The food chain in the deciduous forest has four trophic (nutrition) levels, according to California State University's World Builders project. Each of these levels supports the one above it, and all organisms, when they die, recycle nutrients back into the soil, where plants use them to begin the process anew. In this way, forest wildlife relies on each other for food and to keep destructively high populations in check.
At the base of the food chain, you find plants, the "primary producers," so called because they do not consume other organisms for energy but produce their own energy from sunlight and inputs like water and carbon dioxide. Oak trees dominate the deciduous forests of North America, according to the Marietta College Department of Biology and Environmental Science, along with trees like hickory, walnut, maple and beech. Wildflowers, shrubs and small trees grow in the understory beneath these taller trees. All of these plants produce myriad food sources for other organisms. The oak, for example, provides one of the most important food sources for deciduous forest wildlife: the acorn. Plant stems and foliage offer food sources for animals like deer, while berries and seeds provide for creatures ranging from songbirds to black bears. Organisms that feed on primary producers are known as "primarily consumers."
Primary consumers feed on plant life, and "secondary consumers" feed on the primary consumers. Secondary consumers also comprise a menagerie of critters. Some bird species, for example, feed on insects like caterpillars that eat plants. Animals like field mice that feed on berries and seeds become a food source for secondary consumers like owls and foxes. Fish found in ponds and streams may consume insects, tadpoles or smaller fish that feed on plants. A handful of large predators act as "tertiary consumers" in the deciduous forest, feeding on secondary consumers. Cougars, for example, may eat smaller animals functioning as secondary consumers. Organisms may also play multiple roles. The omnivorous black bear feeds on berries and acts as a primary consumer, as well as consuming other animals that inhabit various levels of the deciduous forest's food chain.
Every autumn, as temperatures drop, the oaks and hickories that dominate the deciduous forest drop their leaves to the forest floor. Forest animals sometimes perish from causes other than becoming a meal for an animal at a higher trophic level. Here, the decomposers like fungi, earthworms and soil microbes step in, breaking down dead plant and animal material into raw nutrients that plants and soil microorganisms can take up. In this way, all organisms in the forest return to the base of the food chain, and the process begins again.