Food Chains & Food Webs (College Board AP Biology)

• 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

• 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)