Forests are filled with an incredible variety of plant and animal life. For example, scientists recorded nearly 10,500 kinds of organisms in a deciduous forest in Switzerland. The number of individual plants and animals in a forest is enormous.
All life in the forest is part of a complex ecosystem, which also includes the physical environment. Ecologists study forest life by examining the ways in which the organisms interact with one another and their environment. Such interactions involve (1) the flow of energy through the ecosystem, (2) the cycling of essential chemicals within the ecosystem, and (3) competition and cooperation among the organisms.
The flow of energy. All organisms need energy to stay alive. In forests, as in most other ecosystems, life depends on energy from the sun. However, only the green plants in the forest can use the sun's energy directly. Through a process called photosynthesis, they use sunlight to produce food.
All other forest organisms rely on green plants to capture the energy of sunlight. Green plants are thus the primary producers in the forest. Animals that eat plants are known as primary consumers or herbivores. Animals that eat herbivores are called secondary consumers or predators. Secondary consumers themselves may fall prey to other predators, called tertiary (third) consumers. This series of primary producers and various levels of consumers is known as a food chain.
In a typical forest food chain, tree leaves (primary producers) are eaten by caterpillars (primary consumers). The caterpillars, in turn, are eaten by shrews (secondary consumers), which are then eaten by owls (tertiary consumers). Energy, in the form of food, passes from one level of the food chain to the next. But much energy is lost at each level. Therefore, a forest ecosystem can support, in terms of weight, far more green plants than herbivores and far more herbivores than predators.
The cycling of chemicals. All living things are made up of certain basic chemical elements. The supply of these chemicals is limited, and so they must be recycled for life to continue.
The decomposers of the forest floor promote chemical recycling. Decomposers include bacteria, earthworms, fungi, and some insects. They obtain food by breaking down dead plants and the wastes and dead bodies of animals into their basic chemicals. The elements pass into the soil, where they are absorbed by the roots of growing plants. Without decomposition, the supply of such essential elements as nitrogen, phosphorus, and potassium would soon be exhausted.
Some chemical recycling does not involve decomposers. Green plants, for example, release oxygen during photosynthesis. Animals-and plants as well-need this chemical to oxidize (burn) food and so release energy. In the oxidation process, animals and plants give off carbon dioxide, which the green plants need for photosynthesis. Thus the cycling of oxygen and carbon dioxide works together and maintains a steady supply of the two chemicals.
Competition and cooperation. Every forest animal and plant must compete with individuals of its own and similar species for such necessities as nutrients, space, and water. For example, red squirrels in a boreal forest must compete with one another-and with certain other herbivores-for conifer seeds, their chief food.
Similarly, the conifers compete with one another and with other types of plants for water and sunlight. This competition helps ensure that the organisms best adapted to the forest will survive and reproduce.Cooperation among the organisms of the forest is common. For many species, cooperation is necessary for survival. For example, birds and mammals that eat fruit rely on plants for food. But the plants, in turn, may depend on these animals to help spread their seeds. Similarly, certain microscopic fungi grow on roots of trees. The fungi obtain food from the tree, but they also help the tree absorb needed water and nutrients.