Section 2: Energy Flow in Ecosystems

Energy flows through an ecosystem, which consists of all parts of an environment in one direction.  It flows from the sun, or chemical compounds, to autotrophs, or producers, and then to the various heterotrophs, or consumers.

Ecologists look at the feeding relationships of organisms to help them understand how energy passes in an ecosystem.  Food chains show how matter and energy move through ecosystems in a series of steps showing which organisms transfer energy by eating and being eaten. For example, in a food chain of grass, a grasshopper, a gecko, and an owl, the grass is first in the food chain.  It uses the sun to produce food instead of eating another organism, so it is a primary producer.  Next, the grasshopper eats the grass, making it a primary consumer.  Then, the gecko eats the grasshopper, which makes it a secondary consumer.  Finally, the owl is a tertiary consumer or the top predator.  Whether an animal at the top of a food chain is killed by another animal or dies on its own, the cycle inevitably begins again.

Feeding relationships among animals are often much more complex than described above.  While food chains show eating patterns, a food web offers all possible feeding relationships at each trophic level in a community.  It links all of the food chains in an ecosystem together.  A food web has more arrows showing many examples of feeding relationships.  Because very few animals feed on only one kind of animal, a food web is necessary to establish the range of food different species eat.

Each organism in a food chain or food web represents a feeding step, or trophic level, in transferring matter and energy.  Energy is always lost between trophic levels.  A biological pyramid is a diagram that shows the amounts of energy or matter at each trophic level.  There are three different types: energy pyramid, biomass pyramid, and pyramid of numbers.  An energy pyramid shows the flow of energy from one trophic level to the next, comparing the energy used by each organism.  With each increasing trophic level, some energy is lost.  A biomass pyramid compares each organism’s biomass, or living organic matter available, at each trophic level.  Finally, a pyramid of numbers shows the total number of organisms at each trophic level.


  1. How does energy flow in an ecosystem?
  2. Compare a food chain to a food web.
  3. What do each of the three types of energy pyramids illustrate?

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