We don't often think about what's happening right under our feet, but there's plenty of life down there. One of the most populous and recognizable inhabitants in the soil is earthworms. These invertebrate creatures are neither insects nor arthropods, like most of our other subterranean residents.
Earthworms are annelids. In Latin, annelida means "little rings," and this is a fitting description of worm anatomy. Worms are composed of many small segments fused together. Worms can have between 100 and 150 of these segments, and they can relax or contract to move the worm through the soil.
As earthworms tunnel their way through the soil, they eat it. They also eat bacteria, fungi and other tiny organisms. The waste they leave in their wake, called casts, returns nutrients to the soil.
Read on to learn even more about these totally tubular creatures.
They aren't native, but they are beneficial — mostly
While some earthworm species are native to parts of the United States, none are native to the upper Midwest. However, unlike some other non-native species, most earthworms are beneficial. Although they are not essential for healthy soil, they are usually an indicator that a soil system is well-functioning.
Many of the non-native earthworms in the United States have been here for centuries. Most are native to Asia and Europe and were likely transported inadvertently on ships.
While it's true that most earthworms enhance soil quality, this is not universal. A few worm species are harmful to soil health. One example is the jumping worm, a large earthworm native to Asia. These destructive worms alter the soil structure, damage plant roots and deplete nutrients in the soil. They were first found in Illinois in 2015, and today they have been identified in more than 35 counties, including Will County.
There are thousands of different kinds
If you've seen one worm, you've seen them all, right? It might seem like that, but there is incredible diversity among earthworm species. The world is home to about 7,000 different kinds of earthworms, and they are divided into 23 families. The United States is home to more than 100 different worm species.
Words to know
Abundant: Existing or available in large quantities.
Arthropod: An invertebrate animal, such as an insect, spider or crustacean.
Beneficial: Favorable or resulting in good.
Burrow: A hole or tunnel dug by an animal.
Destructive: Causing great harm or damage.
Diversity: Having variety among types or qualities.
Indicator: Something that shows the state or level of something else.
Inhabitant: A person or animal that occupies a place.
Invertebrate: An animal lacking a backbone.
Populous: Large in number
Subterranean: Existing or done underground.
Unfathomable: Incapable of being understood.
With so many species, there's a lot of variety among worms. Some are only an inch or two long, while others can be measured in feet. The longest ever recorded was 22 feet long when it was discovered in South Africa. Some may come up above the ground from time to time, but others remain below ground for their entire lives.
One thing that's universal among earthworms is the need for moisture. They will die if they dry out, so they need moist soil. When soil becomes too dry, they will typically burrow farther underground. For this reason, they are most abundant in rainy forested areas, although they can live in many habitats both on land and underwater.
There's a whole lot of worms right under your feet
The number of worms living underground is almost unfathomable, but it's safe to say it's a whole lot. In most soils, earthworms are the most populous subterranean resident. So just how many are down below? A single square yard of farmland in the U.S. can have between 50 and 300 worms living in it, and that's the low end of the population scale. A square yard of woodland or grassland can have 500 worms or more living in it. Think about that on a larger scale: There can be more than 1 million worms living in an acre of land.
All those worms fall into three different groups based on what elevation of the soil they inhabit. Epigeic worms live in the soil at surface level. They are usually smaller species and generally cannot survive in soil with low organic content. Below the surface soil, endogeic worms live in the upper level of soil. They mostly eat the organic matter in the soil. Down even further in the soil are the anecic worms, which include nightcrawlers. These worms eat litter in the surface soil that they pull into their burrows.
They have male and female reproductive organs
Earthworms are hermaphrodites, with all worms having both male and female reproductive organs. However, worms usually need a mate to successfully reproduce. They typically mate when the ground is wet from recent rain.
After mating, they use a slimy mucus to create a cocoon to protect the eggs, which are placed in the soil. It can take anywhere from a few weeks to several months for the eggs to hatch, depending on the species, the soil and the weather conditions. When they do hatch, the worms are fully functioning, but they are smaller than adult worms. They will continue to grow as they get older.
They help prevent pollution and improve soil quality
Worms live in the ground beneath our feet, and all their activity down there has far-reaching effects, even helping prevent pollution. When worms move through the soil, they are improving the soil structure. When it rains, water is able to more quickly seep into the ground through the tunnels and loosened soil. Their underground activity also helps plants grow by creating channels for their roots.
Despite these positive benefits, worm burrows also could make it easier for pollutants like pesticides to get into the groundwater. How pollutants seep into and move through the soil is not an exact process, so it's not known whether the activity of worms could have a negative effect on water quality.
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