10.2 Energy Flow Through Ecosystems

Ecosystem ecology is an extension of organismal, population, and community ecology. The ecosystem comprises all the biotic components (living things) and abiotic components (non-living things) in a particular geographic area. Some of the abiotic components include air, water, soil, and climate. Ecosystem biologists study how nutrients and energy are stored and moved among organisms and the surrounding atmosphere, soil, and water.Ecosystems are powered by two fundamental processes: energy flow and nutrient cycling. These processes are essential for maintaining life and ecosystem stability. Energy flow begins when solar energy is captured by producers through photosynthesis. This energy is then transferred through the ecosystem as organisms consume one another, moving from producers to herbivores, and then to carnivores and decomposers. Unlike nutrients, which are recycled within ecosystems, energy flows in a one-way stream: it enters as sunlight and exits as heat. Nutrient cycling, on the other hand, involves the continuous movement of elements like carbon, nitrogen, and phosphorus between the biotic (living) and abiotic (non-living) components of the ecosystem. Together, these processes link all organisms to each other and to their physical environment, forming the foundation of ecosystem ecology.

Primary Production and Energy Budgets

Nearly all life on Earth depends on energy from the sun. In ecosystems, this energy is first captured by producers through the process of photosynthesis. These producers convert sunlight into chemical energy, which becomes the starting point for energy flow through the rest of the ecosystem.

However, only about 1% of the sunlight that reaches Earth’s surface is actually turned into usable energy by producers. This is because most of the sunlight is either reflected, absorbed as heat, or made up of wavelengths that plants can’t use. Producers also use some of the energy they capture to power their own life processes, like growth and repair.

Primary productivity is the amount of energy that producers make available to the rest of the ecosystem. This energy forms the base of the food web and determines how much life an ecosystem can support. Ecosystems like tropical rainforests and wetlands are highly productive, while deserts and the open ocean have much lower productivity per unit area. Despite its low productivity, the open ocean contributes significantly to global primary production because of its vast size.

Understanding primary productivity helps ecologists estimate how much energy is available to support life and how ecosystems respond to changes in the environment.

Energy Transfer and Ecological Pyramids

As energy flows through an ecosystem, it moves from one trophic level to the next – from producers to herbivores, then to carnivores and eventually decomposers. But this transfer is not very efficient. On average, only about 10% of the energy at one level is passed on to the next. The rest is lost as heat, used for movement, or left behind in waste and undigested material.

This pattern of energy loss can be visualized using an ecological pyramid, where each level represents the energy available to organisms at that trophic level. The base of the pyramid is wide, representing the large amount of energy captured by producers. As you move up to herbivores, then to carnivores, the pyramid narrows because less energy is available at each step.

This explains why ecosystems can only support a few top-level predators, like hawks or wolves. There simply isn’t enough energy to support large populations at the top. It also helps explain why food chains are usually limited to three to five levels because there’s not enough energy left to support another level beyond that.

“18.1 Importance of Biodiversity” from Biology and the Citizen by Colleen Jones is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

“46.2: Energy Flow through Ecosystems” from Unit VIII: Ecology by LibreTexts is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.