Rainforest sits on the equator. Desert sits at about 30 degrees. Tundra sits at the top and bottom of the map. None of that is a coincidence — give me the average temperature and the annual rainfall of a place and I can tell you, roughly, which biome you are standing in. This page shows you how that works and what the examiner wants back.
📚 What you need to know
A biome is a group of ecosystems that share a similar climate, and therefore share similar plants, animals and soils.
Biomes cover huge areas and each one contains many separate ecosystems inside it.
The main groups are freshwater, marine, forest, grassland, desert and tundra, and most of these split further.
Each biome has its own limiting factors, which set its productivity and its biodiversity.
Distribution is set mainly by insolation, temperature and precipitation — the last two are the pair you plot on a graph.
As the climate warms, biome boundaries are moving towards the poles and up mountains.
Two kinds of shift: range shifts (species move) and biome type changes (the biome itself converts to another type).
What a biome actually is
An ecosystem can be a pond. A biome is every pond-like system of that kind across the planet. It is a much bigger unit: a set of ecosystems that ended up looking alike because they formed under the same climatic conditions.
Biomes are named and recognised by their dominant vegetation, because plants respond most directly to heat and water. Get the plants right and the animals, soils and nutrient cycles follow a predictable pattern.
One sentence definition
a biome = a group of similar ecosystems that have developed under similar climatic conditions
Scale check. Ecosystem → biome → biosphere. A single Amazon oxbow lake is an ecosystem; the whole tropical rainforest belt is a biome; every living thing on Earth is the biosphere.
The main biome groups
Six broad groups are enough for SL, and most of them break down into sub-types:
Tundra — Arctic tundra and alpine tundra (high mountains)
Alpine tundra is the one students forget. It exists because climbing a mountain does the same thing to temperature as travelling towards a pole — which is why the top of a tropical mountain can look like the Arctic.
Forest biomes compared
Feature
Tropical rainforest
Temperate forest
Boreal forest
Where
Low latitudes, inside the tropics to about 23.5° N and S: Amazon, Southeast Asia, Congo Basin
Roughly 40°–60° N and S: western Europe, north-east USA, eastern Asia
Roughly 50°–60° N: Canada, Russia, Scandinavia
Rain per year
Over 2 000 mm
About 750–1 500 mm, spread through the year
About 300–900 mm, spread through the year
Temperature
Steady, about 26–28 °C all year
Above 0 °C in winter, about 20–25 °C in summer
Down to about −30 °C in winter, up to about 20 °C in summer
Seasons
Barely any — hot and wet all year
Four seasons of similar length
Mainly two — a long winter and a short summer
Growing season
All year
About 6–8 months
Only about 2–3 months
Soil
Surprisingly poor: heavy rain leaches nutrients and plants take them up fast
Fertile, because leaf litter rots down over autumn and winter
Thin, acidic, often frozen below; litter builds up because decomposition is slow
Life
Around half of all plant and animal species on Earth; mahogany, lianas, orchids, jaguars, toucans
Wide range; oak, beech and birch, with deer, foxes, squirrels and bears
Lowest of the three; conifers, with wolves, reindeer, bears and squirrels
The rainforest soil trap. Huge plant growth does not mean rich soil. In rainforest, almost all the nutrients are locked in the living biomass, not the ground — which is exactly why clearing it for farming fails within a few years.
Grassland biomes compared
Feature
Savanna
Temperate grassland
Where
About 5°–30° N and S, just outside the tropical forest belt: Tanzania, Kenya
About 40°–60° N and S: the pampas of Argentina, the steppes of Russia, the prairies of the USA
Rain per year
About 800–900 mm
About 250–750 mm
Temperature
About 15–35 °C
Wide swing, roughly −40 °C to 40 °C
Seasons
A wet season and a dry season
Four seasons
Growing season
The wet season only, about 4–5 months
Through summer, depending on temperature
Soil
Free-draining with a thin humus layer; nutrients sit near the surface
Deep and fertile — which is why so much of it is now farmland
Life
Grasses, acacia and baobab; the greatest variety of hoofed animals anywhere — zebra, elephant, giraffe
Grasses and sunflowers, with bison, antelope and rabbits
Desert and tundra compared
These two look like opposites but they share a limiting factor: in both, liquid water is hard to get hold of. In a hot desert it evaporates; in tundra it is frozen.
Feature
Hot desert
Tundra
Where
About 15°–30° N and S: the Sahara, the Kalahari and Namib, Australia, the Middle East
North of the Arctic Circle, and around Antarctica
Rain per year
Under 250 mm
Under 250 mm
Temperature
Daytime can reach 50 °C, averaging about 25 °C; nights drop below 0 °C
Below 0 °C for 6–10 months of the year
Seasons
Summer and winter
A long winter and a brief summer
Growing season
All year, but only where water can be found
Just 6–10 weeks
Soil
Dry and infertile, with almost no organic matter
Thin and infertile, sitting on permafrost
Life
Low biodiversity: cacti and yucca, with camels, meerkats, scorpions and spiders
Low biodiversity: mosses, lichens and small grasses, with snowy owls, Arctic foxes, caribou and polar bears
Notice the pattern in every table: whatever is in short supply becomes the limiting factor, and the limiting factor sets both productivity and biodiversity. Water in the desert, warmth in the tundra, light on the rainforest floor.
What decides where a biome sits
Three controls do nearly all the work:
Insolation — the solar radiation reaching the surface. It drives temperature and the rate of photosynthesis.
