Invasive Earthworm Species Colonize Large Swaths of North America, Pose Threat to Native Ecosystems

Soil-dwelling organisms are essential in ecosystems' functioning and delivering ecological services. Because of this, soil taxa, like earthworms, are beneficial in good land practices. However, introducing these organisms in places where native species do not co-evolve with them can lead to catastrophic changes.

Invasion of Soil Dwellers

Although earthworms are primarily invisible and unappreciated, they are precious to farmers and gardeners. Their movement creates tunnels that allow water, air, and nutrients to penetrate, while their waste can be used as a rich fertilizer.

Earthworms also play an essential role in processes that affect aboveground communities and the atmosphere. For instance, earthworms' mechanical movement through the soil may release carbon dioxide, but the longer-term effects of digesting organic matter increase sequestered carbon.

Since the late 1800s, earthworms have been brought to North America from Asia, Europe, South America, and Africa. In some places, introducing non-native species has successfully enhanced the agricultural economy, but they have also been detrimental in other areas. The imported species are more likely to consume aboveground leaf litter than native earthworms, resulting in altered habitat quality.

In the broadleaf forests of the US and Canada, the alien earthworms impact trees like sugar maples. Some invasive species also alter soil properties like pH, nutrients, and texture, which leads to poor crop quality.

Consequences to Native Ecosystems

A recent investigation found that at least 70 non-native earthworm species have invaded the continents, primarily representing an overlooked threat to native ecosystems. Researchers analyzed Stanford University, Sorbonne University, and other institutions. The study "Multiple invasion routes have led to the pervasive introduction of earthworms in North America" provides the largest-ever database of invasive earthworms and warns of the need to understand and manage the terrestrial invaders. Elizabeth Hadly's research team drew on thousands of records from 1891 to 2021 to build a database of native and invasive earthworms.

This was combined with a second database documenting US border interceptions of alien earthworms between 1945 and 1975. Using machine learning, the researchers used the combined databases to reconstruct introduction pathways and the spread of exotic earthworms.

The experts discovered alien earthworm species in 97% of analyzed soil across North America. The foreign occupation was higher in the northern region of the continent and lower in the west and south. Overall, the invasive species represent 23% of North America's 308 earthworm species and account for 12 of the 13 most widespread earthworm species.

In Canada, the proportion of alien earthworms is three times higher than that of native species. Across most of the lower US, US states, and Mexico, there is approximately one invasive species for every two native species.

Not all alien earthworms would bring damage to native ecosystems. However, their large distribution in various native ecosystems makes them worth attention. The study focuses on the significant roles played by alien earthworms in structuring landscapes dominated by humans. The researchers hope their study can raise awareness of the unknown dynamics of alien earthworm introduction in North America.