11. Food chains and webs GapFill

Food chains and food webs

Living organisms in any habitat depend on each other for food, shelter and other things. This interdependence often takes the form of a feeding relationship.

All living organisms need energy. The ultimate source of all this energy is the  airsoilSunwatergrassMoon.

Light energy is trapped by  all living cellsherbivoresbacteriaanimalsfungiplants, and then transferred from organism to organism in a food chain; they use the light energy to make food by photosynthesis, and animals get their energy from eating them or other animals.

A food chain shows what eats what in a particular habitat. For example:

Grass seed  →   mouse   →     robinearthwormbutterflygrass plantbarn owlladybird

Means that grass seed is eaten by mice, etc.

The arrows between each organism in the food chain go from the food to the feeder (in the direction energy flows).

All food ultimately comes from green plants or producers. The other organisms in the food chain are  predatorspreyconsumerscarnivoresreproducersherbivores because they all get their energy by consuming (eating) other organisms.

Trophic levels

The steps in a food chain are called trophic levels. Trophic just means feeding.

Trophic levels can be represented by numbers, starting at level 1 with plants and algae. It works like this:

• Level 1: Producers. Producers are always plants or green algae. All food chains start with them, because they can   produce digestive enzymesmake food by photosynthesiseat carbon dioxiderelease energy from foodcreate energyconsume other plants.
• Level 2: Primary consumers feed on plants. They are  herbivoresdecomposerscarnivoresdetritivoresvegetablespredators, e.g. rabbits, caterpillars, sheep and deer.
• Level 3: Secondary consumers feed on primary consumers. They are carnivores, e.g. cats, foxes and crocodiles.
• Level 4: Carnivores that eat   everythinganimalsother carnivoresprimary consumersplantsproducers are called tertiary consumers.

A top carnivore is at the end of the food chain – no other animals eat it.

All food chains are quite short. There are hardly ever more than   15two3010sevenfour steps because a lot of energy is used up by the organisms at each trophic level to keep themselves alive.

Most animals will eat more than one organism in their natural habitat, so a better way of showing feeding relationships is to draw a   food cyclefood pyramidfood webfood communitysuper chainnutrition diagram to show a series of interconnecting food chains.

Increases or decreases in the population of one organism in a habitat will affect other organisms because their food supply will change.

Environmental  improvementpollutionhealthconservationsciencediversity can have an effect on the whole of a food chain. Toxic chemicals such as pesticides can be passed along food chains because the organisms can't excrete them from their bodies. The concentration of toxins will increase at each level.