Non-Fibre Values - Burrowing an Exotic Food Web
by Frederick W. Schueler, Eastern Ontario Museum of Biodiversity
S&W Report Summer / Fall 2001 (Volume 24)
In 1927, in Life Histories of North American Birds, Arthur Cleveland Bent gives a northern range limit for Ontario Woodcock (Scolopax minor) running through Bracebridge, Madoc, and Ottawa. In 1987 the Atlas of the Breeding Birds of Ontario ran this line through Lake Abitibi, Lake Nipigon, and Lake of the Woods - 500 km north of the 1927 range. It’s not just the clearing of land for agriculture and by forestry that has allowed this long-billed, chunky, secretive Sandpiper to spread north - its predominant food, the Earthworm, had to precede it.

Ever since Darwin published his calculations of the rate at which Earthworms turn over and aerate English soils, these terrestrial Oligochaetes have been considered as beneficent as motherhood or apple pie. Most Canadians don’t even realize that their local Earthworms are European immigrants. Even Agriculture Canada’s Wormwatch web site <www.cciw.ca/ecowatch/wormwatch/english/english.htm> treats Earthwoms as soil amendments rather than as dangerously exotic aliens.

Pleistocene ice sheets wiped out any native Earthworms that may have lived in Canada, leaving North American species, in the family Megascolecidae, only in areas where glaciation was incomplete, such as Vancouver Island and the Richardson Mountains of the Yukon. The twenty or so European species that now live in Canada, in the family Lumbricidae, are thought to have come in the rootstocks of plants imported by settlers or in soil used for ballast in ships. Native North American worms have spread only a little north of the southern limits of glaciation, but the European species have spread widely and rapidly, recently aided by the commercial sale of Earthworms for fishing bait.

Jean-François Desroches is compiling a video of reptiles and Amphibians of Quebec - but he’s reluctant to release it because species after species - frogs, toads, turtles and snakes - is seen gagging down Earthworms. This isn’t just because Earthworms are a convenient food for captive animals - many of these species subsist largely on Earthworms in the wild. The most extreme case is Butler’s Garter Snake, Thamnophis butleri, with a diet in Ontario that is 93% Earthworms. Our soil invertebrate fauna has been profoundly disrupted by the introduction of European Earthworms. Pre-Earthworm forest-floor food chains in southern Ontario are a mystery. Our giant litter-eating Millipedes were presumably much more abundant. Starnose Moles, Condylura cristata, must have relied more exclusively on aquatic prey. Redbelly and Brown snakes (Storeria) must have had to make do with the small native Deroceros slugs, and Robins (Turdus migratorius) must have eaten ‘something else’.

In 1999 Jean-François and Joël Bonin found that populations of Redback Salamanders (Plethodon cinereus) were more abundant in forests with deeper leaf litter and humus. Would the Salamanders decrease with the rapid consumption of the litter by Earthworms, or benefit from feeding on small worms? Probably the former. I’ve often wondered about the worm-cast mineral soils of many southern Ontario woodlots, barely covered with a single layer of leaves. Recently, in Alberta, New York and Minnesota, forest ecologists have documented major changes caused by invading worms in soil structure, erosion, water penetration, nutrient cycling, and the understory shrubs and herbs.

A study by researchers at the University of Minnesota has found dramatic losses of native understory plant species and tree seedlings as exotic Lumbricids invade Sugar Maple/Basswood stands in the Chippewa National Forest of northern Minnesota. Without Earthworms, decomposition of the annual leaf litter in hardwood forests is controlled by fungi and bacteria. Decomposition is slower than accumulation of new litter, resulting in a thick, spongy “duff” on the forest floor. In the first two years of this study, the front of invading Earthworms moved about 10 metres into the forest. The Earthworms ate the organic layer out from under the plants and tree seedlings, replacing the duff with a much denser layer of black earthworm castings. The worm-invaded areas lost a dozen species of spring-flowering forest floor herbs, and Sugar Maple seedling densities dropped from over 100 per square metre to almost none.

These changes don’t seem to reverse with time. Earthworm dominated forests don’t support perennial spring-blooming herbs like Bellworts (Uvularia), Trillium, Yellow Violets (Viola pubescens) or Wild Ginger (Asarum canadense), but fast-growing exotics such as Garlic Mustard (Allaria petiolaris) can often dominate a worm-churned site. And plenty of other alien invertebrates (besides the Insects often recognized as pests) have become important links in forest food chains. Toads and Frogs feed on the large introduced snail Cepaea nemoralis. Slugs of the genus Arion have spread through our forests. All our terrestrial Isopods, Sowbugs or Woodlice, are European introductions.

It’s probably prudent to avoid intentional introductions or the movement of soil into forest interiors, since native species have more chances to adapt if an exotic’s spread is slower, but there’s no way to eliminate any of these established exotics. All we can do is recognize them as non-natives, and their effects as changes from pre-settlement conditions.

Should you wish to contact Fred about this article or information about his work, he can be reached at <bckcd@istar.ca>.

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