How do the world's biomes differ in productivity and structure, and why is the biosphere under threat?
The global distribution of biomes; net primary productivity and its controls; the structure and functioning of major biomes; biodiversity; and the threats to the biosphere from human activity and climate change.
A focused answer to the AQA A-Level Geography 3.1.6 content on the biosphere, covering the global distribution of biomes, net primary productivity and its controls, the structure and functioning of major biomes, biodiversity, and the threats to the biosphere from human activity and climate change.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
AQA section 3.1.6 wants you to understand the global biosphere: the distribution of major biomes, net primary productivity and what controls it, the structure and functioning of biomes, biodiversity, and the threats to the biosphere from human activity and climate change. It scales the systems concepts up from a single ecosystem to the planet.
The global distribution of biomes
Moving from the equator to the poles, the major biomes run: tropical rainforest (equatorial, hot and wet), savanna (seasonal tropics), hot desert (subtropical highs), Mediterranean and temperate grassland/forest (mid-latitudes), boreal forest or taiga (high latitudes) and tundra (subpolar). Their distribution is set by climate, modified by relief, soils and ocean currents.
Net primary productivity and its controls
Net primary productivity (NPP) is the rate at which producers store energy as new biomass after their own respiration, usually measured in grams of carbon or kJ per square metre per year.
The most productive biomes generally support the greatest biodiversity, because more energy and more stable conditions allow more species and more complex food webs.
Structure, functioning and biodiversity
Within a biome, structure reflects its productivity: the rainforest has a tall, multi-layered canopy with rapid nutrient cycling and a huge biomass store, while the tundra is low, sparse and slow-cycling with nutrients locked in litter. Biodiversity (the variety of life) is highest where productivity and stability are greatest, so the rainforest and coral reefs are biodiversity hotspots. High biodiversity supports ecosystem services (climate regulation, pollination, water purification) on which people depend.
Threats to the biosphere
The biosphere faces two interacting pressures:
- Human activity: deforestation (clearing the most biodiverse, high-NPP forests), agriculture and land-use change, pollution, overexploitation (overfishing, hunting) and habitat fragmentation, all reducing biodiversity and ecosystem services.
- Climate change: shifting biome boundaries, disrupting the timing of life cycles, ocean acidification and warming that bleaches coral reefs, and stressing species that cannot migrate or adapt fast enough.
The two reinforce each other: deforestation destroys habitat and releases carbon, worsening climate change, so the combined threat is greater than either alone, and the most productive, biodiverse biomes are most at risk.
Try this
Q1. Define net primary productivity. [2 marks]
- Cue. The rate at which producers store energy as new biomass after their own respiration (per unit area per year).
Q2. Explain why the tropical rainforest has the highest NPP. [3 marks]
- Cue. Warmth, abundant rainfall and year-round sunlight allow continuous, rapid photosynthesis with no seasonal shutdown.
Q3. Outline two threats to the biosphere from human activity. [4 marks]
- Cue. Deforestation (loss of high-NPP, biodiverse forest) and overexploitation or pollution (reducing biodiversity and ecosystem services).
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 2019 (style)6 marksExplain why net primary productivity varies between biomes.Show worked answer →
A 6 mark "explain" question (AO1). Net primary productivity (NPP) is the rate at which producers store energy as biomass after their own respiration. It depends mainly on temperature, water availability and light/nutrients.
The tropical rainforest has the highest NPP because warmth, abundant rainfall and year-round sunlight allow continuous, rapid photosynthesis. Deserts and tundra have low NPP, limited by water shortage and cold temperatures respectively. Temperate forests are intermediate, productive in the growing season but limited by winter cold.
Markers reward linking the climatic controls (temperature, water, light) to NPP and ranking biomes accordingly. Top answers note that nutrient availability and the length of the growing season also matter.
AQA 2021 (style)9 marksAssess the threats posed to the biosphere by human activity and climate change.Show worked answer →
A 9 mark "assess" question (AO1 plus AO2): reach a judgement. Human activity threatens the biosphere through deforestation (loss of the most biodiverse, high-NPP forests), agriculture and land-use change, pollution, overexploitation and habitat fragmentation, all reducing biodiversity and ecosystem services. Climate change shifts biome boundaries, disrupts the timing of life cycles, acidifies oceans and stresses species that cannot migrate fast enough.
The judgement: the two interact and reinforce each other (deforestation both destroys habitat and worsens climate change by releasing carbon), so the combined threat is greater than either alone, and the most biodiverse, high-productivity biomes (rainforest, coral reef) are most at risk. Reward a calibrated conclusion that links the threats and prioritises the most vulnerable, high-value biomes.
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Sources & how we know this
- AQA A-level Geography (7037) specification — AQA (2016)