Invasive earthworms are reshaping North American ecosystems

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 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 against invasive earthworms

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.

Key findings and implications 

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 remarkable world of earthworms

As discussed above, earthworms, both invasive and native, play a pivotal role in maintaining the health and fertility of our soil.

These invertebrates, belonging to the class Oligochaeta, thrive in moist environments, burrowing through the earth to create intricate tunnel systems.

Their activities significantly benefit the ecosystem, making them unsung heroes of the agricultural world.

Earthworms consume organic matter, including dead leaves and decaying plant materials, as they navigate through the soil.

This diet not only helps to break down organic waste but also aids in the recycling of nutrients. As they digest this material, earthworms produce castings — nutrient-rich waste that serves as a powerful natural fertilizer.

These castings enrich the soil, boosting its fertility and enhancing plant growth.

Invasive earthworms and biodiversity

The physical movement of earthworms through the soil is equally beneficial. Their burrowing action creates channels that improve soil structure by increasing aeration and drainage.

This enhanced soil environment allows for better root penetration and water retention, crucial factors for healthy plant development.

Moreover, the tunnels they create serve as conduits for root growth, enabling plants to access nutrients and water more efficiently.

Earthworms also play a critical role in the soil food web. They serve as a food source for a variety of predators, including birds, rodents, and insects, thus contributing to the biodiversity of their ecosystems.

Their presence and activity indirectly support a wide range of above-ground biodiversity, highlighting their importance in sustaining both soil health and overall ecosystem balance.

Supporting agriculture and combating climate change

The impact of earthworms extends beyond their immediate environment. By improving soil health, they support agriculture and horticulture, ensuring that soil is fertile and productive for food crops and ornamental plants alike.

Their contribution to carbon sequestration, through the incorporation of organic matter into the soil, further underscores their role in combating climate change.

Despite their silent work beneath our feet, earthworms face threats from human activities, such as the excessive use of pesticides and habitat destruction.

Recognizing and mitigating these threats is essential to preserving the health of our soils and the broader environment.

In summary, earthworms are vital yet often unnoticed engineers of the earth. Their continuous labor transforms the soil, making it a hospitable place for plants to thrive.

By understanding and appreciating the role of earthworms, we can better steward the land, ensuring the health and vitality of our planet for future generations.

The study is published in the journal Nature Ecology & Evolution.

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