FOOD CHAINS AND FOOD WEBS

Food chains

A food chain shows the different organisms that live in a habitat, and what eats what.

Producers and consumers

A food chain always starts with a producer, which is an organism that makes food. This is usually a green plant, because plants can make their own food by photosynthesis.

A food chain ends with a consumer, which is an animal that eats a plant or another animal. Here is an example of a simple food chain:

grass cow human

Take care - the arrow points to the organism that is doing the eating. If you get the arrows the wrong way round, instead of showing that humans eats cows, you are showing that cows eat humans, and that cows need to beware of snarling, meat-eating grass!

The plant is the producer and the animals are consumers.
Notice that the first consumer in the chain is also called the primary consumer, the next one is the secondary consumer and the one after that is the tertiary consumer.

A consumer that eats plants is called a herbivore, and a consumer that eats other animals is called a carnivore. An omnivore is an animal that eats plants and other animals.

Predators and prey

• A predator is an animal that eats other animals
• The prey is the animal that gets eaten by the predator.

Pyramids of numbers

The population of each organism in a food chain can be shown in a sort of bar chart called a pyramid of numbers. The more organisms there are, the wider the bar. The producer in the food chain always goes at the bottom of the pyramid of numbers.

Think about this food chain:

clover snail thrush hawk

Clover is a plant and it is the producer in this food chain. Its bar goes at the bottom of the pyramid.

Energy is lost to the surroundings as we go from one level to the next, so there are fewer organisms at each level in this food chain. A lot of clover is needed to support the snail population. A thrush eats lots of snails, and a hawk eats lots of thrushes, so the population of hawks is very small.

Sometimes the pyramid of numbers doesn't look like a pyramid at all. This could happen if the producer is a large plant such as a tree, or if one of the animals is very small. Remember, though, that whatever the situation, the producer still goes at the bottom of the pyramid.

Food webs

When all the food chains in a habitat are joined up together they form a food web. Although it looks complex, it is just several food chains joined together.

Some organisms will have more than one predator, but others have just one predator.

This leads to some interesting effects if the population of a particular organism in the food web decreases. Some animals can just eat more of another organism if food is in short supply, while others may starve and die. This in turn can affect the populations of other organisms in the food web.

What would happen if the producer died?

If the producer, so the organisms that feed on it would have no food. They would starve and die unless they could move to another habitat. The other animals in the food web would die, too, as their food supplies died out. The populations of the consumers would fall as the population of the producer fell.

Toxic materials in the food chain

Toxic materials are poisonous. Some quickly break down into harmless substances in the environment. Others are persistent and do not break down. Instead, they accumulate in the food chain and damage the organisms in it, especially the top predators. Mercury and DDT are two persistent toxic materials.

Mercury

Mercury compounds were used until quite recently to make insecticides (chemicals that kill the insects that damage crops) and special paints that stop barnacles growing on the hulls of ships.

Unfortunately, when it gets into the food chain mercury damages the nervous systems and reproductive systems of mammals, including humans.

Tiny plankton in the sea absorb the mercury compounds. When the plankton are eaten by small fish, the mercury they contain stays in the fish. As the fish need to eat a lot of plankton, the concentration of mercury in them becomes higher than its concentration in the plankton.

The larger fish eat many small fish, and in turn larger fish still, such as the tuna fish, eat many of them. In this way, the concentration of mercury in the tuna becomes large. As people who eat a large amount of contaminated tuna may become ill, mercury is now banned from many chemicals.

DDT

DDT is a banned insecticide, which accumulates in birds and causes weakness in the eggs they produce.