Ecosystems

What is an ecosystem?

An is a community of plants and animals and the environment they live in. It has:

(living) parts: plants, animals, fungi and bacteria;
(non-living) parts: water, soil, sunlight, temperature and air.

The parts depend on each other: they are . Ecosystems exist at every scale, from a pond or a hedgerow to a tropical rainforest covering several countries.

Key terms

A UK pond: food chains and webs

A freshwater pond is a small-scale UK ecosystem.

make their own food from sunlight by photosynthesis: pondweed and algae.
Consumers eat other living things: primary consumers eat producers (water snails, tadpoles); secondary consumers eat them (sticklebacks, dragonfly nymphs); a tertiary consumer such as a heron eats those.
, bacteria and fungi, break down dead plants and animals.

A shows one line of feeding; a shows how chains link:

The arrows point from the food to the feeder: the direction energy flows.

Key terms

Nutrient cycling and change

keeps the pond going: plants take nutrients from the water and mud; animals get them by eating; when plants and animals die, decomposers release the nutrients back into the water, ready to be used again.

Changing one part affects the others. For example:

Fertiliser washes off nearby fields. The extra nutrients make algae grow fast and cover the surface. When the algae die, decomposers use up the oxygen in the water, and fish die.
If herons disappeared, sticklebacks would multiply, eat more water fleas, and the algae the fleas eat would spread.

Exam tip

Follow a change step by step through the web: what increases, what decreases, and why.

Key terms

Global ecosystems

Large-scale ecosystems, or , follow a pattern on a world map, mainly controlled by climate:

Ecosystem	Where	Climate
Tropical rainforest	Near the equator: the Amazon, the Congo, South-east Asia	Hot and wet all year
Hot desert	Around 30° north and south: the Sahara, central Australia	Hot and very dry
Tropical grassland (savanna)	Between the rainforests and deserts	Hot, with a wet and a dry season
Temperate deciduous forest	Mid-latitudes, including the UK	Mild, with rain all year
Boreal (coniferous) forest	Northern Canada, Scandinavia, Russia	Cold winters, short summers
Tundra and polar	Around the Arctic and Antarctica	Very cold, little rain or snow

Key terms

Exam practice

Plan your own answer first, then open the model answers to see what each level looks like.

4 marks · about 4 minutes

Explain how a change to one part of a small-scale ecosystem can affect other parts.

Explain means: Give the reasons why something happens, developing each point with "because" or "so".

Link it

Use the facts from:

A UK freshwater pond (A freshwater pond in the UK, Any time of year)

Level 1 (Basic)· 2/4 marks (band 1–2)

If something changes in a pond, it affects other things. For example, if the fish die, the herons have less food.

Why this level: One simple link in a food chain, but the change isn't explained or followed through. To reach Level 2: name a cause of change and trace its effects step by step, saying why each happens.

Level 2 (Clear)· 4/4 marks (band 3–4)

In a pond, fertiliser washing in from nearby fields adds nutrients, so algae grow quickly and cover the surface. When the algae die, decomposers break them down and use up the oxygen in the water. Fish such as sticklebacks die from lack of oxygen, so herons, which feed on them, have less food. One change spreads through the whole food web.

Why this level: A clear chain of effects, each step explained with "so", from a named small-scale ecosystem. Full marks.

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