Global Distribution and Characteristics of Ecosystems

From local ecosystems to global biomes

An ecosystem includes living organisms and the non-living environment with which they interact. Ecosystems exist at different scales: a pond is small, whereas tropical rainforests extend across enormous areas of several continents.

A large-scale ecosystem is called a biome. Biomes are usually named after their dominant vegetation, such as grassland or coniferous forest. Their climate influences which plants can grow; the vegetation then provides habitats and food for animals. This is why places far apart can have similar ecosystems if their climates are similar.

Why biomes form broad geographical belts

Latitude is an important influence on temperature. Near the Equator, the Sun’s energy is concentrated over a smaller area. Towards the poles, it is spread over a larger area, so temperatures are generally lower. This helps explain the broad change from tropical forests to temperate vegetation and then cold environments.

Rainfall matters as well as temperature. Near the Equator, warm, moist air rises, cools and produces heavy rainfall. Around 20–30° north and south, sinking air discourages cloud formation, creating dry conditions. A hot, wet climate can support dense forest; a hot, dry climate cannot.

These are broad patterns, not continuous or perfectly symmetrical strips. Several other influences change conditions:

• Altitude: temperatures fall with height, so high mountains can support cold-environment vegetation even at low latitudes.
• Continentality: inland areas have hotter summers and colder winters than places strongly influenced by the sea.
• Ocean currents: warm or cold currents affect temperatures along nearby coasts.
• Relief: mountains force air to rise and produce rainfall. Land on the sheltered side may lie in a dry rain shadow.

For example, temperate grasslands occur in continental interiors, while forests grow in wetter temperate regions. Latitude alone therefore cannot explain every biome boundary.

Tropical environments: rainforest, savanna and desert

Tropical rainforests occur close to the Equator, including in South America, Africa and South-east Asia. They are hot and wet throughout the year, with temperatures commonly around 26–28°C and annual rainfall around 2,000 mm or more. These conditions allow continuous plant growth, producing dense, layered vegetation and very high biodiversity. The many plants provide a wide range of habitats and food sources.

Savanna, or tropical grassland, occurs broadly between equatorial rainforests and hot deserts, including in central Africa, South America and northern Australia. It remains warm but has distinct wet and dry seasons. Grasses dominate, with scattered trees such as acacia and baobab rather than continuous forest. African savannas support grazing animals such as zebras and elephants, alongside predators such as lions. The long dry season restricts growth and makes fires more likely.

Hot deserts are concentrated around 20–30° north and south, including parts of Africa, Australia and the Americas. They usually receive less than 250 mm of precipitation annually. Days can be extremely hot, but temperatures fall sharply at night because there is little cloud cover. Vegetation is sparse and biodiversity is relatively low. Plants and animals must cope with very limited water; examples include cacti in American deserts and animals such as scorpions and camels.

The key contrast is not simply that all three are hot. The amount and seasonal pattern of rainfall help determine whether dense forest, grassland or sparse desert vegetation develops.

Temperate environments: seasonal climates and contrasting vegetation

Temperate biomes occur between the tropical and polar regions. Their climates have more marked seasonal temperature changes than equatorial rainforest climates.

These contrasts show why average temperature alone is not enough to describe a biome. Mediterranean vegetation must survive summer drought, while temperate grasslands experience strong seasonal temperature contrasts. Deciduous forests have enough moisture to support extensive tree cover, but their growing conditions change through the year.

From northern forests to tundra and polar ice

Coniferous forest, also called taiga, forms a broad northern belt through Canada, Scandinavia and Russia, roughly around 60° north. Winters are long and very cold, while summers are short and milder. Trees such as pine and spruce have needles and are mostly evergreen, allowing them to use the short growing season without first replacing all their foliage. Biodiversity is generally lower than in temperate forests; animals include wolves, bears and squirrels.

Tundra occurs at high northern latitudes, including northern Canada and northern Europe, beyond much of the coniferous forest belt. Conditions are cold, windy and relatively dry, with a brief growing season. The ground is frozen for much of the year; when surface ice thaws, waterlogging can occur. Low-growing grasses, mosses and lichens survive, but the conditions do not support extensive forests.

Polar environments occur within the Arctic and Antarctic circles, especially on the ice-covered land of Greenland and Antarctica. Temperatures are extremely low and precipitation is limited. A desert is defined by its dryness, not its heat, so polar regions can be described as cold deserts. Permanent ice supports very little vegetation, unlike the seasonal plant growth found in tundra.

Recognising and explaining a distribution pattern

When reading a biome map, start with its key to identify the ecosystems shown. Locate the Equator and compare the pattern moving north and south. Look for concentrations, such as rainforest near the Equator, deserts in subtropical regions and coniferous forest across northern continents. Then identify exceptions or interruptions rather than assuming each biome forms an unbroken belt.

A useful explanation links location → climate → vegetation. For example, equatorial locations receive concentrated solar energy and frequent heavy rainfall. Warm, wet conditions allow plants to grow throughout the year, helping to explain dense rainforest vegetation and high biodiversity. In contrast, sinking air in subtropical regions limits rainfall, helping to explain the sparse vegetation of many hot deserts.