Opening a can of worms – Lecture report written by Linda Mosquin
As a gardener I have dug up many earthworms in our flower and vegetable gardens and have long considered the earthworm to be a friend, always marveling at its ability to break up, aerate and improve the soil. Or, as our Mississippi Valley Field Naturalist’s speaker Dr. Paul Gray of the Ontario Ministry of Natural Resources(OMNR) more largely described them, as “. . . ecological engineers famous for their ability to ingest and integrate soils through different layers, for their contribution to agricultural productivity, for their role as food for wildlife and for use by anglers as fish bait.” Many animals eat earthworms…. think crows, gulls, skunks, flickers, robins and others. In Ontario the business of exporting worms to the United States is valued at 110 million dollars a year and involves a migrant work force picking worms at night. And with agricultural fields and pastures in Ontario using more than 5 million hectares of soil, the earthworm would appear to be a seemingly benevolent creature.
Indeed, the lowly earthworm helps the economy, but as Ontario Ministry of Natural Resources (OMNR) Paul Gray recently told an MVFN audience, the evolving earthworm story in North America is much more complex. He is co-author of the 2012 report: Implications of a Potential Range Expansion of Invasive Earthworms in Ontario’s Forested Ecosystems.
Indeed, the earthworm helps the economy, but as our speaker pointed out, with warmer climate change due to humans burning fossil fuels, the evolving earthworm story in Canada/North America is more complex and darker. Native Ontario earthworms, where they existed, are believed to have been eradicated with the Wisconsin glaciers 10,000 years ago and southern native species did not manage to re-colonize this area. Thus our forests developed in the absence of earthworms until they arrived with soils (for ballast) and plants brought here by European settlers. So at present 17 European non-natives and two North American (non-native to Ontario) earthworm species thrive in the province. Some of these earthworms are invasive and with our warming climate it is becoming more apparent that there is a potential for range expansion of these worms in Ontario’s forested ecosystems. Already much damage has been done to the forest habitat around the Great Lakes.
Gray presented a number of detailed charts depicting the warming trend for climate change in Ontario. The International Panel on Climate Change models show the warmest trends of between 9-10 degrees, in the northern latitudes, and 4-5 degrees in our area by the end of the century. Earthworms can be killed by freezing but they have developed systems to avoid this happening. As climate warms they will continue to move forward into our northern forests.
Earthworms are classified into three ecological groups, namely: endogeic, these are rich soil feeders, topsoil dwellers, have no pigmentation, make horizontal burrows, and are small (approx.7.5-12.5 cm). Epigeic earthworms are top-litter feeders and dwellers; they are pigmented, make no burrows and are the smallest (at 7.5cm). Anecic earthworms are larger (12.5-20 cm) earthworms which are litter and soil feeders and dwellers, dorsally pigmented, and make extensive permanent vertical burrows.
Given the different strata of the soil the different earthworms groups reside in and their burrowing habits, it is no surprise that the impact on forests from earthworms is greatest when all three kinds of earthworms are present. As they move into a forest one can see the edge of the healthy, rich, thick horizon zone meeting the edge of the ‘denuded’ soil caused by earthworms eating up much of the available organic material. Organic layers are lost, protozoa are eaten, micro-arthropod eggs are damaged and micro-fauna are preyed upon. Plant communities are weakened and often destroyed. Invasive earthworms do most damage to hardwood forests, such as those consisting of maple, basswood, red oak, poplar, or birch species. So a forest that once had a lush understory ends with a single species of native herb and essentially no tree seedlings. Over time (spreading 5 to ten meters a year) earthworms change the forest soils from a fungal to a bacterial dominated system which hastens the conversion of leaf litter to mineral compounds, starving the plants of organic nutrients. This change in soil eliminates seedlings, ferns, and wildflowers. There is evidence emerging that changes caused by alien earthworms can even eventually affect small mammal, bird and amphibian populations and increase the impacts of herbivores like white-tailed deer. Invasive plants such as buckthorn and mustard garlic can establish a roothold in a diminished ecosystem. These species reduce and destroy habitat for native species and are a serious threat to biodiversity and the health of our forests. Once established, earthworms are virtually impossible to eradicate.
Much of Paul Gray’s presentation on earthworms is based on American information since most of the research has been done there, although one of the best known books on earthworms, “The Earthworms (Lumbricidae and Sparganophilidae) of Ontario” (1977) was written by a Canadian, John W. Reynolds.
Gray described the findings from a multi-species invasion of earthworms at a site in Timmons he worked on in 2011. Nineteen species were identified at this site. Along with other researchers, he has developed a ranking system for the risk posed by these species as a preliminary ‘Invasion Index’ for earthworms in Ontario. Categories included in the ranking system are abundance, distribution, reproduction, transportability as bait, most northerly isotherm (temperature) and pH tolerance. Earthworms like a neutral pH but can exist in a wide range of acidic soil. As the soil becomes less acidic they will find it easier to establish themselves. The species were ranked low, medium or high for invasion potential. The full details of their findings are contained in the OMNR report released in 2012: “Implications of a Potential Range Expansion of Invasive Earthworms in Ontario’s Forested Ecosystems: A Preliminary Vulnerability Analysis” which Gray co-authored with others and which was released in 2012. The speaker had copies of this excellent report at the lecture and it is also available online at the OMNR website http://www.mnr.gov.on.ca/en/Business/ClimateChange/Publication/STDPROD_092882.html.
How do earthworms travel into forested areas in Ontario? With human help of course! Fishermen dumping bait near forested areas, gardeners moving compost, road building, or ATV tire-treads or truck tire-treads which have adult worms and cocoons (egg cases), could all start an earthworm invasion into forested areas. There are some simple things you can do to prevent their further spread. For example, people should take unused fishing bait home and freeze the container for at least a week before discarding the contents, avoid dumping compost anywhere except in your own garden, and wash ATV or other soil-holding vehicle tires before transporting the vehicle. In Minnesota, where extensive research on earthworms has been done it is illegal to dump worm bait.
More research in Ontario on managing invasive earthworms, especially with our warming climate, would be useful. Regulation and education could help prevent alien earthworms from invading Ontario forests. Another route to scientific research that is very much supported by Gray is citizen science. He would be happy to help organize training and seek support for citizen groups that would like to become involved in collecting data about invasive earthworms. If you are interested in starting or joining such a group consider contacting Dr. Gray at To help citizens become informed on earthworms there are various sites on the internet which offer additional information such as http://www.naturewatch.ca/english/wormwatch/ and http://www.nrri.umn.edu/worms/default.htm
Paul Gray (right) of the Applied Research and Development Branch, OMNR, in lively discussion with MVFN President Ken Allison (left) and member Neil Carleton after his presentation on earthworms. Photo Pauline Donaldson