Global Distribution and Factors Affecting Biomes

From ecosystems to global biomes

An ecosystem includes living organisms, such as plants, animals and bacteria, and the non-living surroundings with which they interact. Plants depend on water, light and soil; animals depend on plants or other animals for food. These connections mean that a change in the physical environment can affect the whole ecosystem.

A biome is a large-scale ecosystem with a characteristic climate, vegetation and wildlife. Tropical forests, temperate forests, boreal forests, tropical and temperate grasslands, deserts and tundra are the main groups considered here. Their distribution means where they are found across the world.

Climate creates the broad global pattern. Local factors, including soils and altitude, help explain why vegetation can differ between places within the same broad climatic region.

Why temperature and rainfall control plant growth

Plants need suitable temperatures and a supply of water to grow. Climate therefore influences which plants can survive, how quickly they grow and how long their growing season lasts. The vegetation then influences the food and habitats available to animals.

Temperature varies with latitude. Near the Equator, the Sun’s energy is more concentrated, producing warm conditions throughout the year. Towards the poles, solar energy is less concentrated and temperatures are generally lower. At higher latitudes, cold winters and reduced winter daylight limit photosynthesis and shorten the growing season.

Rainfall affects water availability. Warm conditions alone do not produce a forest: plants also need enough water. Hot, wet conditions support tropical rainforest, whereas hot conditions with very little rainfall support desert vegetation. Seasonal rainfall matters too. Tropical grasslands have wet and dry seasons, so plants must survive periods when water is scarce.

Atmospheric circulation helps explain these differences. Near the Equator, warm, moist air rises, cools and condenses, producing clouds and heavy rainfall. Around 30° north and south, descending air discourages cloud formation and rainfall, helping explain the broad belts of hot deserts. These are general patterns, not continuous bands covering every place at the same latitude.

Locating the main biomes on a world map

A world biome map uses a key to show which colours or patterns represent each biome. Begin with the key, then identify the Equator and latitude lines. Use continent outlines and country labels to turn a coloured area into a geographical description.

The main patterns to recognise are:

• Tropical forests: close to the Equator; the Amazon in Brazil is an important example.
• Tropical grasslands: farther from the Equator, often between tropical rainforest and desert regions; East Africa is an example.
• Deserts: many hot deserts lie around 30° north and south; Australia’s Great Sandy Desert is an example.
• Temperate forests: in the middle latitudes, including the UK and other parts of Europe.
• Temperate grasslands: often in continental interiors, including the central USA’s prairies and the steppes of southern Russia. The Pampas in Argentina are another example.
• Boreal forests: mainly at high northern latitudes, around 60° north, including extensive areas of Russia.
• Tundra: farther north than boreal forest, towards the Arctic.

For a concrete map-reading example, consider the Amazon. Find South America, locate Brazil and identify the tropical-forest shading using the key. Relate that area to the Equator. A useful description is: tropical forest occurs in equatorial South America, including the Amazon in Brazil. The explanation is separate: year-round warmth and abundant rainfall allow sustained plant growth.

Apply the same method to other areas: identify the biome, locate it using latitude and named places, then explain the climatic conditions. Use the actual map to judge its extent and boundaries. These broad location examples do not replace the detail of a particular map, and biome boundaries are often gradual rather than sharp lines on the ground.

How soils change what can grow

Soil supplies plants with water, nutrients and anchorage for their roots. Its depth, acidity and drainage affect which vegetation can survive. Different plants tolerate different soil conditions, so two places with similar climates may still support different vegetation.

Deep, nutrient-rich soils can support more plant growth than thin, nutrient-poor soils. Thin soils offer less space for roots and smaller stores of water and nutrients. Soil acidity also matters because plants differ in their tolerance of acidic conditions.

Drainage controls how long water remains in the soil. Rapidly draining soils may leave plants short of water, while frequently waterlogged soils can prevent many trees from growing and favour vegetation adapted to wet conditions.

Soils and vegetation also influence each other. Dead leaves and other plant material enter the soil as litter and release nutrients as they decompose. However, dense vegetation does not necessarily mean fertile soil. In tropical rainforests, heavy rainfall causes leaching, washing nutrients out of the soil despite the supply of decaying litter. Boreal forest soils may be thin and acidic, limiting the plants that can grow there.

Why vegetation changes with altitude

Altitude is height above sea level. As altitude increases, temperature generally falls. A mountain can therefore have colder conditions than nearby lowland areas at the same latitude.

Colder conditions shorten the growing season and limit the plants able to survive. Steep slopes and thinner soils can also restrict dense forest. With increasing height, vegetation may change from forest to scrub or grasses and then to sparse, cold-tolerant vegetation.

Mount Kilimanjaro in Tanzania illustrates this local influence within a tropical region. Its lower slopes support tropical rainforest, while higher areas have scrub and increasingly sparse vegetation. Latitude alone cannot explain this change: the mountain’s increasing altitude creates colder growing conditions.

Connecting global and local controls

Think of biome distribution at two scales. Climate explains the broad world pattern, especially through temperature, rainfall and seasonality. Soils and altitude modify that pattern locally, affecting water, nutrients and growing conditions.

An explanation should follow the connection all the way to vegetation: increasing altitude lowers temperature, which shortens the growing season and limits tree growth. This is more useful than simply naming altitude as a factor.