Introduction and Distribution
Earthworms were initially introduced to North America by European settlers during the 1600s in dry ballast from ships. In the modern day, most reintroductions occur from gardening/agriculture, or bait worms being discarded. The Post Quaternary Introduction Theory for megadrile migration proposes that the glaciation period North America experienced approximately 10,000 years ago exterminated earthworm species native to North America, meaning the majority of earthworms found there are introduced species from Europe. The most prevalent species being from the genus Lumbricus.
Introduced earthworms have a larger geographic distribution than native species. A 2022 mapped the Relative Alien Species Richness (RASR) of earthworms across North America1.
RASR: the proportion of non-native species versus total number of species across a given geographical area. It can help indicate how prevalent an invasive species is in a region.
Figure 1: "(A) Map of predicted Relative Alien Species Richness of Earthworm in North America (RASR). (B) Statistical distribution of the predicted values of RASR across the geographical units." source
The study discovered that non-native species comprise a majority of species across the continent, with a median RASR of 73%. The continent, however, is divided, with the northern part averaging a RASR >50%, and the southern and western parts averaging a RASR <50%. Meaning, there is a greater proportion of non-native species present in the northern parts of the continent. On average, non-native worms cover a larger geographic range than native worms1.
Earthworm Habits
Introduced earthworms affect the ecosystem in a variety of ways. They are considered ecosystem engineers due to how they change the environment they invade; earthworms alter the distribution of nutrients across soil layers, disrupt root systems and fungal networks, reduce the level of forest floors, and decrease biodiversity both above and underground4.
There are three broad categories of earthworms are organized into based on their burrowing and feeding habits: Epigeic, Endogeic, and Anecic species.
Epigeic: these species live in the surface litter of the forest floor, or the few centimeters of soil and do not make permanent burrows. They feed on surface litter and any fungi or microorganisms in the litter.
Endogeic: these species make extensive burrows in the first 50 cm of soil. They ingest soil and feed on organic matter, fungi, and microorganisms found in the soil.
Anecic: these species make permanent vertical burrows up to 2 m deep in soil, but feed on surface litter. These species are responsible for heavy soil mixing from the different layers of soil and surface litter.
Figure 2: The difference in burrowing habits between types of earthworms. source
Impact on Soil Composition and Biodiversity
Surface litter is important for North American hardwood forests, where it retains moisture and the decomposing matter acts as a nutrient store for developing seedlings and is a habitat for many animals. Earthworms are often viewed as beneficial due to how they aerate the soil: in agricultural settings, where the soil is frequently depleted of nutrients and compacted, earthworms do have benefits. Soil compaction is typically not an issue in American hardwood forests, though, due to the combination of microbes and invertebrates that have evolved for these ecosystems, the litter is broken down slowly over time5. As ecosystem engineers, earthworms affect biodiversity above and belowground; earthworm burrowing activity mixes litter and soil vertically and horizontally, resulting in nutrients being redistributed2, 3.
Areas with high levels of earthworm invasion experience heavy reduction, or elimination, of the forest floor. Earthworms consume the surface litter and replace it with worm castings, changing the density and nutrient composition of the soil.
Redistribution of nutrients influences both biotic and abiotic factors, which in turn negatively impacts plant communities, fungal communities, and soil-fauna. Earthworm invasion has been shown to reduce mycorrhizal fungi, and disrupt hyphae networks through burrowing activity3. Trees also concentrate the majority of their fine roots in the litter layer, where they function to transport nutrients like carbon, nitrogen, and phosphorus to the tree.
Figure 3: "(a) Understory vegetation and herb layer in areas without earthworms. (b) Understory vegetation and soil surface at a site invaded by earthworms." source
Soil Composition
When earthworms mix the soil layers, they alter carbon, nitrogen, and phosphorus cycling. Surface layer soil contains an estimated ⅔ of the total terrestrial carbon pool; in the presence of earthworms carbon previously at the surface layer of soil is moved into deeper layers, where it becomes inaccessible to plants' fine root systems, located higher up in the soil column. It should also be noted that earthworms have been observed selectively removing surface litter materials, based on the C:N ratio. They are shown to preferentially remove materials with low C:N ratios, leaving materials with high C:N ratios7.
C:N ratio: the carbon to nitrogen ratio of an organic material determines its rate of decomposition. Materials with a higher ratio will take longer to decompose.
Earthworms cause an increase in nitrogen and nitrogen mobilization in soil, causing C:N ratios to be thrown off. C:N ratios prefer more carbon to nitrogen, so when earthworms alter these ratios to favor nitrogen, they are changing the way forest floors decompose, and the relationships between soil microbes and fungi. Nitrogen is brought deeper into the soil from surface litter materials being moved down the soil column. When earthworms replace the forest floor with their casts they cause there to be an excess of nitrogen at the surface, and due to the pores their burrows create at the surface, can lead to nitrogen runoff during heavy rains7.
Trees acquire up to 80% of their phosphorus from minerals found in surface litter. Phosphorus in surface litter is mineralized when soil microbes break it down into nutrients able to be absorbed by plant roots. Earthworms affect phosphorus cycling similarly to nitrogen, where they increase mineralization rates deeper in the soil, moving the location of phosphorus around and causing it to be less available to plants in some cases, or for there to be excess leading to runoff7.
Biodiversity
Earthworm invasion decreases the biodiversity of areas they invade, and can facilitate further invasions. Earthworm activity is implicated in the decrease of biodiversity in hardwood forests they invade. Their activity facilitates increased herbivory aboveground — the reduction of the forest floor causes understory plants to become more susceptible to deer grazing3. The combination of earthworm invasion and deer grazing is implicated in the population reduction of native plant species8. Increased grazing activity is believed to facilitate non-native plant invasion by reducing native plant presence, causing disturbance in the habitat and leaving it vulnerable to invasion8.
Shannon Index: a way of measuring species diversity in an area. A higher Shannon index indicates greater biodiversity, whereas a lower Shannon index indicates reduced biodiversity.
Belowground, earthworms have been shown to greatly reduce soil macro and microfauna diversity in areas of high invasion. A 2021 study found that “...abundance, richness and Shannon index were significantly lower in high versus low invasion status areas…” meaning that there is a strong relationship between high levels of earthworm invasion, and a decrease in soil fauna diversity3.
The disruption of mycorrhizae associations by earthworm burrows is directly implicated in the decline of species with strong mycorrhizal associations, like sugar maple, and colonization by amycorrhizal species like Japanese barberry8. Changes to nitrogen and pH levels caused by earthworm activity are also related to facilitating plant invasion, by altering the soil conditions to those more favorable for invasive species8.