The glaciers that crawled across Canada into the northern tier of the lower 48 states during the most recent ice age wiped out earthworms in those areas. In other parts of the U.S., you may find native earthworm species, but the worms living in the regions scoured by glaciers are invaders from overseas, brought here intentionally by early settlers on the assumption that the worms would improve the soil, or carried accidentally in shipments of plants or even in dirt used as ballast in ships.
The northern forest evolved after the glaciers retreated, yielding an ecosystem that does not benefit from earthworms. These forests require a deep layer of slowly decomposing leaves and other organic matter called “duff” that overlays the soil. When earthworms invade these forests, they quickly eat up the duff, with the result that nutrients become less available to young, growing plants and the soil, instead of aerating and loosening, becomes more compact.
The combined effects of such developments have resulted in damage to trees such as sugar maples and to many forest herbs and understory plants, such as trillium, rare goblin ferns, trout lilies and other forest-floor species. In some areas, oak forests have been overrun by buckthorn, and in others the presence of earthworms has allowed the invasion of Japanese barberry.
As duff disappears, so do the insects and other small creatures that depend on it for survival, with the result that animals such as salamanders lose a key food source and face population declines. Earthworm burrows also may speed the passage of water through forest soil, another change that might be a benefit to farmland or a garden with compacted soil but that is a negative in a northern forest.
Charles Darwin published The Formation of Vegetable Mould Through the Action of Worms in 1881. It was his last scientific book, and his most successful. In it, he explains that the very ground we walk upon has passed through the bodies of worms and emerged as castings. He also estimated that there are more than 53,000 worms at work in any given acre of land, and reported that they had turned a rocky field behind his uncle’s house into smooth soil over the course of many years. He was fascinated by the work of the earthworm, which he called an “unsung creature which, in its untold millions, transformed the land as the coral polyps did the tropical sea.” On the surface, the study of earthworms seems to have little to do with the work on evolution and natural selection that made him famous. But this book, too, was about the inexorable processes of nature that, over long spans of time, can bring about dramatic changes.
The book started as a paper, which Darwin presented before the Geological Society of London in 1837. In 1842, Darwin spread a layer of chalk fragments over a pasture near his house and observed the worms’ effect on it for almost 30 years. He placed a large, flat stone – which he dubbed the worm stone-in a field and measured the movement of soil as the worms digested the earth beneath the stone. He also kept worms inside the house, examining the effects of bright light and sound. He figured out through trial and error what the worms’ favorite food was: carrots. He was fond of the worms, which were unmoved by art or music – much as Darwin himself remained unmoved by the arts.
By 1881, Darwin’s health was failing, and he remarked to a friend that he wanted to complete his book on worms before he joined them in the local cemetery. He pushed his publisher to bring the book to press as soon as possible. When it was first published, the work of 44 years, Worms was a best-seller, and Darwin received a surprising amount of fan mail. He died six months after the book was published.
-The Writer’s Almanac, October 10, 2016