Charles Darwin didn’t spend his final decades studying giant squids or exotic primates. He watched worms. For 39 years, the biologist who gave us natural selection tracked the habits of earthworms, eventually publishing The Formation of Vegetable Mould, Through the Action of Worms, With Observations on their Habits in 1881. It sounds tedious. It isn’t.

Darwin argued that these “lowly organized creatures” had played a more significant role in the history of the world than many other animals. He was right. Their sheer numbers dictate the health of local ecosystems, though recent science suggests their impact isn’t always benign.

The Biomass of Underground Life

There are over 3,000 species of earthworms globally. They range from tiny 0.4-inch crawlers to massive 9-foot monsters, the latter often reported in anecdotal sightings across various continents. But size isn’t the story; density is.

In a single hectare of land, you might find between 500,000 and 2 million individual worms. To put that in perspective, the total wet biomass of these subterranean dwellers can equal ten times the weight of all animals living aboveground on that same plot. That is an immense amount of biological machinery working below your feet.

Invasive Species and the Ice Age Gap

We assume these worms are native everywhere. That is false. In many northern regions of North America, earthworms were absent just a few hundred years ago. The last ice age wiped out most native species. What you see today in temperate woodlands are immigrants from Europe and Asia.

They arrived hitching rides in the soil of imported plants during the 19th and 20th centuries. Since then, they have flourished. Humans aided their spread by releasing them as fish bait or accidentally transporting them via car tires. This rapid colonization has altered soils that evolved without them, a change that researchers are only now beginning to fully understand.

Beyond Regeneration

Most people know earthworms can regenerate segments. Some joke they play pinochle on a corpse’s snout. Both are trivial facts compared to what they actually do. Their ability to reshape soil structure, cycle nutrients, and influence plant growth is far more complex.

“It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organized creatures.”

We are still uncovering the full scope of their influence. How do these immigrants affect native plant communities that never evolved with them? What happens to the carbon sequestration rates in soils suddenly teeming with new diggers? The answers are messy. They challenge our simple view of nature as a static balance.

The ground beneath you is not inert. It is moving. And it is largely thanks to creatures we often mistake for dirt.