Physical History

Energy Flows in Ecology


First published on May 17, 2019. Last updated on February 6, 2021.

Energy flows through ecological networks such as food webs. Generally, sunlight flows into plants that create sugars. Animals eat sugars. Both plants and animals expel heat into their environment.

Organisms connected by arrows, such as puffin to fox

Marine food web in Alaska (Source: US Govt.)

Food webs are generally energy webs. Energy in the form of high order photons flows to plants and phytoplankton that produce sugars and starches. Other organisms and animal eat plants and phytoplankton to gain energy. Predators eat those animals to gain energy. All plants, animals and organisms give off some heat into the atmosphere.

The Jouleis the standard unit of energy, but for food, the calorie and Calorie are often used. A calorie is sufficient energy to raise one gram of water one degree Celsius (1 K). A Calorie (with a capital C) is equal to 1000 calories, and also known is a kilocalorie.

If the energy leaving the food web is the same as that entering it, then the temperature will generally stay the same (after being adjusted for season and weather). However, a food web will typically store some of the energy into biomass, which contains varying amounts of energy. Organism bodies contain some energy, such as the cellulose that makes up much plant structure, such as cell walls. Proteins also contain energy. Fruits contain considerable energy in the form of sugar, typically 4 Calories per gram. Seeds contain tremendous amounts of energy in the form of oils (typically 9 Calories per gram) and starches.

Food webs can also release more energy than is imputed for relatively short amounts of time, such as during forest fires.

Energy typically enters and leaves an ecological system in the form of higher energy photons (visible light) and leaves in the form of lower energy photons (but possibly more of them). Yet, there are alternatives. Some of the energy may leave in the form of “waste” biomass. Such as dead plant structures, dead animal bodies and dead bacteria that get stored in the soil or ocean sediments, and eventually become nutrients, minerals or fossil fuels. Alternatively, some energy may enter an ecosystem in the form of high energy molecules, such as near ocean thermal vents.

Energy typically travels through an ecosystem in the form of chemical energy, such as sugars, carbohydrates, fats and proteins.

Note: only part of physical energy can be utilized by living organisms and in industrial processes. The useful part is called exergy. A related quantity, emergy, is the amount of energy consumed in these processes.


Further Reading


  • Aydin Tözeren, Stephen W. Byers, New Biology for Engineers and Computer Scientists. Pearson Prentice Hall, 2004.


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