At least 70 species of imported earthworms have colonized large areas of North America and represent a threat largely overlooked for native ecosystems.
An analysis, published in Nature Ecology & Evolution led by the universities of Stanford and the Sorbonne, provides the database largest ever carried out on this type of worms.
And he warns about the need to better understand and manage the invaders among us.
“Earthworms tell the story of the Anthropocene, the era we live in,” the study’s lead author, Elizabeth Hadly, a professor of Environmental Biology at the Stanford School of Humanities and Sciences, said in a statement.
“It is a story of global homogenization of biodiversity by humans, often leading to the decline of unique local species and disruption of native ecosystem processes,” he said.
Mostly invisible and underappreciated, earthworms are worth their weight in gold to farmers and gardeners because their movement creates tunnels that allow air, water and nutrients to penetrate, while their waste serves as rich fertilizer.
They also play a central role in many processes that impact surface communities and the atmosphere.
For example, although the mechanical movement of them through the soil may initially release carbon dioxide, the long-term impacts of the digestion of organic material result in a net increase in carbon. kidnaped where these “worms” are present.
They have come from different continents
Since the late 19th century, 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 successfully improved the agricultural economy.
However, in other cases they have been harmful. These transplants are more likely to consume aerial leaf litter than native earthworms, altering habitat quality in a way that can harm native plants, amphibians, and insects.
In the broadleaf forests of the northern United States and Canada, the impact of exotic earthworms on the soil stresses trees such as sugar maples by altering their soil microhabitat.
This, in turn, triggers a series of impacts on the food web that help invasive plants spread.
Ironically, for a creature synonymous with soil improvement, some exotic earthworms can alter soil properties such as nutrients, pH and texture, leading to poorer quality crops, among other impacts.
They reproduce without fertilization from a male
Alien worms have a clear advantage. Unlike most of our native species, many female species of exotic worms can produce offspring without fertilization from a male.
In addition, climate change opens new niches for their colonization in northern areas of the continent, where the permafrost is melting and where there are no native earthworms.
Despite all this, only a limited number of studies have documented the spread of exotic worms and none have covered the dynamics of colonization on a large spatial scale or large number 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 that documents interceptions of alien earthworms in the US border between 1945 and 1975.
Earthworms in 97% of soils studied
With the help of machine learning, the team used the combined databases to reconstruct the presumed pathways of introduction and spread of exotic earthworm species.
They found exotic species of earthworms in 97% of the soils studied in North America, with exotic occupancy highest in the northern part of the continent and lowest in the south and west.
Exotic species generally account for 23% of the continent’s 308 earthworm species and account for twelve of the thirteen most widespread earthworm species.
And by 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 worms is three times that of native worms. In most of the lower 48 states and Mexico, there is approximately one exotic worm for every two native species.
They threaten ecosystems
“These proportions are likely to increase because human activities facilitate the development of exotic species that threaten native earthworm species, a phenomenon that is still largely overlooked,” said study lead author Jérôme Mathieu. .
Not all aliens will threaten native ecosystems.
However, according to researchers, their wide distribution and unknown impact on a variety of native ecosystems, such as grasslands and coniferous forests, mean they deserve close attention.
Among other solutions, they suggest that policymakers focus on prevention, such as encouraging the use of native minosas for composting and fishing bait, and early detection through regular monitoring and citizen science.