I last blogged about bioinvasions by earthworms in September 2023. My concern was that the USDA Animal and Plant Health Inspection Service (APHIS) was refusing to regulate worms’ introduction based on an inadequate risk assessment. I complained that APHIS’ principal concern was that introduced worms or the soil they were transported in might transport pathogens. APHIS’ website did not mention introduced earthworms’ well-established ability to disrupt the soil and possibly cause undesirable impacts on plant growth and diversity.
I also cited a study by USDA Forest Service scientists regarding interactions between long-term, continuous stressors, including earthworm invasions, and short-term gap-forming events. Reed, Bronson, et al. (2025) reported finding lower earthworm biomass and density in both deer exclosures and canopy gaps. They hypothesized that in canopy gaps higher deer browse pressure changed plant community and soil properties. These changes then affect soil-dwelling fauna like earthworms. They believed the higher worm densities in closed-canopy sites might be the result of greater volumes of nutrient-rich tree leaf litter which provides both shelter and food. The closed canopy sites also lacked recent soil disturbances. (Other authors cited in that blog describe varying impacts of deer browsing on plant communities and succession.)
A study I have not previously included in my blogs focused on invasions in the forest understory of the Katharine Ordway Natural History Study Area, which is in eastern Minnesota on the bluffs of the Mississippi River. The now dominant oaks are being replaced by a more mesic forest with a growing proportion of Ulmus americana, U. rubra, Acer negundo, Prunus serotina, Tilia americana, and Fraxinus pennsylvanica. (I note that three of these species are being suppressed by two introduced organisms, “Dutch” elm disease and emerald ash borer).
Davis et al. (2015) found that all common herb layer species – native and non-native – established in microsites favorable to plants in general. Under the forest’s mostly closed tree canopy, light levels played almost no role in determining where herbaceous understory plants grew. However, abundant earthworms and deer were deemed to be larger factors explaining most of the changes in native herbs’ abundance and distribution. The earthworms’ effect arose from changes to the soil and litter environment.
I have not found a more recent analysis of the Ordway forest understory, so I don’t know whether these trends continued. I also wonder whether the conclusions about herb species’ interactions would have differed if the non-native species studied were those that completely dominate the ground level, such as Ficaria instead of Alliaria petiolata; or a dense shrub layer, e.g., Rosa multiflora.
A new analysis provides a large-scale description of worm invasions. McCay et al. (2026) report that about 300 species of earthworms inhabit North America, of which about 70 are non-native. They sought to determine the relative importance of Pleistocene glaciation in explaining earthworms’ present assemblages. Their study sampled 94 sites across 13 states of the eastern and central U.S. over a twelve year period (2013 – 2025). Seventy-four of the sites were located in previously-glaciated locations, 20 sites in non-glaciated places.
The study confirmed the presence of 16 species; only three were considered to be native to North America (Aporrectodea rubidus, Eisenoides carolinensis and E. lonnbergi). Minimum species richness at individual sites ranged from zero to eight species. Species richness was markedly greater in the glaciated regions of the Midwest and Northeast. By both dry biomass and numbers, European lumbricids dominated throughout. Lumbricus species constituted 45% by number and 69% by biomass across all sites. Their dominance was highest in the Midwest Glaciated region.
Native earthworms were rare, constituting only 2 or 3% at those sites where present. Invasion by Asian pheretimoid megascolecid (“jumping worm”) species has just begun; they were present at only 10% of study sites.
Earthworms – apparently both native and non-native – were most abundant and diverse in the Midwest Glaciated region (Ohio, Michigan, Minnesota, and Wisconsin), where non-indigenous earthworm invasion is more recent than in the Northeastern Glaciated Region (Massachusetts and New York). Earthworms were least common and diverse in areas south of the terminus of the Pleistocene glaciation. McCay et al. (2026) speculated that development of forests in northern North America over thousands of years with few or no earthworms might have led to accumulations of organic matter in forms useful to earthworms (“banking” of soil organic matter), which supported a pulse of earthworm proliferation following colonization.
Some other factors play a role. Earthworm presence – measured by biomass – and diversity increased with higher soil pH. Many earthworm species are known to be sensitive to soil acidity. Epigeic earthworms (those that feed on leaf litter) are often more tolerant of acidic conditions than endogeic earthworms (which live in and consume soil).
The study did not measure co-occurring variables such as calcium abundance, buffering capacity, or exchangeable aluminum. All might more directly affect worms’ physiology than pH per se.
