Physical Decomposers

MiteThe larger fauna in the heap include mites, millipedes, centipedes, sow bugs, snails, slugs, spiders, springtails, beetles, ants, flies, nematodes, flatworms, rotifers, and most important, earthworms. Collectively, these are called the physical decomposers since they bite, grind, suck, tear, and chew the materials into smaller pieces, making them more suitable for the chemical work of the microscopic decomposers.

All of the organisms, from the microscopic bacteria to the largest of the physical decomposers, are part of a complex food chain in your compost pile. They can be categorized as first-, second-, and third-level consumers, depending upon what they eat and by what they are eaten.

First-level consumers attract and become food of second-level consumers, which are in turn consumed by third-level consumers. The organisms comprising each level of the food chain serve to keep the populations of the next lower level in check, so a balance can be maintained throughout the compost. Soil ecologist Dr. Daniel L. Dindal gives an example in Ecology of Compost:

Mites and springtails eat fungi. Tiny feather-winged beetles feed on fungal spores. Nematodes ingest bacteria. Protozoa and rotifers present in water films feed on bacteria and plant particles. Predaceous mites and pseudoscorpions prey upon nematodes, fly larvae, other mites and collembolans. Free-living flatworms ingest gastropods, earthworms, nematodes and rotifers. Third-level consumers such as centipedes, rove beetles, ground beetles and ants prey on second-level consumers.

The following is a rundown of some of the larger physical decomposers that you may find in nearly any compost pile. Most of these creatures function best at medium or mesophilic temperatures, so they will not be in the pile at all times.


mites Mites are related to ticks, spiders and horseshoe crabs because they have in common eight leg-like, jointed appendages. They can be free-living or parasitic, sometimes both at once. Some mites are small enough to be invisible to the naked eye, while some tropical species are up to 1/2 inch in length. Mites reproduce very rapidly, moving through larval, nymph, adult and dormant stages. They attack plant matter, but some are also second-level consumers, ingesting nematodes, fly larvae, other mites and springtails.


millipede The wormlike body of the millipede has many segments, each except the front few bearing two pairs of walking legs.The life cycles are not well understood, except that eggs are laid in the soil in springtime, hatching into small worms. Young millipedes molt several times before gaining their full complement of legs. When they reach maturity, adult millipedes can grow to a length of 1 to 2 inches. They help break down plant material by feeding directly on it.


Centipede Centipedes are flattened, segmented worms with 15 or more pairs of legs — one pair per segment. They hatch from eggs laid during the warm months and gradually grow to their adult size. Centipedes are third-level consumers, feeding only on living animals, especially insects and spiders.

Sow Bugs

The sow bug is a fat-bodied, flat creature with distinct segments. Sow bugs reproduce by means of eggs that hatch into smaller versions of the adults. Since females are able to deposit a number of eggs at one time, sow bugs may become abundant in a compost pile. They are first-level consumers, eating decayed vegetation.

Snails and Slugs

slug Both snails and slugs are mollusks and have muscular disks on their undersides that are adapted for a creeping movement. Snails have a spirally curved shell, a broad retractable foot and a distinct head. Slugs, on the other hand, are so undifferentiated in appearance that one species is frequently mistaken for half of a potato. Both snails and slugs lay eggs in capsules or gelatinous masses and progress through larval stages to adulthood.

snail Their food is generally living plant material, but they will attack fresh garbage and plant debris and will appear in the compost pile. It is well, therefore, to look for them when you spread your compost, for if they move into your garden, they can do damage to crops.


spider Spiders, which are related to mites, are one of the least appreciated animals in the garden. These eight-legged creature are third-level consumers that feed on insects a0nd small invertebrates and they can help control garden pests.


springtailSpringtails are very small insects, rarely exceeding 1/4 inch in length. They vary in color from white to blue-grey or metallic and are mostly distinguished by their ability to jump when disturbed. They feed by chewing decomposing plants, pollen, grains and fungi.


The rove beetle, ground beetle and feather-winged beetle are the most common beetles in compost. Feather-winged beetles feed on fungal spores, while the larger rove and ground beetles prey on other insects as third-level consumers.

