Under a leafy canopy, the shaded forest floor is a rich ecosystem teeming with life. Here in the leaf litter, millions of small organisms – fungi and bacteria, springtails and mites, spiders and centipedes – are all part of a rich food web. These busy creatures have an important role in the flow of energy through the forest, for many of them feed on dead plant and animal debris, releasing the nutrients so other living things can grow and thrive.
All the leaves, twigs, feathers, insect parts, and other debris that falls on the forest floor form the leaf litter, a very important part of the forest. This blanket of dead organic matter provides homes and nesting material for many small mammals and birds. It protects young seedlings, keeps the soil from being washed away, and helps to hold moisture and moderate temperature. It is also home to millions of small, little-known organisms that perform a big task.
About two tons of debris falls on an acre of deciduous forest every year, yet we rarely find more than a few inches of leaf litter. What happens to it and why doesn’t it build up? As each leaf, twig, or feather falls to the ground, it is attacked by a host of decomposers, organisms that break down dead plant and animal matter. Chewed and shredded by tiny animals like mites and millipedes, ground up in the intestines of worms, digested by bacteria and fungi, the litter is reduced to smaller and smaller fragments until finally the nutrients return to the soil.
Exploring the leaf litter layer by layer gives us clues about the process of decomposition. On top we find leaves that are dry and brown but still largely intact. Under these, the leaves are damper and have holes where they’ve been chewed. Bacteria and fungi coat the surfaces, beginning to break them down chemically, while mites, millipedes, and other tiny invertebrates chew holes or shred them as they eat. Now the bacteria and fungi find places to get inside the leaves. Under this layer we may find skeletonized leaves, in which only the intricate webbing of veins remains intact. Next, we come to a layer of ragged bits and pieces of leaves, dark in color and slimy from a coating of bacteria. Under the leaf litter is a rich, spongy layer of soil called humus, composed of tiny fragments of organic matter, all that remains after the decomposers have done their work.
Among the damp leaves, a tangle of white, root-like fungal threads, called hyphae, can often be seen. Unlike plants, fungi cannot make their own food since they lack chlorophyll. Instead, their hyphae, sometimes forming a fuzzy white mat, secrete chemicals that dissolve dead wood and leaves and absorb nutrients from them. Mushrooms are the ephemeral fruiting bodies of the fungus, sprouting up only when the conditions are right to put out spores. Many miles of fungal hyphae thread their way throughout the leaf litter, though most are too small to see without magnification. They perform an important function, breaking down tougher materials such as the lignin in wood that other organisms like insects can’t digest.
As we sift through the leaf litter, we find numerous small invertebrates, animals without backbones, like earthworms, snails, slugs, and arthropods, invertebrates with hard outer skeletons and jointed legs, such as insects, spiders, mites, and millipedes. The millipede has a body like a long tube made of many segments. As its name implies, it has numerous legs, two pairs on each body segment – though none actually has a thousand! When it moves, a millipede appears to glide along like a train on its tracks. When alarmed, it curls up into a tight ball, legs tucked in, and can emit a bad odor. Millipedes, like mites and many other leaf litter animals, eat decaying leaves, chewing holes or shredding them as they feed.
Another common litter animal, but one that is much harder to view, is the springtail. About the size of a flea, springtails seem to appear and then disappear as they hop about. A tiny appendage on the end of its abdomen, the furcula, folds back under its belly. When released, it flings the springtail into the air like a catapult, a good way to escape predators. A springtail can jump many times its body length, as high as a man jumping over a ten-story building! Springtails are among the most numerous animals in the leaf litter and soil, feeding primarily on fungi. In late winter, springtails, also called “snow fleas,” appear like a grinding of pepper on the snow.
Dead leaves piled into small mounds are signs of earthworm activity. Earthworms make burrows in the soil and then gather dead leaves for food and to cover their tunnel entrances. They are extremely efficient decomposers of decaying leaves. With the help of grit swallowed during tunneling, they grind leaves in their gizzards, extracting nutrients in the process. Their droppings or “castings” are small, round pellets that enrich the soil. Earthworms are not native to our forests but arrived here with colonists. Although they are beneficial in gardens and fields, they can be harmful in the forest. Ecologists fear that they digest the leaf litter so quickly that little is left to support the community of smaller decomposers that normally play this role, upsetting the balance in this ecosystem.
Such a wealth of small animals provides ample food for predators. Centipedes look much like millipedes, but they have only one pair of legs on each segment. Their behavior is markedly different as they dart through the litter, hunting for prey. Larger centipedes can inflict a painful bite, even on people! Besides centipedes, numerous spiders, some ants, tiny nematode worms, and many kinds of beetles, flies, and their larvae are predators of smaller creatures in the leaf litter.
Another leaf litter predator is the red eft, an immature stage of the red-spotted newt. The eft eats a large quantity of tiny springtails and other small prey. It spends from two to five years living in the forest before migrating to a pond where it changes into an aquatic newt. Its bright orange color, like a neon sign, signals to birds and other predators that it is poisonous to eat. Pointy-nosed shrews are another forest floor predator with a big appetite, often feeding on prey like mice, as large or larger than themselves.
The woodcock or “timber-doodle,” as this funny bird is sometimes called, looks a bit like a softball with a long bill. Well-camouflaged in its mottled brown plumage, the woodcock nests and forages in open woods near wetlands. With its long, flexible bill, it probes under the leaf litter for worms, a favorite food, and other critters. The woodcock performs an odd dance-like motion while foraging. It rocks its body back and forth, keeping its head still, and presses the ground with its feet. This may make the worms move around underground, giving away their location.
The forest leaf litter supports an abundant community of living things, including many decomposers that return nutrients to the soil. Microscopic bacteria and fungi, litter critters and their predators, trees, soil, raindrops, and dappled sunlight – all are part of the rich ecosystem underfoot. Many exciting discoveries await those who explore the leafy layers that litter the forest floor.
Johnson, Elizabeth A. and Catley, Kefyn M. Life in the Leaf Litter. New York: American Museum of Natural History, 2002.
Kricher, J. Peterson Field Guide to Eastern Forests. New York: Houghton Mifflin Co., 1998.
Nardi, James B. Life in the Soil. Chicago: University of Chicago, 2007.
Tugel, A.J., A.M. Lewandowski, and D. Happe-vonArb, eds. Soil Biology Primer. Ankeny, IA: Soil and Water Conservation Society, 2000.