Charles Darwin’s Affair with Earth Worms

(Featured image courtesy of Gilles San Martin, Flickr user.)

We always attribute On the Origin of Species as Charles Darwin’s profound accomplishment and stride in advancing scientific thought—to include biology in the context of evolution, specifically.  However, the guy kept busy after his most controversial book. He watched dirt form.

I began to read a book on the soil in honor of the United Nation’s International Year of the Soil, and oddly enough, the book launched into biology. I could learn the principles entwining human history with the ground, but David R. Montgomery saw into the future and knew a biologist would read his Dirt: the Erosion of Civilizations. Such an odd beginning prompted my interest in this book and some complimentary online research.

As a true naturalist, Darwin took advantage of the abandoned farmland around his household and observed as Nature took over. Rocks disappeared. Worms shoved dirt to the surface. But what was really going on? He hypothesized that worms develop new layers of soil. Little earthworms could provide fresh earth on the surface of the countryside. By 1881, six months before his death, he wrote an entire book on the topic.

Agriculture heavily impacts local earthworm populations. A plowed cornfield can have 10 individuals per square meter while a manure-fertilized dairy cattle pasture can have up to 1,300 worms per square meter. (Photo courtesy of Yun Huang Yong)

Today earthworms are known as ecosystem engineers. Worms can eat organic material such as leaf litter and mineral content such as gravel and break them down into healthy soil by the time they are excreted. An oversimplification perhaps, but it’s an amusing image. An image of a worm-shaped engineering firm combined with a manufacturing facility isn’t as amusing.

Darwin noticed the rocks that were on his property before his travels were gone when he returned. After launching his worm studies he dug to show the soil profile to see the rocks inches below the surface. After further studies, including constructing his own isolated worm farms to measure how much the worms ate and ejected, he concluded the critters created a few inches of soil per century.

earthworm cast
An example of what to look for on the surface of evidence for fresh “soil” samples i.e. earthworm casts. (Photo courtesy of U.S. Dept. of Agriculture’s Natural Resources Conservation Service, South Dakota)

Earthworms live within the first few feet below the ground—even the first few inches if that is all that is available before the bedrock. Darwin referred to them as Nature’s plows because the passages they formed allowed subterranean and surface gases to exchange. Water could more easily percolate to deeper plant roots. Meanwhile, the worm poop, known more technically as casts, is fodder for rich and yet not fully quantified populations of bacteria and fungi. The bacteria and fungi then release the mixture of organic and mineral nutrients that plants can utilize, and support animal life above.

Irrelevant to soil science but equally interesting, Darwin also experimented with behavioral responses to music and food. The worms would not respond to the piano unless directly in contact with the instrument to sense the vibrations (a common sense in invertebrates and regarded as separate from sound). They also preferred wild cherries to carrots.


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