Groups of biomes at Higher Level: The same six groups as SL, with different real examples: Canada's biomes, Borneo, the Sonoran Desert, the Yamal tundra, the prairies, the Serengeti and the Great Barrier Reef.
Practise this as you read
- Link each biome's limiting factor to its productivity and its biodiversity.
- Quote both temperature and precipitation whenever you describe or compare a biome.
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Same climate, same kind of ecosystem: A biome is a large-scale ecosystem type shaped mainly by climate. Biomes fall into six broad groups, and each group has its own limiting factors, productivity and biodiversity.
The points to remember
- A biome is shaped mainly by climate: temperature and precipitation.
- Six groups: freshwater, marine, forest, grassland, desert and tundra.
- Each group splits into subcategories: tropical rainforest, temperate forest, boreal forest.
- Climate decides the plants; the plants decide the animals, with their food and shelter.
- Each group has its own limiting factors, productivity and biodiversity.
Remember it as: Water, woods, grass, sand and snow: fresh, salt, forest, grassland, desert, tundra.
On land (terrestrial)
- Forest: tropical rainforest, temperate deciduous, boreal (taiga)
- Grassland: savanna, temperate grassland
- Desert: hot desert, cold desert
- Tundra: Arctic, alpine
In water (aquatic)
- Freshwater: lakes, rivers, wetlands
- Marine: open ocean, coral reefs, estuaries, mangroves
Real example: Canada crosses five biomes: tundra in the far north, a wide belt of boreal forest, the prairies (temperate grassland), temperate rainforest on the wet Pacific coast and deciduous forest near Toronto, with the freshwater Great Lakes in between.
Name it in full: Say 'tropical rainforest', not 'rainforest'; 'coniferous forest', not 'forest'. A name without its qualifier does not count.
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Why is one biome lush and another bare? Plants need light, warmth, water and nutrients. The one in shortest supply is the limiting factor. It sets the biome's net primary productivity, and so how much life it can feed.
The points to remember
- Plants need light, warmth, water and nutrients; the scarcest one limits growth.
- Productivity is highest where it is warm and wet all year, so photosynthesis never stops.
- It is lowest where it is too cold (tundra) or too dry (desert).
- Biodiversity is highest where conditions are stable and resources plentiful.
- It is lowest where conditions are extreme or change a lot with the seasons.
- Limiting factors keep populations small and food chains short.
Real example: on Alaska's Arctic coast the ground is frozen for about nine months, so tundra plants make only about 140 g per m² a year; the rainforest of Borneo makes over 2,000.
Remember it as: Warm and wet: busy. Cold or dry: quiet.
Tropical rainforest is the most productive and most diverse biome on land. Almost everything about it follows from its climate: hot and wet all year, with no cold or dry season to stop growth.
Tropical rainforest: the points to remember
- Found near the equator, between the tropics: hot all year (over 24 °C) with little seasonality.
- Abundant rain (over 2,000 mm) and high humidity; high insolation all year.
- Highest NPP on land: a major carbon sink that releases a lot of oxygen.
- Highest biodiversity: many endemic species, complex food webs, biodiversity hotspots.
- Stratified: emergent trees above a dense canopy, tall straight trunks with buttress roots.
- Fast decomposition and nutrient turnover, but thin, nutrient-poor, acidic soils.
- Limiting factors: light below the canopy and nutrients in the soil.
Real example: the rainforest of Borneo is home to orangutans found nowhere else; the tallest known tropical tree, a yellow meranti in Sabah, is about 100 m tall. Kuching gets about 4,000 mm of rain a year.
Be exact: 'Found in the tropics' alone is not a feature. Say what the climate, soil, plants or productivity are like, each in its own sentence.
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In a hot desert, water decides everything. Rain is rare and unreliable, so plants are few and small, and everything alive is built to find, store or save water.
Hot desert: the points to remember
- Water is the limiting factor: under 250 mm of rain a year, often much less.
- Very hot days, often over 40 °C, but cold nights: clear skies let heat escape.
- Very low NPP (about 90 g per m² a year): plants are small and far apart.
- Low biodiversity, but species are specialised: they store water, save water or hide from the heat.
- Slow decomposition (too dry), so soils are thin and poor in organic matter.
Real example: the Sonoran Desert of Arizona and Mexico gets 75-300 mm of rain in two short seasons. The saguaro cactus can hold hundreds of litres after one storm.
