Ecosystems, food chains and food webs

Last time

Before we start: last time, in Energy flow, pyramids and population growth, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Who eats whom?

A fox eats a rabbit, the rabbit eats grass, and the grass takes its energy from the Sun. Every living thing is linked to others like this. Today: the words population, community and ecosystem, then food chains and the web of feeding links.

Four key words

A population is a group of organisms of one species, living in the same area at the same time. A community is all the populations of different species living together in the same area. An ecosystem is a unit that includes the community and the non-living environment, which interact. A habitat is the place where an organism lives.

An example

All the frogs in a pond are one population. All the frogs, fish, plants and insects together are a community. Add the water, light and temperature, and the whole pond is an ecosystem.

Food chains

A food chain shows the transfer of energy from one organism to the next, each one feeding on the one before. It starts with a producer, usually a green plant, which makes organic nutrients by photosynthesis. The arrows point to the eater, showing the direction of energy flow.

Trophic levels

A trophic level is the position of an organism in a food chain. Producers are level one. Herbivores are primary consumers. Carnivores that eat them are secondary consumers, then tertiary consumers. Decomposers break down dead organisms and waste. They return nutrients to the soil.

A food web

Here is a small food web. Grass is the producer. The rabbit and the grasshopper both eat grass, the bird eats the grasshopper, and the fox eats the rabbit and the bird.

Chains and webs

Real organisms rarely eat just one thing. A fox eats rabbits and birds, so the chains join up into a web. A food web shows all the feeding links in the ecosystem, so we can predict the effects of change. If one organism is removed, a web shows which others lose food or lose a predator.

Predator and prey

A predator eats other animals, called its prey. If prey numbers rise, predators have more food and their numbers rise later. As predators increase, they eat more prey, so prey numbers fall again. The two populations rise and fall in a cycle. Always say what happens to the food source and to the predator when you explain a change in numbers.