Géographie

Biomes: Climate First, Country Second

A biome is defined by climate and vegetation, not by borders. The six major biomes set against their rainfall and temperature, and why they sit where they do.

Biome

A biome is a large-scale region of the Earth defined by its climate and by the community of plants and animals that the climate supports.

Où les élèves le rencontrent: Grade 6 geography, and again in the ecosystems unit at GCSE, where the question is usually to explain why a biome sits where it does rather than simply to name it.

La réponse en bref
A biome is a very large region defined by its climate and by the plant and animal life that climate supports — tropical rainforest, hot desert, savanna, temperate forest, boreal forest, tundra. Biomes follow rainfall and temperature rather than borders, so one country can hold several and one biome can cross many.

Un exemple

Pakistan alone contains hot desert, temperate forest and high-altitude tundra.

The Thar and the deserts of Balochistan are hot desert; the coniferous slopes above Murree, Swat and Chitral are temperate forest; and above the tree line in the Karakoram the vegetation is tundra, kept low and sparse by cold rather than by drought. One passport, three biomes — which is the clearest possible demonstration that a biome is not a kind of country.

Two numbers decide almost everything

Plot the world's land on a chart with annual rainfall along one axis and average temperature along the other, and the biomes sort themselves into patches on it. Wet and hot gives rainforest. Dry and hot gives desert. Wet and cool gives temperate forest. Dry and cold gives tundra. The rough figures below are typical rather than exact, and the boundaries between them are broad zones rather than lines.

Two of the definitions are genuinely precise, and worth knowing for that reason. A desert is usually defined as receiving under 250 mm of rainfall a year, and tundra as a place where the warmest month averages below 10 °C — which is also, more or less, the reason no trees grow there.

Tropical rainforest — over about 2 000 mm of rain a year, warm all year round, no real dry season
Savanna, or tropical grassland — roughly 500 to 1 500 mm, nearly all of it in one wet season, followed by a long dry one
Hot desert — under about 250 mm a year, with a very large difference between day and night temperatures
Temperate deciduous forest — roughly 750 to 1 500 mm spread through the year, with a winter cold enough to make broadleaved trees drop their leaves
Boreal forest, or taiga — low rainfall, long winters far below freezing and short summers, dominated by conifers that keep their needles
Tundra — under about 250 mm, so as much a cold desert as a wet place, with the ground frozen for most of the year

Biome, ecosystem, habitat: three rungs of one ladder

The three words are often used as if they were synonyms, and the difference between them is scale. A habitat is where a particular organism lives: a fallen log, a hedgerow, a rock pool. An ecosystem is a community of living things together with the physical conditions they live in: a pond, a woodland, a coral reef. A biome is the global category that many similar ecosystems belong to.

Which is why the boreal forest of Canada, Scandinavia and Russia is one biome and not three. Different countries, different species in the detail, same climate and the same response to it.

Why the deserts sit where they do

Latitude sets the temperature, and the circulation of the atmosphere sets the rainfall. Near the equator, air heated by the strong overhead sun rises, cools and drops its moisture as it goes, which is why the rainforest belt lies where it does. That air travels away from the equator high up and descends again at around 30° north and south — and descending air warms, so it takes up moisture rather than releasing it.

Follow that band of descending air around the globe and you find the great hot deserts: the Sahara, the Arabian Desert, the Thar, the deserts of Australia and northern Mexico. Altitude then repeats the whole pattern in miniature, which is why a day's climb in the Karakoram can take you from subtropical valley through temperate forest to tundra.

Questions fréquentes

Is a biome the same as an ecosystem?

No — a biome is much bigger. An ecosystem is one community of living things and their surroundings, such as a single pond or wood. A biome is the worldwide category that thousands of similar ecosystems fall into. Every rainforest ecosystem in Brazil, Congo and Indonesia belongs to the same rainforest biome.

How many biomes are there?

There is no agreed number, because the boundaries are drawn by people rather than by nature. Different schemes recognise six, nine or more than a dozen, depending on how finely they split categories such as grassland and scrub. School courses generally work with six to ten, so follow the list your syllabus uses.

Which biome is Pakistan in?

Several, which is the useful answer. The southern and western plains are hot desert, much of the country is dry grassland and scrub, the northern mountain slopes carry temperate and coniferous forest, and above the tree line in the Karakoram and Himalaya the vegetation is alpine tundra.

Can a biome change?

The boundaries can and do move, because they are set by climate. A shift in rainfall or temperature moves the edge between savanna and desert, or between boreal forest and tundra, and clearing land for farming can convert one to another far faster than climate alone would.

Dernière mise à jour 21 août 2026