Link the factor to the result: Write the chain: little water, so little photosynthesis, so low NPP, so few species. A feature on its own is one point; the link shows you understand it.
The tundra lies beyond the tree line, near the poles and on high mountains. Cold and darkness are the limiting factors: plants have only a few warm weeks to grow.
Tundra: the points to remember
- Low mean temperatures: below 0 °C for most of the year, a short growing season.
- Low precipitation, mostly frozen (snow); permafrost below the surface.
- Limited sunlight at high latitude: long dark winters.
- Low primary productivity: small plants, lichens and mosses, no trees.
- Low species diversity and short food chains.
- Slow decomposition in the cold: dead matter builds up as peat.
- Animals migrate or hibernate to survive the winter.
Real example: on the Yamal Peninsula in Arctic Russia, the Nenets people move their reindeer up to 1,000 km a year, between summer tundra pastures and the forest edge in winter. Lemmings stay active under the snow.
Tundra is dry, not wet: Tundra gets about as little precipitation as a desert. 'High rainfall' is wrong; say 'low precipitation, mostly as snow'.
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Between the tropics and the poles lie the biomes most people live in: forests where there is enough rain for trees, grasslands where there is not. Each one is a subcategory of the forest or grassland group.
Two more forests and two grasslands
- Boreal forest (taiga): long cold winters, short summers; conifers; moderate NPP, low diversity.
- Temperate deciduous forest: mild, rain all year; trees drop leaves before the cold winter; fertile soil.
- Temperate grassland: moderate rain (250-750 mm), too dry for most trees.
- Savanna: hot, with a wet season and a long dry season; grass with scattered trees.
- Grasslands are kept open by grazing and fire; they grow in a season, not all year.
Boreal forest (taiga)
- Winters below -20 °C
- Needle-leaved conifers shed snow
- Slow decomposition, acid soil
- NPP about 800
Temperate deciduous forest
- Mild, 500-1,500 mm rain
- Broad leaves fall in autumn
- Fast leaf litter decay, fertile soil
- NPP about 1,200
Real example: the North American prairies were kept treeless by fire and by millions of grazing bison; their deep, fertile soils now grow wheat. The Serengeti savanna greens in the rains and dries out from June, when about 1.5 million wildebeest move on to find grass.
Most of the Earth is covered by water, and water biomes are grouped too. Here the key abiotic factor is salinity.
The points to remember
- Water biomes are grouped by salinity: freshwater (under 0.5 parts per thousand) and marine (about 35).
- Freshwater: lakes, rivers and wetlands. Marine: oceans, coral reefs, estuaries, mangroves.
- Limiting factors in water: light (falls with depth), nutrients, dissolved oxygen, temperature.
- Shallow, sunlit, nutrient-rich water (reefs, estuaries, marshes) is very productive; the open ocean is not.
- Mangroves: warm tropical coasts, tides, salty, anaerobic mud; very productive, store much carbon.
Real example: the Great Barrier Reef, off Queensland, Australia, is the largest coral reef system on Earth. Its water is warm, clear and shallow but poor in nutrients, yet the reef recycles them so well that it is one of the most productive ecosystems there is.
Mangroves in the exam: Treat mangroves as a biome shaped by abiotic factors: salinity, flooding by the tides, and warm temperatures.
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Many questions hand you a figure and ask you to name a biome or describe its climate. The marks go to exact names and exact numbers read from the figure.
Reading a biome figure
- On a biome diagram, find the temperature on one axis and the precipitation on the other.
- Quote both values for each biome, with units: '20-30 °C and 0-50 cm'.
- On a map, use the key: name the colour exactly as the key does.
- Name the biome in full: 'coniferous forest', 'tropical rainforest', not 'forest' or 'trees'.
- From a climate graph, give numbers: 'over 24 °C every month', 'about 1,800 mm a year'.
- Give one answer when asked for one: only the first is marked.
Real example: Utqiagvik, Alaska, has a mean of about -11 °C and 12 cm a year: tundra. Manaus, at 27 °C and 230 cm, is tropical rainforest: two biomes at opposite corners of the diagram.
Both values, every time: To distinguish two biomes, give the temperature AND the precipitation of each. 'The desert is hot and the tundra is cold' leaves out precipitation and loses the marks.
How this comes up: Paper 2, Section B (a): identify four characteristic features of a named biome [4].
Tropical rainforests grow in a belt around the equator, from the Amazon to the Congo basin and Borneo.
Identify four characteristic features of the tropical rainforest biome.
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