Plant material drawn into earthworm burrow
Whitney Cranshaw, Colorado State University, Bugwood.org
Earthworms are not native to the Great Lakes region of North America. The current population, which initially arrived with European settlement, is slowly changing the face of native forests.
Keywords: Megascolecidae, glaciation, Great Lakes region, Lumbricidae, decomposition, leaf litter accumulation, forest floor, soil horizons
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Extent of glaciation in Great Lakes region
Any native species of earthworms (in the family Megascolecidae) that may have lived in the region were extirpated when glacial ice sheets covered the Upper Midwest 11,000 to 14,000 years ago. Natural recolonization by earthworms is very slow, so forests of the Great Lakes region developed in the complete absence of earthworms. All earthworms now in the region are exotic, and most are European (in the family Lumbricidae). They continue to be transported through a range of human activities such as the dumping of unused fishing bait, transport of compost and mulch, and anything else that moves soil. Lacking a powerful detrivore such as earthworms, annual leaf litter decomposition in hardwood forests is controlled by fungi and bacteria. Decomposition is slower than accumulation of new litter, resulting in the formation of a thick, spongy forest floor that provides protection from predation and extremes in temperature and moisture to seeds of understory plants. Earthworms remove the forest floor by eating it and by mixing it into the upper mineral soil horizon. As a result, it is difficult for plants and animals adapted to forest floor conditions to survive following earthworm invasion.