The big idea: A biome is a large region of the Earth that has a particular climate and a characteristic community of plants and animals.
Examples you should know are hot desert, tropical rainforest, temperate forest, grassland and tundra.
What decides which biome forms in a place is mostly two abiotic (non-living) factors: temperature and precipitation (rainfall). Together these set the conditions that plants — and the animals that depend on them — must live in.
- Biome
- A large geographical region with a particular climate and a characteristic community of organisms (for example tropical rainforest or tundra).
- Climate
- The typical pattern of temperature and precipitation in a region, averaged over many years.
- Abiotic factor
- A non-living physical or chemical feature of the environment, such as temperature, rainfall, light or soil.
- Precipitation
- Water that falls onto the land, mainly as rain (also snow). It determines how much water is available to organisms.
- Temperature
- How hot or cold a region is. It affects which organisms can survive and how fast biological processes happen.
Why temperature and rainfall matter most: Plants are the base of almost every community, and plants are limited above all by how warm it is and how much water there is.
So if you know a region's temperature and its precipitation, you can predict which biome — and therefore which kind of vegetation — it will have.
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Because temperature and precipitation are the two factors that matter most, scientists classify biomes by plotting them on a chart with temperature on one axis and rainfall on the other.
Each biome occupies its own region of that chart, so a place's climate values point to a single biome.
| Abiotic factor | What the axis usually shows | Effect on the biome |
|---|---|---|
| Temperature | Mean annual temperature (hot at one end, cold at the other) | Sets which plants can grow and how fast; cold limits the tundra |
| Precipitation | Mean annual rainfall (wet at one end, dry at the other) | Sets water availability; low rainfall gives deserts, high rainfall gives rainforest |
How the two factors sort the biomes: Follow the two axes from one extreme to the other:
Hot + very dry → hot desert. Hot + very wet → tropical rainforest.
Moderate temperature + moderate-to-high rainfall → temperate forest. Moderate temperature + a drier season → grassland.
Cold + low rainfall → tundra.
So two numbers — a temperature and a rainfall — are enough to place a region into a biome.
Each biome sits in a region of the temperature–rainfall climate space. Read off a point's temperature and rainfall to find its biome.
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| Biome | Temperature | Precipitation (rainfall) | Typical vegetation |
|---|---|---|---|
| Hot desert | Hot | Very low | Sparse, drought-adapted plants (e.g. succulents) |
| Tropical rainforest | Hot | Very high | Dense, tall, evergreen broadleaf trees |
| Temperate forest | Moderate (warm summers, cool winters) | Moderate to high | Deciduous broadleaf trees |
| Grassland | Moderate | Moderate (a dry season) | Grasses, few trees |
| Tundra | Cold | Low | Low mosses, lichens and small shrubs |
Temperature axis
- Runs from hot to cold
- Hot end → deserts and rainforest
- Cold end → tundra
- Sets which plants can survive and how fast they grow
Precipitation axis
- Runs from wet to dry
- Wet end → tropical rainforest
- Dry end → desert
- Sets how much water is available to organisms
Climate decides the community: A biome's climate decides its vegetation, and the vegetation decides which animals can live there.
That is why hot, wet rainforest is packed with tall evergreen trees and huge biodiversity, while cold, dry tundra has only low mosses, lichens and small shrubs — the same two abiotic factors explain both.
A memory hook: Two axes, two questions: how HOT? how WET?
Hot + wet = rainforest; hot + dry = desert; cold = tundra. Everything in between (temperate forest, grassland) sits in the moderate middle.
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How this is tested: On Paper 1A this micro almost always appears as a data question: you are shown a graph with temperature on one axis and precipitation on the other, with points (or shaded zones) for different biomes, and asked to identify which point represents a named biome such as temperate forest.
The skill is to read both coordinates of a point and match them to the biome whose climate fits — moderate temperature and moderate-to-high rainfall for temperate forest.
On Paper 2 the same idea can be tested by asking you to state the two abiotic factors used to classify biomes, or to explain why a biome's climate gives it its characteristic vegetation.
IB-style question — read a temperature-vs-precipitation biome graph
A biome graph plots mean annual temperature on the x-axis and mean annual precipitation on the y-axis. Four points are marked: P (hot, very dry), Q (hot, very wet), R (moderate temperature, moderate-to-high rainfall) and S (cold, low rainfall). Identify which point represents temperate forest. [1]
How to read the graph
- Recall the climate of a temperate forest. It has a moderate temperature (warm summers, cool winters) and moderate-to-high rainfall — not extreme on either axis.
- Read each point's two coordinates. P is hot and very dry (a hot desert); Q is hot and very wet (tropical rainforest); S is cold with low rainfall (tundra).
- Match the moderate-moderate point. Only point R has a moderate temperature with moderate-to-high rainfall, so R is the temperate forest. (1 mark for R.)
Final answer
Point R — it has a moderate temperature with moderate-to-high rainfall, which is the climate of a temperate forest.
Why this scores the mark: The trick is to read both coordinates of each point, not just one.
Temperate forest is the middle climate — moderate on both axes — so you find it by ruling out the extremes (hot/dry, hot/wet and cold) and taking what is left.
| Biome | Temperature | Precipitation (rainfall) | Typical vegetation |
|---|---|---|---|
| Hot desert | Hot | Very low | Sparse, drought-adapted plants (e.g. succulents) |
| Tropical rainforest | Hot | Very high | Dense, tall, evergreen broadleaf trees |
| Temperate forest | Moderate (warm summers, cool winters) | Moderate to high | Deciduous broadleaf trees |
| Grassland | Moderate | Moderate (a dry season) | Grasses, few trees |
| Tundra | Cold | Low | Low mosses, lichens and small shrubs |