, Jérôme Mathieu. , associate professor of ecology at the Sorbonne, who led the research while a visiting professor in Hadly’s laboratory.

Not all alien earthworms will threaten native ecosystems. However, researchers say their wide distribution and unknown impact on a range of native ecosystems, such as grasslands and coniferous forests, deserve special attention. Among other solutions, they suggest policymakers focus on prevention, such as encouraging the use of native worms for composting and fishing bait, as well as early detection through regular monitoring and citizen science. .

By raising awareness of the mostly unknown dynamics of the introduction of exotic earthworms to North America, this study highlights the essential role they play in structuring ecosystems and in their function in our landscapes dominated by l ‘man.

“This is most likely the tip of the iceberg,” said study co-author John Warren Reynolds of the Oligochetology Laboratory and New Brunswick Museum in Canada. “Many other soil organisms may have been introduced, but we know very little about their impacts.”

Co-authors of the study also include Carlos Fragoso of Red de Biodiversidad y Sistemática, Instituto de Ecología AC, Mexico.