The flow of energy through ecosystems comes in a variety of forms (Focus: Key Figure 57.1). The most obvious source is the sun, which delivers radiant energy that warms Earth and provides energy for photosynthesis. Autotrophs such as plants, algae, and photosynthetic bacteria capture that radiant energy and fix it into organic compounds that make up the tissues of organisms (see Figure 57.1A). Energy can also come from the oxidation of inorganic nutrients by archaea and bacteria through the process of chemosynthesis, which involves using energy from inorganic compounds to take up CO₂ and produce carbohydrates (see p. 544). Chemosynthesis typically occurs in ecosystems that lack sunlight and have high concentrations of particular inorganic compounds, such as hydrothermal vents, hot springs, and soils. These two forms of energy capture and carbon fixation are both involved in a process known as primary production. Ecologists use the term “primary productivity” to indicate the rate of primary production over time.

A third source of energy in an ecosystem is the product of primary production—the organic compounds synthesized through photosynthesis and chemosynthesis—that is made available for organisms at higher trophic levels in food webs (see Figure 57.1B). This source of energy, known as secondary production, is contained within the tissues of organisms and is used to power the metabolism of heterotrophs, or the consumers within food webs, which include herbivores, carnivores, omnivores, parasites and pathogens, and eventually decomposers. Metabolism involves converting the organic compounds in food into useable energy—primarily ATP—by processes such as glycolysis (see Key Concept 9.1). The main by-products of metabolism are inorganic compounds, CO₂, and water. The energy generated by metabolism to support the “work” that organisms do in their daily life is eventually lost from the ecosystem in the form of metabolic heat.