After it rains, earthworms often appear on sidewalks and roads. We step around them, or sometimes over them, without thinking very much about the small creature in front of us.
But an earthworm can tell us something about a much bigger world.
Earthworms spend almost their entire lives inside soil. As they move, they create tiny passages that allow air and water to travel through the ground. They eat dead leaves and other organic material and return nutrients to the soil. Their activity changes the structure of the soil and helps create conditions where plants and many other organisms can live.
Scientists sometimes call earthworms ecosystem engineers because they do not simply live in their environment. They help shape it.
They can also tell us something about the condition of the soil itself.
Earthworms live in constant contact with the ground around them. Changes in moisture, temperature, chemicals and pollution can affect their survival and reproduction. Because they are sensitive to these changes, scientists can study earthworm populations as one way to understand whether soil is healthy or under stress. They have even been used to study contamination from heavy metals.
This matters because soil is easy to overlook.
We see trees, flowers, animals and rivers. Soil is simply what lies underneath them. Yet healthy soil stores water, cycles nutrients and supports much of the food we eat. A surprising amount of life exists below the surface where we rarely look.
Earthworms are not a perfect measure of soil health. In some ecosystems where they are not native, introduced earthworms can actually disrupt the natural balance. Nature is rarely as simple as saying that more of one species is always better.
But that may be the more interesting lesson.
The health of our planet is often revealed by relationships between small things. A worm, a handful of soil, a plant root and millions of microorganisms may be part of the same living system.
We tend to notice environmental change when something dramatic happens. A forest disappears. A river dries up. A species becomes endangered.
Sometimes, though, the earlier signals may be much quieter.
We may just need to look beneath our feet.
Daniel Kim, April 2025