Ecology describes the many ways in which organisms interact with one another and with their physical environment. Organisms do not exist in isolation, but rather live as members of biological communities. These communities may be composed of a few species or many thousands, but in each case the member species interact with one another in a wide variety of ways. Communities are often discernable at a very local level—a salt marsh, tidal pool, or hardwood forest, for example. The interacting units in communities are individuals and the populations they form. Therefore ecology is directly relevant to the evolutionary processes acting on species through their populations.

The network of interactions between all these species (animal and plant, multicellular and unicellular, eukaryotic and prokaryotic) in a community is often described as a food web. Fundamentally, it represents the paths that energy takes as it is transferred from one organism to another after being captured from the sun or a chemical source such as the chemicals emitted near deep-sea hydrothermal vents. All food webs begin with an energy source. Energy is initially captured by autotrophs (“self-feeders,” also called producers), organisms with the ability to capture energy and create biomass from it. The most visible autotrophs to us land-dwelling humans are green plants and algae, which use photosynthesis to capture energy from the sun. Some bacteria use a similar process called chemosynthesis to gather chemical energy.

Once energy has entered the food web through the autotrophs, it is captured by organisms at the next level, heterotrophs (“other feeders,” or consumers). Heterotrophs largely are animals. Unable to gather energy directly, they must harvest it from other life forms. Heterotrophs can be divided into two basic types: those that consume primary producers (herbivores) and those that eat other heterotrophs (carnivores). In its simplest form, a food web might consist of a primary producer, a primary consumer (herbivore), and a secondary consumer (carnivore), each gathering energy from the level immediately below its own. However, each energy transition within the food web is inefficient. Whereas plants capture about 10% of the sun’s energy that reaches them, an herbivorous animal can only acquire 10% of the plant’s energy (and therefore only 1% of the sun’s). Therefore, each trophic level of the food web can support fewer and fewer heterotrophs, and the top predators within a food web typically are the rarest members of the community. In most food webs heterotrophs feed on more than one trophic level, and there are also parasites that steal energy from both autotrophs and heterotrophs.

This system has another element that is less obvious and thus often unappreciated in its importance - the decomposers. These organisms—typically bacteria and fungi—feed on the decaying remains of dead autotrophs and heterotrophs. By capturing energy and nutrients from these dead organisms, decomposers provide a means to return it to the system through the soil, where it can be used by autotrophs once again.

Energy is not the only thing that cycles through living organisms and their ecological communities. Carbon is an important element for all living things because it is a basic building block of all organic molecules. Carbon enters food webs from the Earth’s crust and the atmosphere (as carbon dioxide), and returns through decomposition and animal respiration. The water cycle is equally crucial for living organisms, along with the nitrogen, phosphorus, and sulfur cycles.

Communities are part of a hierarchy of ecological organization. At broader spatial scales, communities are part of larger units, called biomes, with distributions dictated by climatic conditions regionally. Examples include the tropical rainforest, tundra, and desert.