Food Chains & Food Webs (College Board AP® Biology)
Study Guide
Written by: Phil
Reviewed by: Lára Marie McIvor
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 level | Name of trophic level | Description of organisms in trophic level |
|---|---|---|
1 | Producers | Plants and algae produce their own biomass using inorganic nutrients and energy from sunlight |
2 | Primary consumers | Herbivores feed on producers (plants and algae) |
3 | Secondary consumers | Predators feed on primary consumers |
4 | Tertiary consumers | Predators feed on secondary consumers |
5 | Quaternary consumers | Predators feed on tertiary consumers |
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
Examiner Tips and Tricks
Don’t forget – animals (known as consumers) can be at different levels within the same food web as they could be omnivores (animals that can eat both plants and animals) or could be predators that eat both primary, secondary and/or tertiary consumers!
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)
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