A biome is a group of ecosystems that share a climate and, because of that, share a look. You do not need to memorise every number in the tables below — you need to be able to say why a rainforest sits where it does and a tundra does not, and what happens to both as the climate shifts.
📘 What you need to know
A biome is a group of similar ecosystems that developed under similar climatic conditions.
Biomes are defined by their dominant vegetation, climate and abiotic factors, and cover large areas.
Main groups: freshwater, marine, forest, grassland, desert and tundra.
Each has characteristic limiting factors, productivity and biodiversity.
Distribution is set mainly by insolation, temperature and precipitation.
Global warming causes range shifts (species move) and biome type changes (the biome itself converts).
Boundaries are moving poleward and upward in elevation.
What decides where a biome forms
Three abiotic factors do most of the work:
Insolation — the amount of solar radiation reaching the surface. It sets temperature and the rate of photosynthesis.
Temperature — controls photosynthesis and respiration in plants, and metabolic rate in animals.
Precipitation — controls water availability, which is the limiting factor across huge areas of the planet.
Put temperature and precipitation together and you can predict the biome. That is the single most useful idea on this page.
A deliberate simplification of the usual climate diagram. Real boundaries blend into each other, but the pattern is right: warm and wet gives rainforest, cold and dry gives polar desert.
If a question gives you a temperature and a rainfall figure and asks which biome it is, do not panic about exact boundaries. Place it roughly on this grid and justify your answer with the two numbers. The reasoning is what is marked.
Forest biomes
Feature
Tropical rainforest
Temperate forest
Boreal forest
Location
Low latitudes, within the tropics; Amazon, Southeast Asia, Congo Basin
40 to 60 degrees north and south; western Europe, northeast USA, eastern Asia
50 to 60 degrees north and south; Canada, Russia, Scandinavia
Annual precipitation
Over 2000 mm
750 to 1500 mm, all year round
300 to 900 mm, all year round
Temperature
26 to 28 degrees C
Above 0 in winter, 20 to 25 in summer
Around minus 30 in winter, up to 20 in summer
Seasons
No real seasons; hot and wet all year
Four seasons of similar length
Two main seasons: winter and summer
Growing season
All year
6 to 8 months
2 to 3 months
Soils
Fairly infertile: heavy leaching and fast nutrient uptake by plants
Fairly fertile: organic matter decomposes over autumn and winter
Poor and often acidic, with permafrost and a thick litter layer
Biodiversity
Very high; roughly half of the world’s species. Mahogany, lianas, orchids; toucans, jaguars, frogs
High, but below rainforest. Beech, oak, birch; deer, squirrels, bears
Lower than temperate forest. Conifers; reindeer, wolves, bears
Notice the pattern running left to right: as you move away from the equator, the growing season shortens, precipitation falls and biodiversity drops. That trend is worth far more in an exam than any individual figure.
Grassland biomes
Feature
Savanna
Temperate grassland
Location
5 to 30 degrees north and south, bordering tropical forest; central Africa such as Tanzania and Kenya
40 to 60 degrees north and south; the pampas of Argentina, steppes of Russia, plains of the USA
Annual precipitation
800 to 900 mm
250 to 750 mm
Temperature
15 to 35 degrees C
Minus 40 to 40 degrees C
Seasons
Wet season and dry season
Four seasons
Growing season
During the wet season, 4 to 5 months
During summer, depending on temperature
Soils
Free draining, thin humus layer, most nutrients near the surface
Fertile
Biodiversity
Wide range of species and the greatest diversity of hoofed animals. Grasses, baobab, acacia; zebras, elephants, giraffes
Large numbers of plants and animals. Grasses and sunflowers; bison, antelopes, rabbits
Desert and tundra biomes
Feature
Hot desert
Tundra
Location
15 to 30 degrees north and south; Sahara, Kalahari, Namib, Australia, Middle East
North of the Arctic Circle, and Antarctica
Annual precipitation
Below 250 mm
Less than 250 mm
Temperature
Daytime can reach 50 degrees C, averaging around 25; nights below 0
Below 0 degrees C for 6 to 10 months
Seasons
Summer and winter
Winter and summer
Growing season
All year
Only 6 to 10 weeks
Soils
Infertile and dry
Thin, infertile, with permafrost beneath
Biodiversity
Low. Cacti and yucca; spiders, scorpions, camels, meerkats
Same low rainfall, opposite reason for low productivity. In a hot desert, water is the limiting factor. In the tundra, water is technically scarce too, but the real limits are low temperature and permafrost, which lock the water away and shorten the growing season to weeks.
How global warming is moving biomes
Because biome distribution follows climate, changing the climate moves the biomes. Two things happen, and exams want both named:
Range shifts — species move to new areas as their current habitat becomes unsuitable.
Biome type changes — a whole biome converts into a different one, for example forest becoming savanna, or tundra becoming forest.
The direction of movement is consistent: boundaries shift poleward and upward in elevation. Warmer biomes such as tropical rainforest and savanna expand, while colder biomes such as tundra and boreal forest contract.
The consequences follow on:
Species arriving in new areas meet unfamiliar competition, predators and disease.
Populations decline, and in some cases species go extinct — especially those with nowhere further to go, such as mountain-top and polar species.
Ecosystem services suffer, including water regulation, nutrient cycling and carbon sequestration.
WORKED EXAMPLE
A region has a mean annual temperature of 27 degrees C and annual precipitation of 2400 mm, with no dry season. Identify the biome and justify your answer.
Step 1: Read the temperature27 degrees C is hot and consistent, so this is a low latitudeStep 2: Read the precipitation2400 mm is above 2000 mm, with no dry seasonStep 3: Combine themHot plus very wet plus no seasons gives a year-round growing seasonTropical rainforestSavanna would need a distinct dry season, which is absent here
WORKED EXAMPLE
Explain why tundra species are especially at risk as global temperatures rise.
Step 1: What happens to the biomeWarming allows boreal forest to expand northward into tundraStep 2: Where can tundra species go?poleward – but the Arctic Ocean ends the landStep 3: State the consequenceTheir range is squeezed against a hard boundary, so there is no refugeRange contraction with nowhere to shift, raising extinction risk
💡 Exam tip
Justify biome identification using both temperature and precipitation, plus seasonality if you have it.
Learn the trend across the tables (growing season, rainfall, biodiversity) rather than every number.
Name the limiting factor when asked why a biome has low productivity: water in deserts, temperature and permafrost in tundra.
Use the terms range shift and biome type change — both are specific mark-scheme wording.
State the direction of movement: poleward and upward in elevation.
⚠ Common mix-up
Biome and ecosystem used as the same word. A biome contains many ecosystems across a large geographic area.
Assuming rainforest soil is rich. It is relatively infertile — nutrients are in the living biomass, not the soil.
Thinking savanna and temperate grassland are interchangeable. Savanna has a wet and dry season; temperate grassland has four seasons.
Saying biomes simply “die” under warming. They shift and convert — describe the movement, not just loss.
Ignoring elevation. Biomes move up mountains as well as toward the poles.
Up next: Circulating Air and Ocean Currents — the reason deserts sit at 30 degrees and rainforests sit on the equator.
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