Energy flow in an ecosystem refers to the transfer and consumption of energy that takes place within a food chain. The energy flow takes place via the food chain and food web. During the process of energy flow in the ecosystem, plants being the producers absorb sunlight with the help of the chloroplasts and a part of it is transformed into chemical energy in the process of photosynthesis. All living organisms can be classified as producers or consumers, and those producers and consumers can then be classified as part of a food chain. Energy flows within ecosystems because the organisms in those ecosystems are highly interdependent.

The unidirectional flow of energy and the successive loss of energy as it travels up the food web are patterns in energy flow that are governed by thermodynamics, which is the theory of energy exchange between systems). The first law of thermodynamics states that energy is neither created nor destroyed; it can only be converted from one form to another. This is true in energy flow in the ecosystem. The second law states that there is a loss of energy at each step of energy flow. This law also stands true in ecology as there is a progressive decrease in energy at each trophic level.

At the base of the pyramid are the producers, who use photosynthesis or chemosynthesis to make their own food. Herbivores or primary consumers make up the second level. Secondary and tertiary consumers, omnivores, and carnivores follow in the subsequent sections of the pyramid. At each step up the food chain, only 10 percent of the energy is passed on to the next level, while approximately 90 percent of the energy is lost as heat.

The flow of energy is a function of primary productivity with temperature, water availability, and light availability. Much variation in the flow of energy is found within each type of ecosystem, creating a challenge in identifying variation between ecosystem types. For example, among aquatic ecosystems, higher rates of production are usually found in large rivers and shallow lakes than in deep lakes and clear headwater streams).

In summary, energy flow in an ecosystem is the transfer and consumption of energy that takes place within a food chain. The energy flow takes place via the food chain and food web, and the unidirectional flow of energy and the successive loss of energy as it travels up the food web are patterns in energy flow that are governed by thermodynamics. The flow of energy is a function of primary productivity with temperature, water availability, and light availability.