The Difference Between Red Wigglers and Other Earthworms
Many people are surprised to learn that there are different "kinds" of earthworms.
There are many species of earthworms, but almost all of them can be classified into three groups based on their environmental or ecological niche.
Composting worms are 'Epigeic' Earthworms.
Epigeic, meaning "upon the earth" is how composting worms are classified.
They prefer to live only the top several inches of the soil profile - in decaying organic matter rather than soil itself. Because they prefer to live in leaf litter, they do not make burrows like other kinds of earthworms.
Being so close to the surface, comparatively, they are more pigmented than other earthworms, which is where the "Red" in "Red Wiggler" comes from.
Composting worms have adapted to eat as fast as possible and reproduce as fast as possible because there is lots of available food, but living only in the top few inches of the soil profile leaves them exposed to environmental changes more than other earthworms.
If their area goes through a drought, the organic matter layer, where these guys live, is going to be the first to dry out. Their thought process is "better eat all this food as fast as possible and lay as many cocoons as possible in case things dry up and wipe us out!"
Or so I assume that's part of their thought process, I haven't mastered worm telepathy quite yet. (Note to self: Pitch Earth-Man to Marvel. Just like Aquaman, but with earthworms instead.)
Additionally, they don't mind roommates. Composting worms can share the same cubic foot of material with thousands of other worms.
This is why these worms do so well in composting contexts. They're designed for it.
Endogeic and Anecic Earthworms - The Classic "Nightcrawlers"
The kinds of earthworms that do prefer to live in the soil, are Endogeic and Anecic Earthworms. This means "within the earth" and "emerge from the earth" respectively. These are the ones you see in your yard as you are digging, or you find after a heavy rain.
They make burrows throughout the soil that can be multiple feet deep, and are the "earth worker" earthworms.
They can grow to be quite massive compared to composting worms. They are built to tunnel through soil. They need to have a lot of muscle to do that.
In the North Americas, these Anecic earthworms are invasive and do harm to our forest soil profiles by mixing the different horizons together.
They don't do very well in composting contexts as they prefer permanent burrows, aren't adapted to live in that much organic matter, don't eat or reproduce quickly, and don't like being confined and sharing space with other earthworms.
This infographic helps put it all together.
