Ecosystems » Global Ecosystems

From a pond to the whole planet

In the last lesson you studied a small ecosystem, a pond. Ecosystems also exist at a much larger scale. The biggest natural ecosystems cover whole continents and are called biomes. In this lesson you will get an overview of six of them: where each one is found, and its main climate and vegetation.

Key terms:

• Biome: a large scale natural global ecosystem, with its own typical climate, plants and animals.
• Distribution: where something is found across the world.
• Latitude: how far north or south of the equator a place is.

Why biomes are where they are

The two biggest influences on a biome are temperature and rainfall. Both change with latitude. Places near the equator get the Sun's rays most directly, so they are hot. Places near the poles get weaker rays, so they are cold. Rainfall follows the global circulation of the air, which is why very wet and very dry places form in bands around the Earth.

Plants grow to suit their climate, and animals depend on those plants. That is why the world's biomes form a pattern that roughly follows lines of latitude. Mountains and ocean currents can change the pattern a little.

The six biomes at a glance

Tropical rainforest

Rainforests sit in a belt either side of the equator. The Sun is strong all year, so there is no cold season and plants can grow all year round. Warmth and heavy rain give the most dense vegetation on land. The vegetation is tall, evergreen forest. Rainforests contain more plant and animal species than any other land biome.

Hot desert

Hot deserts are found further from the equator, where dry air sinks and little rain falls. There are no clouds to keep heat in at night, so the temperature can drop sharply after dark. Few plants can cope, so the ground is often bare sand or rock, with sparse plants such as cacti and thorny shrubs.

Tundra and polar

Tundra lies close to the poles. It is too cold and windy for trees. Much of the ground is frozen for most of the year and only thaws on the surface in the short summer. The vegetation is low shrubs, mosses and lichens. The very coldest places, such as inland Antarctica, are covered by ice and have almost no plants at all.

Temperate forest

The UK is in the temperate forest biome. The climate is mild, with no extremes of heat or cold. The four seasons change what the forest looks like: leaves open in spring, fill the trees in summer, and fall in autumn. Much of the original forest has been cleared for farming and towns, but this is still the natural biome for the UK.

Grassland and savanna

Grasslands are found where there is too little rain for a forest but too much for a desert. The savanna is tropical grassland with a clear wet season and dry season. Grasses grow fast in the wet season and turn yellow and dry in the dry season. Trees are few and far apart because the long dry season makes it hard for them to survive.

Savanna is the tropical type of grassland. Temperate grasslands, such as the prairies of North America and the steppe of Asia, are found in continental interiors, with hot summers and cold winters.

Coral reefs

Coral reefs are a marine biome, so they are in the sea, not on land. Corals are tiny animals that build hard skeletons. They need shallow water so sunlight can reach tiny algae living inside them. They need warm, clean, salty water, which is why reefs are found in tropical seas and not near the poles. Reefs have very high biodiversity.

Exam-style question

(a) Name the biome found in the UK. [1 mark]

(b) Describe the climate of a tropical rainforest. [2 marks]

(c) Explain why few trees grow in the tundra. [2 marks]