The global distribution of biomes

A biome is a large-scale community of plants and animals adapted to a particular climate. CCEA names four you must locate: tundra, tropical rainforest, hot desert and temperate grassland.

• Tundra — high latitudes (around the Arctic); very cold, low precipitation, permafrost, thin acidic gley/podsol soils, low biomass.
• Temperate grassland — mid-latitude continental interiors (Prairies, Steppes, Pampas); hot summers, cold winters, moderate rain; deep, fertile chernozem soils.
• Tropical rainforest — around the Equator; hot and wet all year; high biomass, leached soils.
• Hot desert — around 20–30° latitude beneath descending air; arid, sparse xerophytic vegetation.

Matching biome to soil

Climate change and the tundra

The tundra is highly sensitive to warming. CCEA requires a regional-scale case study of a tundra ecosystem to evaluate the actual and potential impacts of climate change.

• Permafrost thaw releases stored carbon dioxide and methane — a positive feedback.
• Longer growing seasons allow shrub encroachment, altering albedo and habitats.
• Species ranges shift; ground instability (thermokarst) damages ecosystems and infrastructure.

Keep it honest: discuss the direction and mechanisms of change and evaluate their severity qualitatively — do not invent precise temperature or carbon figures for your case study.

How a small-scale ecosystem functions

An ecosystem couples living and non-living parts:

• Biotic components — plants, animals, decomposers.
• Abiotic components — climate, soil, water, light.
• Energy and matter move through as inputs, outputs, transfers and stores.

Trophic structure: autotrophs (producers) fix energy; heterotrophs (consumers) eat other organisms; decomposers recycle nutrients. Energy passes up trophic levels, drawn as a pyramid — because only about 10% of energy transfers between levels.

The nutrient cycling model (Gersmehl) stores nutrients in three compartments — biomass, litter and soil — with flows between them and inputs (weathering, precipitation) and losses (leaching, run-off).

Calculate — energy transfer

Calculate — transfer efficiency

Whose climate feature?

Tap a feature, then tap the biome it belongs to.

Why a pyramid?

Seral stages, climatic climax and plagioclimax

Succession is the change in a plant community over time. A sere is the whole sequence; each stage is a seral stage.

• Pioneer species (e.g. lichens on bare rock, or marram grass on dunes) begin soil formation.
• Conditions improve, so more demanding species colonise, until a stable climatic climax community — controlled by climate — is reached.
• Where human activity (grazing, burning) arrests succession, the result is a plagioclimax (e.g. heather moorland).

CCEA asks for a small- or regional-scale succession case study — for example the Umbra sand dunes (psammosere) or a woodland such as Breen Wood. Learn the seral sequence and the controls, using vegetation described qualitatively.

Match the succession term

Tap an item on the left, then its matching partner on the right.

Plagioclimax

The nutrient cycling model in practice

In the Gersmehl model, the size of the biomass, litter and soil stores and the rate of the flows differ between biomes:

• Tropical rainforest: huge biomass store, rapid recycling, but small soil store — so cleared land quickly loses fertility.
• Tundra: slow decomposition in the cold means a large litter store and sluggish flows.
• Temperate grassland: nutrients build up in a deep, fertile soil store.

Link back: the store sizes explain soil fertility and how vulnerable each ecosystem is to disturbance.

Rainforest nutrient store

The big ideas to know

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