Beetles are easily visible creatures with two pairs of wings, the more forward-placed of these serving as a cover or shield for the folded and thinner back-set ones that are used for flying.

A beetle’s immature stage is as a grub that feeds and grows during the warm months. Once grubs are full grown, they pass through a resting or pupal stage and change into hard-bodied, winged adults.

Most adult beetles, like the larval grubs of their species, feed on decaying vegetables, while some, like the rove and ground beetles, prey on snails, insects and other small animals. The black rove beetle is an acknowledged predator of snails and slugs. Some people import them to their gardens when slugs become a problem.


antAnts feed on a variety of material, including aphid honeydew, fungi, seeds, sweets, scraps, other insects and sometimes other ants. Compost provides some of these foods, and it also provides shelter for nests and hills. They will remain, however, only while the pile is relatively cool.

Ants prey on first-level consumers and may benefit the composting process by bringing fungi and other organisms into their nests. The work of ants can make compost richer in phosphorus and potassium by moving minerals from one place to another.


Many flies, including black fungus gnats, soldier flies, minute flies and houseflies, spend their larval phase in compost as maggots. Adults can feed upon almost any kind of organic material.

All flies undergo egg, larval, pupal and adult stages. The eggs are laid in various forms of organic matter. Houseflies are such effective distributors of bacteria that when an individual fly crawls across a sterile plate of lab gelatin, colonies of bacteria later appear in its tracks. During the early phases of the composting process, flies provide ideal airborne transportation for bacteria on their way to the pile.

If you keep a layer of dry leaves or grass clippings on top of your pile and cover your garbage promptly while building compost, your pile will not provide a breeding place for horseflies, mosquitoes or houseflies that may become a nuisance to humans. Fly larvae do not survive thermophilic temperatures. Mites and other organisms in the pile also keep fly larvae reduced in number. Though many flies die with the coming of frost, the rate of reproduction is so rapid that a few survivors can repopulate an area before the warm season has progressed very far.

Nematodes, Flatworms and Rotifers

nematodesNematodes, or eelworms, plus free-living flatworms and rotifers all can be found in compost. Nematodes are the microscopic creatures that can be classified into three categories: (1) those that live on decaying organic matter; (2) those that are predators on other nematodes, bacteria, algae, protozoa and so on; and (3) those that can be serious pests in gardens, where they attack the roots of plants.

Flatworms, as their name implies, are flattened organisms that are usually quite small in their free-living form. Most flatworms are carnivorous. They live in films of water within the compost structure.

Rotifers are small, multicellular animals that live freely or in tubes attached to a substrate. Their bodies are round and divisible into three parts: head, trunk and tail. They are generally found in films of water and many forms are aquatic. The rotifers in compost are found in water that adheres to plant substances where they feed on microorganisms.


If bacteria are the champion microscopic decomposers, then the heavyweight champion is doubtless the earthworm. Pages of praise have been written to the earthworm, ever since it became known that this creature spends most of its time tilling and enriching the soil. The great English naturalist, Charles Darwin, was the first to suggest that all the fertile areas of this planet have at least once passed through the bodies of earthworms.

The earthworm consists mainly of an alimentary canal that ingests, decomposes and deposits casts continually during the earthworm’s active periods. As soil or organic matter is passed through an earthworm’s digestive system, it is broken up and neutralized by secretions of calcium carbonate from calciferous glands near the worm’s gizzard. Once in the gizzard, material is finely ground prior to digestion. Digestive intestinal juices rich in hormones, enzymes and other fermenting substances continue the breakdown process. The matter passes out of the worm’s body in the form of casts, which are the richest and finest quality of all humus material. Fresh casts are markedly higher in bacteria, organic material and available nitrogen, calcium, magnesium, phosphorus and potassium than soil itself. Earthworms thrive on compost and contribute to its quality through both physical and chemical processes.

Both male and female reproductive systems are in one earthworm, but fertilization can occur only between two separate individuals during copulation. The fertilized eggs are deposited and contained in a cocoon, out of which the young worms emerge after 8 to 10 days.

Since earthworms are willing and able to take on such a large part in compost making, wise gardeners adjust their composting methods to take full advantage of the earthworm’s talents.