Photo: Male American Robin (Turdus migratorius) eating an earthworm. Source: Wikipedia.

To be more specific, are earthworms natural?  For the chronically busy and distracted, the short answer to that question is – absolutely not!  Earthworms are not natural!  Not even a little bit!  At least they’re not natural if they’re non-native, introduced earthworms that are devouring the forest floor and radically changing the ecology of some hardwood forests in the northern United States.

Science Underfoot

Surely I must be kidding?  What could be more natural and beneficial than the common earthworm, slowly churning through our compost piles and garden soils, dutifully converting organic waste into productive elements of soil?  They’re everywhere.  You see them when you dig in the garden.  Many birds and other animals love to eat them. After heavy spring and summer rains, they cover our sidewalks and roads, and squish under the feet of the careless.

On the other hand, maybe this is just another Commie, liberal, university, leftist, socialist, hippie, environmentalist plot to alarm a gullible public and scare them into donating more money to “save the world.”  Unfortunately, the truth of the matter isn’t quite that simple.  Many introduced earthworms are not natural in their environmental effects and they are wreaking ecological havoc in some North American forests. Forget environmental opinion. Let’s talk science.  But first, a bit of history.

Cleopatra and Julius Caesar. Painting by Jean-Léon Gérôme. Source: Wikipedia.

Cleopatra – Worm Worshipper

Cleopatra VII was the last pharaoh of ancient Egypt, one of the most famous female rulers in history, and apparently was somewhat of a worm worshipper based upon reports that she considered earthworms to be sacred:

“Did you know the ancient Egyptians were the first to recognize the beneficial status of the earthworm? Cleopatra (69 – 30 B.C.) recognized the earthworm’s contribution to Egyptian agriculture and declared them to be sacred. Removal of earthworms from Egypt was punishable by death. Egyptian farmers were not allowed to even touch an earthworm for fear of offending the god of fertility. A 1949 study by the USDA confirmed that the great fertility of the soil in the Nile valley was due in large part to the work of earthworms.”  Source: Did You Know……. Earthworms

However, Cleopatra is hardly alone in lauding the importance of earthworms to humans and the world.  Aristotle called earthworms “the intestines of the soil” while Charles Darwin studied worms for 39 years and concluded that: “It may be doubted whether there are many other animals which have played so important a part in the history of the world as these lowly organised creatures.”

“It will be difficult to deny the probability that every particle of earth forming the bed from which . . . old pasture land springs has passed through the intestines of worms.”

No one knows for sure when non-native earthworms were introduced to North America, but for the sake of convenience, let’s blame the first pilgrims and colonists for starting the process of bringing in passenger and cargo ships containing rocks and soil for ballast. Introduced earthworms probably got their toehold on the continent when these materials were dumped ashore, and when plants with soil and other cargo accompanied passengers to North America.

Dang Pilgrims! There goes the neighborhood! And there goes some of Mother Nature’s magnificant hardwood forests as well.

It is just possible that John Rolfe was responsible for the worms—specifically the common night crawler and the red marsh worm, creatures that did not exist in the Americas before Columbus…

Most people know him today, if they know him at all, as the man who married Pocahontas. A few history buffs understand that Rolfe was one of the primary forces behind Jamestown’s eventual success. The worms hint at a third, still more important role: Rolfe inadvertently helped unleash a convulsive and permanent change in the American landscape….

As the colonists bitterly came to realize that Virginia had no gold and that the Indians weren’t going to selflessly provide them with all the food they needed, they began to mold the land to their needs. Unable to adapt to this foreign landscape, they transformed it into a place they could understand. In doing so, they unleashed what would become a multilevel ecological assault on North America. Their unlikely weapons in this initial phase of the campaign: tobacco, honeybees, and domestic animals.  National Geographic: America Lost and Found

There is little doubt that the earthworms that you find in your yard, garden, and crawling across sidewalks and streets after rains are all non-native, introduced earthworms.  In reality, they don’t belong here.

Our colleagues at the University of Idaho, Yaniria Sanchez-de Leon and Dr. Jodi Johnson-Maynard, have conducted one of the few detailed studies of earthworms in the Palouse region, and they found that three introduced species (Aporrectodea trapezoides, A. tuberculata, and Lumbricus terrestris – often called the common earthworm, or nightcrawler) dominated the earthworm community, whether samples were taken in planted grasslands in an agricultural setting or in native prairie remnants.

Earthworm Research at Washington State University

In an earlier study of earthworms, in 1999, WSU researchers Mary Fauci and Dr. David Bezdicek, of the Department of Crop and Soil Sciences, sampled earthworms at 46 sites in mostly farm fields and some grasslands in Palouse Prairie, but all of the worms they found were of introduced species. The highest earthworm diversity (8 species of non-native earthworms) occurred right here on the campus of Washington State University in the agronomy teaching gardens, perhaps illustrating the potential to bring in non-native earthworms in cultivated gardens when using plant materials originating from different sources.

So where are all the native earthworms? And isn’t one worm as good or bad as another? First, we don’t have as many native worms as you might suspect.  And second, not all worms are created equal. While some introduced worms may have beneficial effects in the human-created habitats of compost piles, gardens, and agricultural systems, some of our native forest habitats evolved without any earthworms at all.

Distribution of continental ice sheets (white area) during the last Pleistocene glaciation. Source: Wikipedia.

The last Pleistocene glaciation ended some 12,000 years ago (see map). Any native earthworms were destroyed by the permanent ice cover and bordering permafrost, causing distributions to be pushed to the south.  It is easy to see from the map of continental ice sheets that the northeastern and north central United States were devoid of earthworms when the ice finally started melting and forests began their march northward following the retreating glaciers.

Forests in these regions have had thousands of years to evolve and develop communities of trees, shrubs, and understory plants all without any native earthworm community. And now the bad guys show up, and that would be us by the way, not the earthworms!

Worms As Ecological Engineers

Scientists have suspected for some time that introduced earthworms were having negative ecological impacts in these particular North American forests. As introduced earthworms invaded these forests, a series of largely negative ecological changes cascaded throughout the system, changing the structure, function, and even the overall appearance of the forest.

Not all worms are created equal because different species of worms occupy different habitats and eat different organic materials as food.  Some species of worms specialize by living in different layers or types of soil, or specialize in eating different foods such as animal manure vs. decaying logs, vs. leaf and organic matter on the soil surface.

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