Energy flow

The Sun is the original source of energy for almost all organisms on the Earth. The way energy passes through an ecosystem is described as energy flow.

Plants trap sunlight energy in chlorophyll and use it in photosynthesis to make food (sugar and starch) that can be passed onto animals.

Animals/consumers obtain the food made by plants/producers through feeding.

Herbivores (plant eating animals) are primary consumers.

Carnivores (animals who eat other animals) are secondary consumers while those that feed on secondary consumers are tertiary consumers.

This sequence of feeding (producers, primary consumers, secondary consumers and tertiary consumers) shows the flow of energy.

The different stages in the feeding sequence are also referred to as trophic levels. Producers are trophic level 1; primary consumers are trophic level 2 and so on.

Food chains and food webs

Specific feeding sequences (and therefore flow of energy) can be represented as a food chain.

A 'grass seeds-vole-barn owl' food chain

Arrows between each organism show the direction of:

  • Feeding (consumption).
  • Energy flow.
  • The transfer of substances such as carbon and nitrogen.

Food webs show a number of food chains that are interlinked due to the organisms involved living in close proximity.

Food web

Most organisms will have more than one food source that can be seen in a food web but not a food chain.

Energy loss

Energy is lost between each trophic level.

From the Sun to the plant (producer), energy is lost when light is reflected off the leaf or passes through the leaf missing the chloroplasts. However, with no shortage of sunlight, this is not an issue.

Between each trophic level only 10-20% of the energy is transferred – a loss of 80-90%.

Energy flow

Energy is lost due to:

  • The whole organism not being eaten (skeleton and fur left behind).
  • Not all the food being digested – some passes out of the animal in excretion or egestion.
  • Energy being lost as heat in respiration and therefore not being passed onto the next level. Respiration provides the animal with energy for movement, reproduction and growth.

Shorter food chains are more energy efficient as energy is lost at each stage.