B10 Energy Flow in Ecosystems

Animals, plants and where they live

What you need to know...

(the numbers in bold are the points from the curriculum...)

  1. 1 State that the Sun is the principal source of energy input to biological systems.

  2. 2 Define the terms:

    • food chain as a chart showing the flow of energy (food) from one organism
      to the next beginning with a producer (e.g. mahogany tree → caterpillar → song bird → hawk),

    • food web as a network of interconnected food chains showing the energy flow through part of an ecosystem,

    • producer as an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis,

    • consumer as an organism that gets its energy by feeding on other organisms,

    • herbivore as an animal that gets its energy by eating plants,

    • carnivore as an animal that gets its energy by eating other animals.

    1. 3 Describe energy losses between trophic levels.

    2. 4 Define the terms:

      • decomposer as an organism that gets its energy from dead or waste organic matter,

      • ecosystem as a unit containing all of the organisms and their environment, interacting together, in a given area, e.g. decomposing log or a lake,

      • trophic level as the position of an organism in a food chain or food web.

    3. 5 Explain why food chains usually have fewer than five trophic levels.

    4. 6. Describe the carbon cycle

    5. 7. Discuss the effects of combustion of fossil fuels and the cutting down of forests on the oxygen and carbon dioxide concentrations in the atmosphere


This topic is about life and how living organisms depend on eachother and the environment for what they need. All the energy that allows digestion, respiration, movement and all life processes (MRS GREN) ultimately comes from the sun! In this topic we learn how this happens and follow the flow of this energy as it is moved through the food web.

But what happens when different organism need the same things? Thats when it starts to get really interesting, but first a quick introduction to see how life fits together and to look at some of the definitions of key words that we need to understand and use in this unit...

Food Chains and Food Webs


Some organisms use energy from the sun (this is where the energy that drives the food web comes from) to help produce their food, they are called ___________ .

Most types of organisms (including organisms) need to get their energy by consuming other plants and animals, these organisms are known as ____________ .

Important words we need to know

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Some quick questions

You can think of the word habitat as the address where an animal or plant lives. Name the habitat for:

  1. Tadpole
  2. Gorilla
  3. Polar bear
  4. Lion
  5. Sloth
  6. Kangaroo

What is the difference between a population and a community?

An ecosystem consists of a ___________ and _____________.

A specific part of an ecosystem where one particular type of organism lives is called its ________.


Questions 15.1, 15.2, 15.3 on page 219

Arctic Ecosystem

Can you identify the producers, herbivores, carnivores from this diagram?

Where do you think all the energy used by Polar Bear originally came from?
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  • Plants are called producers because they make (produce) their own food - glucose molecules.
  • Here is a quick reminder of how its done, AND why its so important for organisms that depend on them (like us)


  • Consumers are organisms that eat/consume other organisms

  • They do not produce their own food.

  • Herbivores, carnivores and omnivores are all types of consumers.

Food webs in action

Food webs are diagrams to show how energy moves through the ecosystem, and food webs consist of many interlinked food chains.

Below is a diagram of a food web, fill in the the arrows to show how energy is moved from producers through to the top carnivores.

Don't forget that the arrows are used to show the flow of energy, so they must point in the direction that the energy moves

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Decomposers are organisms that feed on dead/waste material from plants and animals.

They release the nutrients locked inside material such as;

  • Leaves and sticks that have fallen from plants and trees
  • Dead organisms
  • Faeces

Without decomposers so many nutrients would be lost from the environment, watch the short video clip below to find out more...

Producers are the beginning of it all, without plants there would be nothing else! It is producers that use the energy from the sun to create carbohydrates, and these carbohydrates are eaten by primary consumers and then by secondary consumers etc. The energy moves through the food web.

Five trophic levels

  • Energy is lost from each level of the food chain, as energy is used in life processes such as movement, digestion, respiration, faeces.
  • Energy is lost as thermal energy from mammals to their surroundings.
  • Energy is lost in faeces and urine
  • Ultimately, all energy is lost as thermal energy produced by the movement, respiration (and other chemical reactions) and through faeces

All this energy loss means that the higher up the food web you go the less energy is available, this is why food webs can not sustain more than five trophic levels.

Try an exam style question

Food chains and food webs

The Carbon Cycle

  1. The carbon cycle shows how carbon containing compounds move around our biosphere.
  2. Carbon dioxide in the atmosphere is used by plants in photosynthesis, the carbon atoms from CO2 are used to make carbohydrates
  3. These carbohydrates are then eaten by primary consumers such as herbivores, and also by omnivores. The carbon atoms are now used in respiration to make energy and carbon dioxide. The same carbon atoms are exhaled back into the atmosphere.
  4. When an organisms dies it is broken down by decomposers. The decomposers use the carbohydrates from the dead organisms in respiration and release the carbon atoms back into the environment as CO2.
  5. Not all organisms will be broken down by decomposers. Some organisms will decay over millions of years to eventually form fossil fuels. Fossil fuels are a major source of energy and when they undergo combustion they release the carbon that has been trapped for millions of years back into the atmosphere as CO2.

Some of this CO2 is used by plants in photosynthesis and the cycle continues....

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How do you think the carbon cycle is affected by deforestation?

Look at the diagram of the carbon cycle, what would happen to atmospheric levels of carbon dioxide if.....

  • Deforestation increases (so there are less trees doing photosynthesis)
  • Combustion increases (more fossil fuels are being burned)


What are Trophic Levels? | Ecology and Environment | The Virtual School

Coral Reef Ecosystem

Coral Reef Adventure 1-4 HD 720p

Ecological Pyramids (triple award only)

Pyramid of numbers shows how many organisms there are at each trophic level.

Pyramid of biomass shows the weight (g/m2) of the organisms at each trophic level.

Ecological Pyramids | Ecology and Environment | the virtual school

Collecting information

How can we find out about the environment?

Data collection

We can use a data logger to test various conditions from the environment such as

  • carbon dioxide
  • levels of oxygen are changing,
  • pH
  • amount of light
  • Salinity (how much salt there is in solution)