Precipitation — how much water is available, which is the key limiting factor for most land biomes.
Temperature — it sets the rate of photosynthesis and respiration in plants, and the metabolic rate of animals.
Plot the last two against each other and the world’s biomes sort themselves into blocks. This graph is worth memorising in outline, because ESS papers love asking you to read a biome off it.
The blank space above the blocks is not a mistake: cold air simply cannot hold enough water vapour to deliver that much rain, so those combinations do not exist on Earth.
Reading the graph in an exam. Go across to the temperature, up to the rainfall, and name the block. Then justify it with the numbers you used — “hot, over 250 cm of rain, so tropical rainforest” is a full answer.
Biomes by latitude
Because insolation depends on latitude, the biomes line up in bands running east to west. The pattern repeats either side of the equator, which is a strong clue that something global is driving it. (That something is atmospheric circulation, and it is the next page.)
The bands are not perfectly symmetrical, because the southern hemisphere has far less land at 50–60 degrees. Mountains and ocean currents bend the pattern locally too.
Limiting factors, productivity and biodiversity
A limiting factor is whatever is in shortest supply and therefore caps growth. Every biome has one, and it explains almost everything else about that biome.
Hot desert: water is limiting, so productivity is low and few species can cope — low biodiversity.
Tundra: low temperature and permafrost are limiting, so the growing season is weeks long and productivity is tiny.
Tropical rainforest: heat, light and water are all plentiful, so productivity is the highest of any land biome, and biodiversity follows it.
Boreal forest: cold limits decomposition as well as growth, which is why litter piles up on the forest floor.
The chain to remember runs: climate → limiting factor → productivity → biodiversity. Almost any “explain why biome X has low biodiversity” question is answered by walking down that chain.
Biomes on the move
Biome boundaries are drawn by climate, so when the climate shifts, the boundaries shift with it. Two things happen:
Range shifts — species move into new areas as their old habitat stops suiting them.
Biome type changes — the biome itself converts to another type, such as forest drying into savanna, or tundra being taken over by trees.
As global temperatures rise, boundaries are moving polewards and upwards in elevation. Warmer biomes such as tropical rainforest and savanna expand; colder ones such as tundra and boreal forest are squeezed. Tundra has nowhere left to retreat to, which is why it is expected to lose the most.
The consequences are not only about species lists. Moving species meet new competitors, predators and diseases, so populations fall and some go extinct. Ecosystem services shift with them — water regulation, nutrient cycling and carbon storage all change when a biome converts.
There is a nasty feedback here worth a mark in an evaluation question: thawing tundra releases carbon that has been frozen for thousands of years, which warms the climate further, which thaws more tundra.
Worked examples
EXAM Q1
A location has an average annual temperature of 26 °C and 310 cm of rain a year. Identify the biome and justify your answer. [3]
Step 1: read across the temperature axis
26 °C sits in the hot band, past 20 °C.
Step 2: read up the precipitation axis
310 cm is above the 250 cm line, so it is in the wettest block.
Step 3: name it and justify
Hot and very wet all year means no limiting factor on growth.
Tropical rainforestquote both numbers back — the justification mark needs the evidence
EXAM Q2
Explain why tundra has low productivity compared with tropical rainforest. [4]
Point 1: insolation
Tundra is at high latitude, so insolation is low and spread over a wide area, giving low temperatures.
Point 2: growing season
Only 6–10 weeks are warm enough for photosynthesis, against all year in rainforest.
Point 3: water and nutrients
Permafrost locks water as ice and slows decomposition, so nutrient recycling is slow.
Point 4: link it up
Less photosynthesis over less time means far lower net primary productivity.
Low insolation and permafrost limit both the rate and the length of growthfour marks means four separate points, not one point said four ways
EXAM Q3
Outline one range shift and one biome type change that could result from global warming. [2]
Range shift
A species such as a bird or an insect moves further north, or higher up a mountain, tracking the temperature it needs.
Biome type change
Warming lets trees colonise ground that used to be frozen, so tundra becomes boreal forest.
One is species moving, the other is the biome itself convertingthe difference is the whole point of the question — make it obvious
💡 Exam tip
Learn two numbers per biome — a rough rainfall and a rough temperature. That is usually enough to justify any identification.
Always name the limiting factor when you explain productivity or biodiversity. It is the most reliable mark on this sub-topic.
Use real named places (Sahara, Amazon, Siberia) instead of “a hot place”. Examples score.
If asked to describe a distribution, use latitude: “found between about 15 and 30 degrees north and south” beats “near the equator-ish”.
Keep range shift and biome type change clearly separate — questions often ask for one of each.
Remember alpine tundra: altitude does the same job as latitude.
⚠ Common mix-up
Calling a biome an ecosystem. A biome contains many ecosystems; it is the larger unit.
Assuming rainforest has rich soil. The nutrients are in the plants, not the ground.
Mixing up savanna and temperate grassland. Savanna is hot with a wet and dry season; temperate grassland has four seasons and a much wider temperature range.
Saying deserts are always hot. Cold and coastal deserts exist — a desert is defined by low rainfall, not high temperature.
Forgetting tundra is a desert by rainfall. Under 250 mm a year, same as the Sahara.
Writing that biomes “move”. The boundaries shift and species relocate; the map does not slide.
Naming only temperature as the control on distribution. Precipitation matters just as much, and the graph needs both.
Up next: Circulating Air and Ocean Currents — the engine behind every band you just met, and the reason the world’s great deserts all sit at roughly the same latitude.
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