The global distribution of biomes

Overview

What a biome is, where the tropical rainforest, hot desert, temperate grassland and tundra each lie, and why the real pattern does not follow neat latitudinal bands.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is a biome?

   Model answer

   A biome is a very large-scale ecosystem, covering a large part of a continent, in which a distinctive community of plants and animals has developed in response to the climate of that zone. Its vegetation has a characteristic structure and the species within it share adaptations to the same conditions of temperature, moisture and light. Because climate is arranged in broad latitudinal belts, biomes are too, though relief, ocean currents and distance from the sea modify the pattern.

2. 2

   Where in the world is the tropical rainforest biome found?

   Model answer

   In a discontinuous belt straddling the Equator, mostly within about ten degrees north and south of it and extending in places to the tropics. The three great blocks are the Amazon basin of South America, the Congo basin of central Africa, and South East Asia and the islands of Indonesia, Malaysia and New Guinea. Smaller areas occur in Central America, the eastern coast of Madagascar, west Africa and north-east Australia. The distribution follows the zone where the Intertropical Convergence Zone brings high temperatures and heavy convectional rainfall throughout the year.

3. 3

   Where are the hot desert biomes found, and why are they where they are?

   Model answer

   They are centred on latitudes of about twenty to thirty degrees north and south, along and just outside the tropics, and they are typically on the western sides of continents and in continental interiors. The main examples are the Sahara and Arabian deserts, the Kalahari and Namib in southern Africa, the Atacama in South America, the Australian deserts, and the Mojave and Sonoran deserts of North America. They occur there because the descending limb of the Hadley cell creates a belt of subtropical high pressure, in which sinking air warms and dries and cloud cannot form. This is reinforced by great distance from the sea in continental interiors, by rain shadows behind mountain ranges, and by cold offshore currents that chill and stabilise the air along coasts such as the Atacama and the Namib.

4. 4

   Where is the temperate grassland biome found?

   Model answer

   In the continental interiors of the mid-latitudes, broadly between about thirty and fifty-five degrees north and south. The main areas are the Prairies of the United States and Canada, the Steppes stretching from Ukraine across southern Russia into central Asia, the Pampas of Argentina and Uruguay, the Veld of South Africa, and the Canterbury Plains of New Zealand. They lie too far from the ocean, and often in the rain shadow of mountain ranges, for enough rainfall to support forest, but they receive enough for a continuous cover of grasses.

5. 5

   Where is the tundra biome found?

   Model answer

   In the high latitudes of the northern hemisphere, generally north of about sixty to sixty-six degrees, forming a belt between the northern edge of the coniferous forest, or taiga, and the permanent ice of the Arctic. It covers northern Alaska, the northern mainland and islands of Canada, the ice-free coastal fringe of Greenland, northern Scandinavia, Iceland and a vast area of northern Siberia. It is overwhelmingly a northern hemisphere biome because at the equivalent southern latitudes there is almost no land, only the Southern Ocean; small equivalent areas occur on the Antarctic Peninsula and on sub-Antarctic islands. Alpine tundra also occurs above the treeline on high mountains in lower latitudes.

6. 6

   Why does the vegetation of a biome change as you move from the Equator to the poles?

   Model answer

   Because the two factors that limit plant growth, warmth and moisture, change systematically with latitude. Insolation is intense and year-round at the Equator and becomes progressively weaker and more seasonal towards the poles, so the length of the growing season shortens from twelve months to less than three. Rainfall varies with the general circulation of the atmosphere, being heavy where air rises at the Equator, minimal where it sinks at the tropics, and moderate in the mid-latitude belt of depressions. As warmth and water become scarcer, vegetation becomes shorter, simpler in structure, less diverse and less productive, so tall, multi-layered, evergreen rainforest gives way through desert scrub, grassland and coniferous forest to low, ground-hugging tundra.

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