The quiet invasion of non-native earthworms across North America, once considered benign, has been revealed as a significant threat to the continent’s ecosystems.
According to a new study led by Stanford University, at least 70 species of invasive earthworms have made their way into North American soil. The study presents the largest database of alien earthworms to date, shedding light on a critical yet underappreciated ecological issue.
“Earthworms tell the story of the Anthropocene, the age we live in,” said study senior author Professor Elizabeth Hadly.
“It is a story of global homogenization of biodiversity by humans, which often leads to the decline of unique local species and the disruption of native ecosystem processes.”
Introduction of invasive earthworms
Earthworms, generally hailed for their beneficial roles in agriculture and gardening, are now under scrutiny for their potential as antagonists in native ecosystems.
The introduction of alien earthworm species to North America, a practice dating back to the late 1800s, aimed to exploit their positive contributions to soil health. These creatures aerate the soil, facilitate water and nutrient penetration, and enrich the ground with their castings. However, the study highlights a darker side to this narrative.
Non-native plant species
Non-native earthworms have begun to stress native plants, trees, and wildlife by altering soil properties and encouraging the spread of invasive plant species.
For example, in the northern broadleaf forests of the U.S. and Canada, alien earthworms’ impact on soil stresses trees such as sugar maples by altering the microhabitat of their soils. This triggers a cascading series of impacts on the food web that help invasive plants spread.
Displacement of native earthworms
One of the most striking findings of the research is the sheer scale of alien earthworm colonization, with these species found in 97% of studied soils across North America. These invaders now account for 23% of the continent’s earthworms, showcasing a significant displacement of native species.
The researchers noted that invasive earthworms are at a distinct advantage, considering that many female alien earthworm species can produce offspring without fertilization from a male. Furthermore, melting permafrost in the northern parts of the continent is providing brand new habitats for invasive earthworms.
Urgent need for action
The study’s lead author, Jérôme Mathieu, an associate professor of Ecology at the Sorbonne University, emphasized the urgent need for attention to this phenomenon, pointing out that human activities continue to facilitate the proliferation of these alien species, threatening the survival of native earthworms.
The researchers utilized a vast collection of records spanning from 1891 to 2021, combined with data on U.S. border interceptions of alien earthworms from 1945 to 1975, to map out the introduction pathways and spread of these species across North America.
The experts found that overall, invasive earthworms represent 23% of the continent’s 308 earthworm species, and account for 12 of the 13 most widespread earthworm species.
The proportion of alien earthworms in Canada was found to be three times greater than that of native earthworms. Furthermore, the study revealed that there is about one alien earthworm for every two native species across most of the lower 48 U.S. states and Mexico.
The analysis, bolstered by machine learning techniques, reveals a complex web of introduction and dissemination, highlighting the adaptability and resilience of these organisms. The study also underscores the importance of prevention and early detection in managing the threat posed by alien earthworms.
“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,” said Professor Mathieu.
“This is most likely the tip of the iceberg,” said study co-author John Warren Reynolds. “Many other soil organisms may have been introduced, but we know very little about their impacts.”
The study is published in the journal Nature Ecology & Evolution.
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