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Earthworm Ecological Groups

There are three broad ecological groups of earthworms, distinguished by both color and size. Knowing which ecological groups of earthworm you have can be valuable to know, even if you do not identify the species because each group tends to affect the forest differently. To download a key to ecological groups click here.

major ecological groups

Three major ecological groups of earthworm have been identified based on the feeding and burrowing behaviors of the different species.

Tips for Identifying Ecological Groups

1) Use adult specimens, since juveniles (those without a clitellum) may not be full sized.
2) Use preserved specimens when determining size. The size range listed for each group is based on the length of preserved specimens and you will find that more often than not the earthworms are quite a bit longer, or more stretched out, when they are alive than when they are preserved. If you use live specimens, be sure to keep this in mind when estimating their size.

To find out more about earthworm anatomy click here.

To find out how to preserve your earthworms click here.

Litter and Surface Dwelling Species

Earthworms that live and feed only at the surface, and sometimes only in the litter layer are called epigeic (epi = top; geic = earth).  Epigeic earthworms are small bodied (1-7cm) and reddish-brown in color. There are several species in this group that have been identified in the Great Lakes region and two are very common and widespread (see below).  They are generally found in the surface litter or just beneath it, on top of the mineral soil, but rarely burrow deeply into soil. Epigeic earthworms range in size from 1 to 7 cm long as adults. They are pigmented earthworms, with a red-brown pigment in their skin which tends to be darker on the back and head so the belly is often more beige in color. Scientists think this pigmentation may provide protection from predators and from UV rays, which they are very sensitive to. 

Dendrobaena octaedra

 

This very small species, Dendrobaena octaedra, lives strictly in the litter layer and feeds primarily on fungi and bacteria found there. As a result, this species has less of an affect on the forest floor than many other species of earthworms.

Lumbricus rubellus

 

This moderately sized species, Lumbricus rubellus, is commonly sold as fishing bait and is often called “leaf worms” or “beaver tails”. It lives and feeds in the litter layer and the top few inches of mineral soil, so is sometimes referred to as an epi-endogeic species. However, it does not form permanent burrow systems like most fully endogeic species do.  It feeds primarily on surface litter and organic material found in the top layer of mineral soil. It also can be found grazing in the rhizosphere of plant roots, probably feeding on the fungi and bacteria found there.  As a result if its activities, this species can rapidly remove the forest floor when it invades and is associated with big changes in the forest soils and plant communities.

Soil Dwelling Species

Earthworms that live and feed in the mineral soil, rather than at the surface, are called endogeic (endo = in or internal; geic = earth). Adult endogeic earthworms can range from small to large in size (2-12cm long), depending on the species, but are identified by their lack of skin pigmentation. As a result of having no skin pigmentation, they may appear gray-blue, yellowish, pink or whitish in color. If their gut is full of dark colored soil, they may appear very dark, but don’t be fooled! They lack the red-brown skin pigmentation on the back and head that epigeic and anecic species. Endogeic earthworms live and feed in the mineral soil layers. They ingest mineral soil and digest the organic material and microorganisms found there. They rarely feed at the surface, but may be found under logs and rocks and are often seen at the surface after heavy rains when the surface moisture makes it possible for them to disperse without drying out. Endogeic species on earthworms form a network of permanent burrow systems in the mineral soil, down to about 50 cm deep, that allows them to easily move around accessing the soil resources. Several species endogeic earthworms have been identified across the Great Lakes region, three are relatively common (see below).

Aporrectodea caliginosa

This large, unpigmented species, Aporrectodea caliginosa, is very common in gardens and agricultural fields and is sometimes referred to as “angle worms”. This species is responsible much of the mixing of the mineral soil and contributes to the development of the thick layer of dense, black soil (A horizon) after earthworms invade.

Aporrectodea rosea

This small, unpigmented species, Aporrectodea rosea, is also common in gardens and agricultural fields. Live specimens of this species can sometimes be distinguished by a very pink head and tail. While this species is common across the region, its populations seem to be more patchily distributed, so it’s not clear what their role is in the overall impacts of exotic earthworm invasions.

Octolasion tyrtaeum

This moderately sized, unpigmented species, Octolasion tyrtaeum, is less common than the Aporrectodea species, but is found patchily across the Great Lakes region (along with a closely related species, Octolasion cyaneum). Live specimens of this species can sometimes be distinguished by a bright red-orange clitellum.

Deep Burrowing Species

Earthworms that burrow very deeply (down to 2 meters) but feed on fresh surface litter are called anecic (not sure where that term comes from!). Anecic earthworm species are very large (8-15cm long) and strongly pigmented with the same reddish-brown in color seen in epigeic species. Anecic species of earthworms form vertical, unbranching burrows down to 2 meters into the soil but feeds on fresh surface litter which it pulls down into its burrow. Anecic species of earthworm have the potential to eat huge amounts of litter every year and as a result can have big effects on the forest floor when they invade. In the Great Lakes region, there is only one anecic species of earthworm, the common night crawler (Lumbricus terrestris).

Lumbricus terrestris

This very large, pigmented species, Lumbricus terrestris, is by far the most common earthworm sold for fishing bait across the Great Lakes region. It is also known as the “Canadian Crawler” since the United States imports millions from Canada annually for the fishing industry, but it is an exotic, European species. An established population of nightcrawlers can consume all the leaf litter produced in a forest each year. By preventing any recovery of the forest floor in sites where it has invaded, this species contributes to big long-term effects of earthworm invasion in the forest ecosystem.

Lumbricus terrestris

As Lumbricus terrestris pulls leaves down into its burrow, the leaf stems (called petioles) and fragments of leaves, pile up at the borrow entrance. In addition, nightcrawlers deposit cast material (earthworm poop) at the surface surrounding their burrow entrance. The result is called a midden, a pile of cast material mixed with leave stems and debris about 5 cm in diameter and perhaps 1-2 cm high. Since each burrow contains one individual, counting the number of middens in a given area can give you a good estimate of the number of nightcrawlers present.

 

Effects of Different Ecological Groups of Earthworms in the Forest

Earthworms in different ecological groups have different feeding preferences and burrow systems and this affects the kinds of impacts they have in the forest.  Epigeic species primarily affect the forest floor, endogeic species primarily affect the mineral soil horizons and anecic species can affect both the forest floor and the mineral soil horizon. As you might expect, the biggest effects come when more than one ecological group is present in a forest. However, research has shown that the changes in forest soils, the forest floor, and the understory plant communities are much greater when all three ecological groups than simply the sum of their effects alone. Researchers have examined these synergistic effects by using experiments where earthworms are introduced alone and in combination. This is one reason that even if some earthworm species may be present in a site, by preventing the introduction of even more species you may be able to limit the effects of earthworm invasion to some degree.

To find out more about research on earthworm effects click here.

To download a key to ecological groups click here.

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