The climate decides the biome: Abiotic factors decide which biome develops on land. It is the climate, the long-term average, not one day's weather, that does it.
The points to remember
- Abiotic factors are the determinants of where land biomes are found.
- The main ones are temperature and precipitation, with insolation (sunlight energy).
- Together they set the water and energy available for photosynthesis.
- They decide the soil and the plants; the plants decide which animals can live there.
- For any temperature and rainfall pattern, one natural ecosystem type is likely to develop.
- Warm and wet: dense forest. Dry: grassland or desert. Cold: slow growth, tundra or taiga.
Remember it as: Water and warmth decide the plants; the plants decide the animals.
Real example: crossing the USA westwards at about 40° N, yearly rain falls from over 1,000 mm near the Appalachian Mountains to under 400 mm at the foot of the Rockies. The biome changes with it: deciduous forest, then tall-grass prairie, then short-grass prairie.
Factor plus effect: Name the factor and what it does: 'low temperature slows photosynthesis', not just 'temperature'. If temperature or rainfall changes, the biome boundaries shift too.
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The biome graph puts the two main factors together. Each area is one biome.
How to use the biome graph
- Across: mean annual temperature (°C). Up: annual precipitation (mm).
- Plot the place's two numbers: the area the point falls in is its likely biome.
- Hot and very wet: tropical rainforest. Hot and dry: hot desert.
- Coldest: tundra; a little warmer: boreal forest. Mild and wet: temperate forest.
- Mean annual temperature = the 12 monthly means added up, divided by 12.
Real example: Manaus, Brazil, averages 27.4 °C and 2,385 mm a year, so it plots in tropical rainforest. Cairo, Egypt, averages 22.3 °C but only about 25 mm, so it plots in hot desert.
Name it in full: Give the biome's full name from the chart, and only one answer. Near a boundary, quote both values to show which side the point falls on.
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Each abiotic factor can be a limiting factor. Where one is scarce, productivity is low and the biome is simple.
How the climate limits productivity
- Low temperature slows enzymes and photosynthesis, and the growing season is short.
- Below 0 °C water freezes, so roots cannot take it up; permafrost blocks roots and drainage.
- Low precipitation gives too little water for photosynthesis: plants are small and far apart.
- Low insolation (a low sun and short winter days) gives little energy for photosynthesis.
- Warm, wet and sunny all year gives the highest productivity: tropical rainforest.
| Factor | Iqaluit, Canada | Effect |
|---|---|---|
| Temperature | Below 0 °C for about 8 months | Short growing season; slow photosynthesis |
| Water | About 400 mm a year; frozen in winter | Little water plants can use |
| Insolation | About 4 hours of daylight in December | Little energy in winter |
| Ground | Permafrost | Shallow roots only; no trees |
Real example: Iqaluit, in Canada's Arctic, lies in the tundra: mosses, lichens and dwarf shrubs, but no trees, because the cold, the frozen ground and the dark winters limit growth.
Use the data you are given: Tie each way to the data: 'below 0 °C for five months, so water is frozen'. 'High rainfall' is never a reason a cold biome is unproductive.
Water is the factor that differs most from region to region. How much falls, how much evaporates, and how much stays frozen decide the biome.
Water and the biome
- Tropics with high rainfall and transpiration all year: tropical rainforest.
- Where evaporation exceeds precipitation, water limits growth: desert.
- Near the poles most water is frozen (ice, permafrost), so little is left for plants: tundra.
- Mid-latitudes with moderate rain and transpiration: temperate forest or grassland.
- Where water inputs exceed outputs, or the water table is high: wetland.
- Mountains: the windward side is wet (forest); the leeward side is in a rain shadow (dry).
| Place | Rain a year | Biome |
|---|---|---|
| Hoh Rainforest (west, windward) | About 3,500 mm | Temperate rainforest |
| Sequim (north-east, leeward) | About 400 mm | Dry grassland and oak woodland |
Real example: wet winds from the Pacific rise over the Olympic Mountains, Washington, cool and drop their rain on the west side. The air sinks and warms on the east side, so Sequim lies in a rain shadow.
Link every point to a biome: In a longer answer, pair each water fact with the biome it produces: 'evaporation exceeds precipitation, so water limits growth and a desert forms'. Facts with no biome score less.
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On a mountain the temperature falls as you climb, so the vegetation changes in bands. A vegetation map read with an elevation map shows it.
Altitude and maps
- Climbing a mountain is like travelling towards a pole: it gets colder, so biomes change in bands.
- The highest land has the coldest biome: alpine grassland, scrub, or bare rock and ice.
- Use two maps together: find the highest zone on the elevation map, then read its vegetation.
- Name the vegetation exactly as the key does: 'subtropical rainforest', not 'rainforest'.
Mount Kinabalu, Borneo, from foot to summit
Below 1,200 m
Lowland rainforest, very tall and rich in species.
1,200-2,000 m
Montane forest of oaks and laurels, shorter trees.
2,000-2,800 m
Cloud forest: small trees covered in mosses.
2,800-3,400 m
Subalpine scrub of low, twisted shrubs.
Summit, 4,095 m
Bare granite with a few grasses.
Real example: Mount Kinabalu in Malaysian Borneo rises from rainforest to bare granite in about 15 km. Each band holds different species, which is one reason the mountain is so rich in plants.
Copy the key exactly: If the highest zone is 'grassland and woodland', give both words. 'Forest' alone, or 'rainforest' when the key says 'subtropical rainforest', is too vague.
You may be asked to draw a climate graph for a biome from a table of monthly data. It uses the same layout as the graphs in the papers.
Drawing a climate graph
- Draw 12 bars for monthly precipitation, on a left axis in mm.
- Plot 12 points for mean monthly temperature on a right axis in °C, and join them.
- Label the months, both axes with units, and give a title naming the place.
- Add the total precipitation and the mean temperature: they place it on the biome graph.
Real example: Manaus's graph shows the tropical rainforest pattern: a flat temperature line near 27 °C and rain every month, adding up to 2,385 mm.
Two axes, two scales: Choose a scale for each axis that uses most of the height, and start the temperature axis below zero if any month is below freezing.
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How this comes up: Paper 2, Section B (b): explain, with named biomes, how abiotic factors decide where biomes are [7].
Tropical rainforest, hot desert and tundra are found in very different parts of the world.
Explain how abiotic factors determine the distribution of three named terrestrial biomes.
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