Why do so many earthworms appear after it rains?

By Kevin Byrne, AccuWeather staff writer

Following a heavy rain, there are often some noticeable changes to your surroundings, whether it’s a peculiar smell in the air or a change in temperature.

In some cases, in addition to dodging puddles, you might have to avoid long, stretched-out earthworms that have surfaced from under the ground.

But why do these earthworms, which are an integral part of the soil ecosystem, frequently pop up above ground after it rains? Researchers from Penn State University say they move to the surface to avoid suffocation.

earthworms in soil

(Photo/ John Shepard/iStock /Getty Images Plus)

“Earthworms can live under submerged conditions if the oxygen content of the water is high enough. In most cases, however, earthworms will die when exposed to excessive waterlogging.”

Earthworms and soil have a symbiotic relationship, according to Izolda Trakhtenberg.

Trakhtenberg, also known as the “Earth Lady,” has worked in soil science and Earth Science Education at NASA for over 15 years. Today, she facilitates workshops and assemblies about soil science to students of all ages.

“Worms play a role in aerating the soil as they burrow through it,” Trakhtenberg said. “And they receive the benefits of that aeration since they breathe air in through their moist skin. If the soil is too wet, they can't access sufficient air. That's one of the reasons they rise to the soil surface after heavy rains.”

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Earthworms breathe through their skin. Too little moisture around them in the soil, and the worms can't breathe. Too much moisture around them, and there isn't enough air for them to breathe, she said.

According to the Penn State researchers, there are more than 1,000 types of earthworms, but mainly they are classified into three types: litter dwellers, topsoil dwellers and subsoil dwellers.

An ideal soil for growing food contains 45 percent minerals, 5 percent organic matter and 25 percent each of air and water, Trakhtenberg explained. Worms need the air space in the soil to get the oxygen they need to survive, she added.

"If the soil becomes over-saturated with water, that can harm both plants and the billions of animals that call the soil home. Also, a saturated soil might cause flooding and run-off on the surface," she said.

Others have different theories as to why earthworms come above ground.

Jeff Neal, operator of The Critter Depot, said there are several known theories, ranging from the vibration of raindrops disrupting the worms' comfort level and forcing them to move to a migration strategy.

"It's difficult to travel in dry, compact soil," he said. "So when there's excess water, it helps them travel greater distances."

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