Yes, Earthworms are an Invasive Species in Eastern North America!

Ontario has no native earthworm species.

During the Pleistocene Epoch that began about 2.6 million years ago and lasted until about 11,700 years ago, ice sheets covered Ontario. When they retreated, no earthworms were present in Ontario. Over the next 10,000 years, Ontario ecosystems evolved without earthworms. Their introduction over the past few centuries is dramatically impacting Ontario ecosystems. Gardeners need to reduce their spread to protect Ontario forests and grasslands.Earthworms move through the soil, ingesting organic and mineral matter as well as microorganisms. As they do, they mix soil layers and create new soil profiles. They change the physical, chemical and biological activity of soils. They impact soil microbia and reduce mycorrhizal diversity. Plant species supported by new soil systems change.

As plant populations change, associated pollinators, herbivores and predators are all impacted.

As plant populations change, associated pollinators, herbivores and predators are all impacted. Thus with the introductions of multiple earthworm species, major changes to the structure and function of our north-temperate forests can result in what is termed “invasional meltdown.” In our northern forests, More soils predominate. These are soils in which organic matter is practically unmixed with lower mineral soils. The uppermost layer of a healthy native temperate forest floor is covered in organic matter (called detritus, duff or the O horizon). Within three to four years after introduction, earthworms can transform More soils to Mull soils, characterized by mixed organic mineral layers. As the duff layer decreases, plants whose roots feed in that organic layer begin to diminish.

Native plants like spikenard (Aralia racemosa), Solomon’s seal (Polygonatum pubescens), bellwort (Uvularia grandifolia), nodding trillium (Trillium cernuum), large-flowered trillium (Trillium grandiflorum), and goblin fern (Botrychium mormo) slowly disappear. Invasive species, like Rhamnus cathartica (buckthorn) and Lonicera x bella (honeysuckle) benefit and thrive.

While it is impossible to get rid of them, gardeners must take great care not to transport plants to new locations that might spread worms further.

Ecosystem disruption ensues. While it is impossible to get rid of them, gardeners must take great care not to transport plants to new locations that might spread worms further. There is considerable research about the negative impacts of earthworms. But there is so much more to learn. Here’s a good overview: Implications of a Potential Range Expansion of Invasive Earthworms in Ontario’s Forested Ecosystems: A Preliminary Vulnerability Analysis. http://www.ontla.on.ca/library/repository/mon/26002/314892.pdf

Cathy Kavassalis – Halton Master Gardener

More Reading:
Addison, J.A. (2009) Distribution and impacts of invasive earthworms in Canadian forest ecosystems. Biol Invasions .11: 59. https://doi.org/10.1007/s10530-008-9320-4Hendrix, P.F. & Baker, G.H. & Callaham, Mac & Damoff, George & Fragoso, Carlos & González, Grizelle & James, Samuel & Weyers, Sharon & Winsome, T. & Zou, Xiaoming. (2006). Invasion of exotic earthworms into ecosystems inhabited by native earthworms. Biological Invasions. 8. 10.1007/978-1-4020-5429-7_9.Bohlen PJ, Scheu S, Hale CM, McLean MA, Migge S, Groffman PM, Parkinson D. (2004) Non-native invasive earthworms as agents of change in northern temperate forests. Frontiers in Ecology and the Environment.;2:427–435 Stable URL: http://www.jstor.org/stable/3868431Edwards, C.A., Bohlen, P.J., (1996). Biology and Ecology of Earthworms, third ed. Chapman and Hall, London, EnglandEvers, A. K.; Gordon, A. M.; Gray, P. A.; Dunlop, W. I. (2012) Implications of a Potential Range Expansion of Invasive Earthworms in Ontario’s Forested Ecosystems: A Preliminary Vulnerability Analysis. Ministry of Natural Resources Climate Change Research Report.CCRR-23 vii + 31 pp http://www.ontla.on.ca/library/repository/mon/26002/314892.pdfFrelich L.E. et al. (2006) Earthworm invasion into previously earthworm-free temperate and boreal forests. In: Hendrit P.F. (eds) Biological Invasions Belowground: Earthworms as Invasive Species. Springer, DordrechtHendrix P.F. (2006) Biological invasions belowground—earthworms as invasive species. In: Hendrix P.F. (eds) Biological Invasions Belowground: Earthworms as Invasive Species. Springer, DordrechtHendrix, Paul & Callaham, Mac & Jr, & Drake, John & Huang, Ching-Yu & James, Samuel & Snyder, Bruce & Zhang, Weixin. (2008). Pandora’s Box Contained Bait: The Global Problem of Introduced Earthworms. Annual Review of Ecology Evolution and Systematics. 39. 593-613.. 10.1146/annurev.ecolsys.39.110707.173426. https://www.researchgate.net/publication/259116970_Pandora’s_Box_Contained_Bait_The_Global_Problem_of_Introduced_EarthwormsMadritch, M.D. & Lindroth, R.L. (2009) Removal of invasive shrubs reduces exotic earthworm populations. Biol Invasions 11: 663. https://doi.org/10.1007/s10530-008-9281-7Resner, K., Yoo, K., Sebestyen, S.D. et al. (2015). Invasive Earthworms Deplete Key Soil Inorganic Nutrients (Ca, Mg, K, and P) in a Northern Hardwood Forest. Ecosystems 18: 89. https://doi.org/10.1007/s10021-014-9814-0Reynolds, J.W. 1977a. The Earthworms (Lumbricidae and Sparganophilidae) of Ontario, Life Sci. Misc. Publ., Roy. Ont. Mus., 141 pp. https://archive.org/stream/earthwormslumbri00reyn/earthwormslumbri00reyn_djvu.txtSzlavecz, K., Placella, S. A., Pouyat, R. V., Groffman, P. M., Csuzdi, C., Yesilonis, I. 2006. Invasive earthworm species and nitrogen cycling in remnant forest patches. Applied Soil Ecology. 32: 54-62. https://naldc.nal.usda.gov/download/27458/PDF

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