Food Chains & Food Webs (College Board AP® Biology)

Energy Flow Through Food Chains

• Plants are found at the start of almost every food chain

• This is because plants can convert light energy into usable forms which supports higher trophic levels in the food chain

• Chemical energy is stored in a plant’s biomass (after respiratory losses have been taken into account)

  • This is the energy that is available to a plant for growth and reproduction

• This chemical energy also represents the energy that is available to organisms in higher trophic levels in the ecosystem

  • Trophic levels describe the position of an organism in a food chain, web or pyramid

  • Trophic levels can be represented by numbers or by the name of that trophic level eg. plants and algae are in trophic level 1, otherwise known as producers

  • Further trophic levels are numbered subsequently according to how far the organism is along the food chain

• Like plants and other producers, these consumers store chemical energy in their biomass

Trophic levels table

Trophic Levels in a Simple Food Chain Diagram

Trophic levels for a simple food chain - the blue arrows show how the chemical energy originally produced by the primary producer (grass) is transferred to other organisms in the community

Decomposers

• Chemical energy stored in producers is also available to another group of organisms known as decomposers

  • The two main groups of decomposers are bacteria and fungi

• Decomposers carry out a very important function in ecosystems – they break down dead plant and animal material (in the process gaining the chemical energy still stored in the dead matter)

• They do this by:

  • Secreting digestive enzymes onto the surface of the dead organism

  • These enzymes break down the dead matter into small soluble food molecules

  • These molecules are then absorbed by the decomposers

• This process of decomposition also helps to release organic nutrients back into the environment (eg. the soil) which are essential for the growth of plants and other producers

Net Energy Transfer

• The ability of an organism to obtain energy when required and expend it during necessary activities is key to survival

• If an organism obtains more energy than it expends, then there is a net energy gain

  • This excess energy can be stored in the organism as fat 

  • Or alternatively, used in the growth of the organism

• Conversely, if an organism expends more energy than it obtains, then there is a net energy loss

  • This can have negative effects on the organism as a shortfall in energy means

    • Less energy available for normal function

    • Organisms may then use stored energy to make up the shortfall

    • This can lead to a loss in mass

  • Prolonged periods of net energy losses can result in death

Net Energy Transfer & Ecosystems

• A food web is a network of interconnected food chains

• Food webs are more realistic ways of showing connections between organisms within an ecosystem as animals rarely exist on just one type of food source

A food web shows the interdependence of organisms

• Food webs give us a lot more information about the transfer of energy in an ecosystem

• They also show interdependence - how the change in one population can affect others within the food web

• On an ecosystem level, an increase in energy availability from sunlight may increase the growth of producers

  • Populations of primary consumers may then increase

  • Interdependent organisms within the ecosystem will be impacted

  • The size and number of other trophic levels will be affected

• For example, in the food web above, if the population of earthworms decreased:

  • The population of grass plants would increase as there are now fewer species feeding off them

  • The populations of frogs and mice would decrease significantly as earthworms are their only food source

  • The population of sparrows would decrease slightly as they eat earthworms but also have another food source to rely on (caterpillars)