An ecosystem is composed of all interacting organisms (biota) along with their physical and chemical environments. Physical aspects of an ecosystem, called abiotic components, include variables related to temperature, sunlight, soil, and other factors.

Ecosystems vary in size from microscopic environments to large geographical areas. The boundaries of an ecosystem are not usually distinct and the activities of one ecosystem often impact other ecosystems. Some ecologists consider the biosphere as a global ecosystem.

Ecosystem diversity can be measured by the richness of genetic diversity within individual species, by the number of different species present, and the variety of habitats. Diverse ecosystems are believed to be more stable and productive.

Energy and nutrients (chemicals) continuously move through ecosystems. Because energy cannot be recycled, a continuous input is required, almost always by sunlight. Elements such as carbon, sulfur, phosphorus, and nitrogen are recycled within ecosystems through biogeochemical cycles. Both energy and chemicals are transferred by photosynthesis and feeding relationships. The flow of carbon through ecosystems closely parallels the flow of energy.

Over long periods of time, ecosystems change in appearance and composition. Succession is the regular replacement of populations in a habitat. Some communities may follow a recognizable sequence of change ending in a relatively stable community referred to as a climax community. Recently, the concept of climax community has generated considerable debate as the endpoints of a succession depend on the introduction of complex environmental factors.

Chemical elements essential to life are available in limited amounts and must be cycled between living organisms and the environment. Because these processes involve both chemical and biological processes, they are called biogeochemical cycles. Elements such as carbon (from carbon dioxide), hydrogen, and nitrogen move between the atmosphere and organisms, while elements such as phosphorus, calcium, potassium, magnesium, sodium, and iron enter into organisms from the soil. The four primary biogeochemical cycles are water, nitrogen, carbon, and phosphorus.

The water or hydrologic cycle refers to the continuous circulation of moisture on earth, particularly between the atmosphere and the earth's surface. Solar radiation provides the energy for the water cycle. Water changes between gaseous and liquid states through the processes of transpiration, evaporation, and precipitation. Transpiration is the loss of water vapor by plant parts (mostly through tiny pores, known as stomata). Only a small amount of water is involved in a chemical process that occurs during photosynthesis (hydrogen and oxygen are split).

All living things require water.

A major component of the atmosphere, nitrogen is essential for all living things. However, most organisms are unable to use the gaseous forms of nitrogen present in the atmosphere. In order for nitrogen to be usable by most organisms, it must be "fixed," in other words, combined with oxygen, hydrogen or carbon to form other molecules. Nitrogen fixation can happen during rainstorms, which yields nitrate and ammonium ions. Nitrogen also can be fixed biologically by free-living and symbiotic bacteria. Leguminous plants, for example, host nitrogen-fixing bacteria in root nodules allowing them to capture nitrogen and incorporate it into proteins and other molecules.

Unlike other organisms, nitrogen fixing bacteria are able to convert atmospheric nitrogen to ammonia, which then can serve as raw material for the incorporation of nitrogen into other molecules. The other four important steps in the nitrogen cycle are: (1) assimilation (reduction of nitrate ions [NO₃^-] inside plants to ammonium ions [NH₄⁺], which are used to manufacture proteins and other molecules; this conversion requires energy); (2) ammonification (release of excess nitrogen in the form of ammonia [NH₃] and ammonium ions [NH₄⁺] by soil-dwelling bacteria and some fungi during the decomposition of complex organic compounds such as proteins, and nucleic acids); (3) nitrification (the oxidation of ammonium ions or ammonia by free-living, soil dwelling bacteria to nitrates [NO₃^-]; and (4) denitrification (the conversion of nitrate to gaseous nitrogen [N₂] by free-living bacteria in soil; this conversion yields energy and occurs in conditions with low levels of oxygen).

Carbon, in the form of carbon dioxide, comprises about 0.03 percent of the atmosphere. Worldwide circulation of carbon atoms is called the carbon cycle. Since carbon becomes incorporated into molecules used by living organisms during photosynthesis, parts of the carbon cycle closely parallel the flow of energy through the earth’s living systems. Carbon is found in the atmosphere, the oceans, soil, fossil deposits and living organisms. Photosynthetic  organisms create carbon-containing molecules (known as “organic” compounds), which are passed to other organisms as depicted in food webs. Each year, about 75 billion metric tons of carbon are trapped in carbon-containing compounds through photosynthesis. Carbon is returned to the environment through respiration (breakdown of sugar or other organic compounds), combustion (burning of organic materials, including fossil fuels), and erosion.