Food Chains & Food Webs (Cambridge O Level Biology)

Food Chains Definitions Table

Term	Definition
Producer	An organism that can produce its own food, using sunlight or other energy sources
Herbivore	An animal that eats only plants
Carnivore	An animal that eats other animals
Primary consumer	An herbivore that eats producers
Secondary consumer	A carnivore that eats primary consumers
Tertiary consumer	A carnivore that eats secondary consumers
Decomposer	An organism that feeds on dead or decaying matter by breaking it down

Food Chain Diagram

A food chain with three trophic levels

• A food chain shows the transfer of energy from one organism to the next, starting with a producer
• The source of all energy in a food chain is light energy from the Sun
• The arrows in a food chain show the transfer of energy from one trophic level to the next
• Energy is transferred from one organism to another by ingestion (eating)
• In the food chain above:

Position in the food chain	Organism	Explanation
Producer	Grass	Makes its own food using energy from photosynthesis
Primary consumer	Mouse	Eats the producer
Secondary consumer	Barn owl	Eats the primary consumer

Food Webs

• 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

Food Web Diagram

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
• For example, in the food web above, if the population of earthworms were to decrease:
  • 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)

• Most of the changes in populations of animals and plants happen as a result of human impact - either by overharvesting of food species or by the introduction of foreign species to a habitat
• Due to interdependence, these can have long-lasting knock-on effects to organisms throughout a food chain or web