March 9, 2020, delivered a sunny, warm (65° in the shade; 85° in the sun!) hint of the mud season to come. Our lawn was water-logged but no longer snow-covered, and from it a few earthworms crept into the garage to bask on the warm concrete floor. These harbingers of spring, accessible in the melting soil layer, are an early season feast for woodcocks, robins, moles, and even foxes, raccoons, and skunks.
Seeing these active, early earthworms prodded my curiosity about their ecology. The most familiar of our earthworms are the pinkish-brown 3-6 inch garden and compost denizens (Lumbricus terrestris) more commonly known as “night-crawlers.” Do they belong here?
The advance of the last glaciers of the Pleistocene (Ice Age) completely buried, disrupted, and scoured the soils across most of Canada and the northern states, including Vermont. Plants and animals in those soils were erased. Once the ice melted away, about 12,000 years ago, the continent slowly rebuilt its soils, its hydrology, and its ecosystems, but the pre-Pleistocene soil denizens, including earthworms, were gone.
South of the glaciers, earthworms had survived, but those that we find in our gardens, lawns, fields, and forests are invasive newcomers: species transported with crop and garden plants from Europe and Asia in the 17th and 18th centuries. These newbies, like so many invasive species, have filled an important, but perhaps disruptive, ecological niche. In their native European territory, Charles Darwin studied and estimated an average of over 53,000 earthworms per agricultural acre in the UK. More recent estimates in healthy, fertile farmlands there may reach as many as 1.7 million worms per acre!
Farmers, gardeners, anglers, and toddlers find earthworms useful and fascinating. So do spring-fever-struck teenagers! On a sunny, warm May day, at my Pennsylvania high school campus, the green lawn welcomed our English class. The girls chattered and the boys clowned around, none more than classmate Joe. He found a slippery earthworm, dangled it over his open mouth, and promised he would swallow it whole if every member of the class would give him a quarter. We did. He did, without blinking, for $4 and without any discomfort. Joe became a biology professor and the world’s authority on another invasive invertebrate: the destructive Gypsy Moth. A lesson for us all!
Soil scientists and ecologists have reached disturbing conclusions about the impacts of these imported worm species. The traditional understanding of the values of earthworms is that they aerate the soil; decompose decaying plant and animal materials; enrich fertility in farmlands and gardens and are fine fishing bait. These may be true, but evidence also suggests that farm and garden soils and compost can thrive without any earthworms. Additional research is finding several negative impacts of earthworms in agriculture and horticulture. They may disturb the flow of water in the soil, diminish the abundance of organic matter within the soil’s profile, and reduce carbon sequestered in the soil while releasing carbon dioxide to the atmosphere.
The deepest concern about the presence of invasive earthworms is in the condition of forest soils. Vermont’s worms prefer hardwood forests. Conifer forests have more acidic soils which, with tough fallen needles, are less digestible than broadleaf litter. Maple, birch, elm, and ash forests are preferred by earthworms over oak and beech leaf litter. Earthworms also feed on live plants, microscopic invertebrates, and fungi, including the mycorrhizae that provide positive services to plant roots. Vermont’s forests most preferred by earthworms are the same forests that support ephemeral wildflowers; a host of songbird species; lush ferns; and gorgeous fall foliage.
The Biology of an Earthworm
Earthworms belong to a large invertebrate category known as the Annelids or Segmented Worms. Their tubular design is divided into segments that can be recognized by any small child. They possess an active circulatory system that runs the length of the worm. A second full-length tube, the coelom, fills with fluid – a skeleton to stiffen the otherwise floppy body. They are highly sensitive to the ambient soil and water chemistry (salt is toxic to them).
Each segment has tiny hairs that help them move through the soil. For a brain, they have two tiny ganglia that control their neural system. Their feeding and digestion are efficient in their tubular design. Their respiratory system exchanges gases through pores in the skin, which also produces a thin protective mucus coating to control their water balance. Earthworms are without eyes but sense light in neurons distributed over their epidermis.
These worms are hermaphrodites; each worm produces eggs toward one end of the body and sperm toward the other. Mating takes place when two worms align to contact their reciprocal sex cells, exchanging sperm. The results are double, and the tiny eggs become fertile. As hermaphrodites, some earthworms may be parthenogenetic, fertilizing their eggs without a mate. Either way, the fertilized eggs in tiny cocoon pouches are very numerous and lead to a high reproductive rate for earthworms.
Thus, the removal of a damaging earthworm population is probably impossible, but preventing the earthworms from moving into hardwood forests is possible by managing those intended for garden compost, mulch, manure, or fishing bait. Remember, these forests were obliterated by the continental glaciers, too, so the native forests that have recovered have no need for earthworm ecology. Thus, for environmental conservation, we must apply steps to keep the worms from disrupting the soil profile in the woodlands of Vermont. I don’t recommend duplicating Joe’s effort, but toddlers’ handling and curiosity may play an earthworm management part. The best information on the impact of earthworms in Vermont’s forests has been assembled in a thorough research study conducted by UVM and the Vermont Department of Forests, Parks, and Recreation, and is available at: https://fpr.vermont.gov/sites/fpr/files/Forest_and_Forestry/Forest_Health/Library/EarthwormsInForests_final.pdf
I recommend it to all Vermont forest landowners. After reading it, I would vote “Foes.”
