earthworm in dirt

Are Earthworms Indigenous to the Americas?

A reader wrote to us a while back with a simple question: is the earthworm indigenous to the Americas? (Technically, he asked where “earth worms” are from, using two words, but six of one, half a dozen of the other.) Fortunately, we have some familiarity with this topic, and it is actually quite fascinating. In a sense, earthworms are indigenous to the Americas – that is, certain species of earthworm are native to the Americas – but these earthworms no longer exist. The earthworms that we now have are not indigenous to the Americas, but instead came from Europe. This is all a little complicated, so below we provide an overview of where earthworms come from, and also detail the impact that earthworms had upon their arrival to the Americas.

We first had the occasion to examine this topic when reading press reports about Charles Mann’s book 1493, a book about “how European settlements in the post-Colombian Americas shaped the world,” to borrow the book’s promotional copy. Relying on what other experts have claimed, Mann writes that the earthworms we are all so familiar with were actually not in the Americas several hundred years ago. Rather, earthworms were introduced by Europeans who came to explore the New World.

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As you might have guessed, European explorers and settlers didn’t simply bring jars of worms and purposely release them into the wild. Earthworms were instead introduced inadvertently to the Americas, just as so many other things were (including various species of animals and plants, and, more tragically, infectious diseases). The exchange went both ways, with the Americas contributing plenty to Europe. This great (and in many ways tragic) crossing of cultures is called the Colombian Exchange because its beginning is marked by the arrival of Columbus to the Americas. So, the earthworms we see crawling around are of European descent.

How exactly the earthworms arrived in the Americas is a matter of speculation, but one possibility is that earthworms were carried over in ballasts, the barrels that ships used to maintain balance at sea. Ballasts were often filled with soil, which very likely contained earthworms, and when the barrels were opened, earthworms began to take over the Americas. We use the phrase “take over” purposely because earthworms were able to spread quickly thanks to an abundant food source.

Since earthworms weren’t part of the Americas’ ecosystem prior to the arrival of the Europeans, large amounts of leaf litter, which earthworms consume, accumulated. So, when earthworms arrived, they essentially had a backlog of food waiting to be consumed. And consume they did, which had wide-ranging implications for the Americas’ ecosystem. Several plants that depended on the nutrients found in leaf litter could no longer survive once earthworms starting burning through this particular food supply. By and by, new plants like pine trees began to grow and flourish in the New World once old species died off. Overall, however, earthworms are believed to have decreased plant diversity – their consumption of the nutrient-rich decaying matter found on forest floors altered the ecosystem in the direction of less diversity.

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Thus, earthworms came from Europe and literally altered the landscape. Interestingly, though, earthworms were probably in the Americas long before the arrival of Europeans. However, they likely died off during the glacial age, when glaciers covered much of the land. Earthworms cannot survive in these conditions, and so it took the arrival of Europeans to reintroduce earthworms to the Americas, and by the time this happened, the ecosystem had evolved in such a way that lots of plants thrived off leaf litter.

So, once more, earthworms are indigenous to the Americas – they can rightly call this vast land their native home – but they have not been here continuously. The glaciers took them away and the Europeans, for good or ill, brought them back.

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