The ‘Earthworms Are Good ‘ school of thinking is a generally-held and worthy gardening belief. Earthworms are not usually associated with any negative attributes by horticulturalists and soil scientists. Golfers are the only group that I know of who uniformly hate earthworms. They, in their never-ending quest for personal-best low scores, obsessively clear all earthworm casting from meticulously manicured, daily-mown grass greens. Having their balls deflected by worm castings from carefully calculated paths to too-often missed flag-marked sunken cups can involk vehiment condemnation of greens’-keepers professional inadequacies.
Gardeners thankfully have long been taught to make their lawns and garden soils worm-havens to reap the benefits of improved root and thus overall plant growth. Worm castings are seen as proof-positive that the gardener is achieving a gardening 'holy grail' – worm rich soil.
Fine Gardening (February 2005), a trusted source of valuable gardening information, recently produced an excellent summary article extolling the benefits of having vibrant earthworm populations in garden soils. Gardeners were reminded that earthworms, through digestion of undecayed vegable matter, hasten the release of nutrients needed by actively growing plants. Miriads of worm tunnels areate soils facilitating exchange of oxygen and carbon dioxide with the above-ground atmosphere. In the world of horticultural commerce there are many new industrial-scaled businesses bagging certifiably-pure worm castings and worm-digested municipal compost suited to the home gardener.
However, opposed to all the positive attributes of encouraging worm-enriched gardening paractices, there is one contrary earthworm concern emerging in the forestry world. My attention to the negative impacts of earthworms on forest ecosytems started very innocently one evening, at a local garden club's post-meeting pot-luck dinner. A respected visiting lecturer, chatting about problems establishing alpine plants in scree (gravel-based) gardens, mentioned that earthworms are not a part of alpine ecosystems. He conjectured that they might be physically disturbing delicate alpine plant root systems. It was his almost incidental afterthought that sparked my antennae. “Earthworms are now being blamed for destroying some North American forests too” he said.
I have to admit I raised my eyebrows when I heard this. I had an almost empty wine glass in hand, a satiated appetite thanks to a splendid table by accomplished cooks, and a noisy roomful of gardeners trading ‘Did you know … ?’ stories. I wondered if I had just been handed a can of worms – pun intended!
My interest in this catastrophic worm story was heightened by youthful entreprenurial experience - I had a role in sending earthworms into forested areas. In the 1950’s, as an adjunct to many hours spent fishing in nearby southern Ontario lakes, my home-based venture was as a dew worm picker and worm vendor to fishermen. My possible contributions to forest declines, from leftover worms being thrown into woods at the end of fishing trips, were inconsequential I am sure – at least based on the size of my bank account!
Within the week the visiting University of Michigan’s professor emailed me his internet links to the worm world. Geological and ecological worm facts soon opened up like a Pandora’s Box!
Ensuing internet searches revealed that, geologically-speaking, North America had glaciers scour soil from much of the northern states and provinces and deposit it further south. This I knew already. But I had not considered that pre-glaciation worm populations moved with that soil leaving wormless environments for future forests to colonize. Soil building and northward earthworm migrations proceed at geologically-slow rates and, until the arrival of early settlers, northern post-glacial soils and colonizing trees developed in harmony without earthworms. Evolving forest ecosystems adapted to slow soil formation and slow nutrient release from vegetation decomposition in a cool soil environment. With the coming of early settlers, earthworms quickly found their way into our northern ecosystems. These settlers and their decendents both purposefully (for improved agricultural practices) and unknowingly (in soil constituting ships’ ballast) imported earthworms into North America. Fishermen and I have made our more recent contributions too.
This is where the passage of time and continuation of old habits enters the discussion. Historically the worm populations of northern forests’ soils were not much studied. Patterns of forests’ tree composition change were never priority research projects. However, the internet reveals that alarm bells over worm-induced forest declines are beginning to be sounded by various researchers. Increasingly, they report, earthworm-filled northern forest soils are becoming more suited to growing non-native trees and shrubs. It is the simple presence of earthworms in the soil which concerns them. The very attributes which gardeners cherish (worm tunnel aeration, digestion of organic matter and excretion of worm castings making nutrients readily available for easy uptake by roots) are the problems in native forest soils. With more oxygen-rich air entering the duff layer through earthworm-formed tunnels, organic materials accumulated over the centuries is disappearing faster than it is building up. In some cases a decade is all that is required to wipe out milleniums’ worth of organic material accumulations. Forest soils are loosing their thick, fluffy, and moisture-retentive insulation layerof forest debis. They are becoming increasingly nutrient-rich and warmer largely due to the simple presence of earthworms. As in farming areas, forest soils are changing – and not for the better as far as historical tree species composition is concerned.
Gardeners in cottage areas are unsuspectingly further contributing to this accelerated forest ecology change by importing alien trees, shrubs and garden plants to personalize their summer retreats. This plant importation, with unintended plant escapes and introduced pollen pools, further compounds native plant survival in forested areas. This is where selecting native plants for lakeside properties can help maintain traditional forest ecosystems.
Sadly, from our nursery-owner’s perspective, there is yet to be a significant conversion of gardeners to planting native species anywhere – let alone in forested, lake-side areas. We encourage all gardeners to accept their role in maintaining native plants for the benefit of forest-dependent native birds and animals. As fishermen we should not toss unused fish bait worms or mediums used for transporting them (they may contain viable worm eggs) into the woods. Unused worms and transportation soils/mediums taken into remote areas should be brought back to our southern urban residential plots where they can be incorporated into already earthworm-populated compost piles or gardens.
Further Reading:
The following web resources will give interested readers a wealth of literature entreés into the world of earthworm biology and research on earthworm impacts on forest soils.
http://www.nrri.umn.edu/worms/forest.html Worms and Your Forest (source: Minnesota Worm Watch)
http://conbio.net/SCB/Services/tips/2002-9-December.cfm#A3 Non-native Earthworms may be Wiping Out Rare Plants. (Source: December 2002 issue of Conservation Biology: the journal of the Society for Conservation Biology)
http://res2.agr.ca/london/faq/earth-terre_e.htm FAQ - Earthworms (Source: Agriculture Canada.)
http://www.eap.mcgill.ca/publications/eap6.htm Earth Worms: The Agriculturalist’s Friends. Jennifer A. Ramsay and Stuart Hill. From Macdonald Journal 39(10), 6-8 Oct. 1978