Soil structure also matters. Soil-feeding species might be disadvantaged by sandy and gravelly soils, which do not support persistent burrow formation and are often nutrient deficient. On the other hand, they thrive better in clay soils.
While each study participant established at least one site in a relatively undisturbed habitat, e.g., a mature forest, the majority of sites tended to be within a disturbed landscape, less than .8 km from a road.
Earthworm abundance and activity vary seasonally but only 4 sites were observed over more than one season. Across all sites, there was greater species richness in autumn and spring than summer. McCay et al. (2026) think that this might have been because many earthworms aestivate during dry weather, so they are less detectable during summer although they are still present.
Distribution of native North American earthworms suggests there was some natural colonization of previously glaciated landscapes before European people arrived.
McCay et al. (2026) also did not document land use history at the sampling sites, although that can profoundly affect worms and vertical distribution of carbon in the soil. Nor did their habitat classification system [deciduous and coniferous forests, mowed and unmowed grasslands] describe the vegetation in sufficient detail to determine whether food quality might have been a factor. It is recognized that litter varies in C:N ratios, lignin content, and palatability.
McCay et al. (2026) note an interesting pattern at the global scale: Australia, New Zealand, South Africa and North America (U.S. and Canada) have been invaded by the same suite of invasive European earthworm species. One common factor, not addressed by the scientists, is that all were settled by British colonists. Instead, they focus on ecological factors. They suggests that the species’ niche requirements are broad and well-matched to temperate forest soils worldwide, and that climate and soil conditions, rather than biotic resistance from native communities, are primary factors limiting their distribution and abundance.
Another factor supporting this hyptothesis, in their view, is that the composition of earthworm communities was relatively similar above and below the southern glacial limit in North America. Here, European earthworms colonized not just the relatively “open” soils of previously glaciated north but also non-glaciated areas farther south – assisted by human introductions and creation of disturbed habitats. They suggest that the novel species might have benefitted from reduced competition because the European species had functional differences from resident native species.
The current rapid expansion of “jumping” earthworms in North America is a fundamentally different invasion wave than the centuries-old invasion by European lumbricid species. The Asian pheretimoid species reproduce parthenogenetically and have shorter generation times. They achieve high population densities at the soil surface, process organic matter rapidly, and create a loose casting layer, potentially creating different soil conditions in ways not true after invasion by European earthworms. Although it is too early to know the outcome, McCay et al. (2026) note emerging evidence that the Asian pheretimoids might outcompete European earthworms through more flexible resource exploitation, preemptive foraging during the late summer and early fall, and tolerance of environmental stress.
Effects on atmospheric carbon levels
McCay et al. (2026) point out that the introduced earthworms are disrupting the significant pool of accumulated organic matter in previously glaciated soils. As earthworms move in, they facilitate more rapidly cycling but also translocate carbon into deeper soil strata. They conclude that while earthworms initially accelerate carbon loss through increased decomposition and respiration, their long-term effects might protect soil carbon storage. Earthworm casts and burrows can create microsites with different oxygen and moisture conditions that might promote carbon stabilization through different mechanisms than the original forest floor.
SOURCE
Davis, M.A., M.D. Anderson, L. Bock-Brownstein, A. Staudenmaier, M. Suliteanu, A. Wareham and J.J. Dosch. 2015. Little evidence of native and NIS species influencing one another’s abundance and distribution in the herb layer of an oak woodland. Journal of Vegetation Science · June 2015
DOI: 10.1111/jvs.12302
McCay, T.S., Anderson, L.J., C.P. Bloch, A.E. Cahill, S.L. Cooke, B.J. Dolan, K.M. Flinn, D. Garneau, N.J. Hains, K. Hopfensperger, M.Beth Kolozsvary, C. Mankiewicz, S.E. Scanga, J.L. Schafer, E. Schwartzberg, D.A. Scott, K.L. Shea, J. Simmon, J.N. Styrsky. 2026. Earthworm assemblages in the Eastern and Midwestern United States and the legacy of glaciation. Biol Invasions (2026) 28:91 https://doi.org/10.1007/s10530-026-03798-3
Reed, S.P., D.R. Bronson, J.A. Forrester, L.M. Prudent, A.M. Yang, A.M. Yantes, P.B. Reich, and L.E. Frelich. 2023. Linked disturbance in the temperate forest: Earthworms, deer, and canopy gaps
Ecology. 2023;104:e4040. https://onlinelibrary.wiley.com/r/ecy
Posted by Faith Campbell
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For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm
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