This level includes both herbivores and carnivores: nematodes, protozoa, rotifers, soil flatworms, springtails, some types of mites, and feather-winged beetles.
Nematodes are tiny, cylindrical, often transparent microscopic worms, and are the most abundant of the physical decomposers. A handful of decaying compost can contain several million nematodes. Under a magnifying lens, nematodes resemble fine human hair. They can be classified into three categories: 1) those that live on decaying vegetation; 2) those that are predators on other nematodes, bacteria, algae, protozoa, etc.; and 3) those that can be serious pests in gardens where they suck the juices of plant roots, especially root vegetables.
Protozoa are the simplest form of animal organism. Even though they are single-celled and microscopic in size, they are larger and more complex in their activities than most bacteria. Protozoa obtain their food from organic matter in the same way bacteria will, but because they are present in far fewer numbers than bacteria, they play a much smaller part in the composting process.
Rotifers are minute worms which usually have one or two groups of vibrating cilia on the head. Their bodies are round and divisible into three parts: a 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 which adheres to plant substances where they feed on microorganisms.
Flatworms are, for the most part, general scavengers that graze on a wide variety of things including animal matter. As their name implies, flatworms are flat and usually quite small in their free-living form. Most flatworms are carnivorous and live in films of water within the compost structure.
Springtails are extremely numerous in compost. They are very small wingless insects and can be distinguished by their ability to jump when disturbed. They run in and around the particles in the compost and have a small spring-like structure under the belly that catapults them into the air when the spring catch is triggered. They chew on decomposing plants, pollen, grains, and fungi. They also eat nematodes and droppings of other arthropods and then meticulously clean themselves after feeding.
Feather-winged beetles are the smallest of all beetles and possibly of all insects. These beetles are distinguished by their feather-like wings. Some are blind and most live under bark in forests and woodland. Not surprisingly they go unnoticed. Most species feed on fungi.