19.2) Food chains and food webs

19.2) Food chains and food webs


Food chain: shows the transfer of energy from one organism to the next, beginning with a producer.

Food web: is a network of interconnected food chains.

Producer: is an organism that makes it own organic nutrients, usually using energy from sunlight, through photosynthesis.

Consumer: is an organism that gets its energy from feeding on other organisms.

Herbivore: is an animal that gets its energy by eating plants.

Carnivore: is an animal that gets it energy by eating other animals.

Decomposer: is an organism that gets its energy from dead or waste organic material.


Interdependence means the way in which living organisms depend on each other in order to remain alive, grow and reproduce.

Predator is a carnivore that kills and eats other animals.

Scavengers are carnivores that eat the dead remains of animals killed by predators.

Energy is transferred between organisms in a food chain by ingestion.

  • A food chain shows what eats what in a particular habitat. It shows the flow of energy and materials from one organism to the next, beginning with a producer.
  • The arrows between each organism in the chain always point in the direction of energy flow from the food to the feeder.

A food web is a network of interconnected food chains. It shows the energy flow through part of an ecosystem.


Producers are plants that produce food.

Primary consumer are animals that eat the plants.

Secondary consumer are animals that prey on the plant-eaters.

Tertiary consumers are animals that feed on secondary consumers.

Quaternary consumer is an animal that is at the top of the food chain.

Pyramid of biomass:

  • Biomass is the total dry mass of one animal or plant species in a food chain or food web.
  • A pyramid of biomass shows the biomass at each trophic level, rather than the population.
  • Nearly always the correct pyramid shape.
  • more accurate indication of how much energy is passed on at each trophic level.



The effect of over-harvesting:

Over-harvesting causes the reduction in numbers of a species to the point where it is endangered or made extinct. As a result biodiversity is affected.



Introducing foreign species to a habitat:

  • An example of this process was the accidental introduction of rats to the Galapagos Islands.
  • The rats had no natural predators and food was plentiful: they fed on the eggs of bird, reptiles and tortoises along with young animals.
  • The galapagos Islands provide a habitat for many rare species, which became endangered as a result of the presence of the rats.


Energy transfer:

Trophic level: is the position of an organism in a food chain, food web or pyramid of numbers or biomass.

  • Energy decreases as it moves up trophic levels because energy is lost as metabolic heat when the organisms from one trophic level are consumed by organisms from the next level.
  • Energy transfer is inefficient because energy is lost while moving from one trophic level to another. This is because not the entire organism is consumed or digested.
  • The transfer of energy from primary to secondary consumers of probably more efficient, since a greater proportion of the animal food is digested and absorbed that is the case with plant material.
  • It is very unusual for food chains to have more than five trophic levels because, on average, about 90% of the energy is lost at each level.

  • Short food chains are more efficient than long ones in providing energy to the top consumer. On the right side are two food chains and energy values for each level in them. Both food chains have a human being as the top consumer.
  • Ten times more energy is available to the human in the second food chain than in the first.
  • Some farmers try to maximize meat production by reducing movement of their animals (keeping them in pens or cages with a food supply) and keeping them warm in winter. This means less stored energy is wasted by the animals.


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