OxfordAQA International AS and A-level Geography (9635): 3.3.2 Ecosystems under stress – Study Guide

This guide covers section 3.3.2, Ecosystems under stress, of the OxfordAQA International AS and A-level Geography (9635) specification, Version 3.2. It works through every content point from 3.3.2.1 to 3.3.2.7, including the two required case studies and the named coral reef. The section is Section B of Unit 3: Physical geography 2, a 1 hour 30 minute written exam in which you answer all questions in Section A (40 marks) and Section B (40 marks). Unit 3 is part of the full International A-level, where it carries 20% of the qualification.

Use it with the revision notes, the practice questions, the OxfordAQA A-level Geography hub and the checklist.

Coverage table

Syllabus reference	What you must know or be able to do (from the document)	Where assessed
3.3.2.1	Biodiversity; local and global trends; causes, rates and impacts of decline; importance of ecosystems for people; sustainability	Unit 3, Section B
3.3.2.2	Structure, energy flows, trophic levels, food webs; systems concepts; biomass and NPP; succession terms; nutrient cycling; climate-vegetation-soil-topography links; factors of change	Unit 3, Section B
3.3.2.3	Biome concept and global distribution; tropical rainforest and savanna grassland: characteristics, adaptations, human impact, development issues	Unit 3, Section B
3.3.2.4	One sere (lithosere, halosere, psammosere or hydrosere) and its climatic climax; human effects on succession	Unit 3, Section B
3.3.2.5	Coral reef distribution and conditions; a named, located reef: natural and human factors, future prospects	Unit 3, Section B
3.3.2.6	A distinctive local ecosystem: characteristics, adaptations, local factors of change, management and conservation	Unit 3, Section B
3.3.2.7	Case study of a region experiencing ecological change; case study of an ecosystem at a local scale	Unit 3, Section B

For Unit 3 the document gives approximate weightings of 9.5% AO1 (knowledge), 7.5% AO2 (application, analysis, evaluation) and 3.0% AO3 (skills) of the whole A-level.

Biodiversity and why ecosystems matter (3.3.2.1)

Biodiversity is the variety of life in an area. It works at three levels: genetic diversity within species, species diversity, and the diversity of habitats and ecosystems.

Global trends are downward. The WWF Living Planet Report 2024 found an average 73% decline in monitored wildlife populations between 1970 and 2020. Freshwater populations fell most (85%). Latin America and the Caribbean showed the steepest regional fall (95%). The 2019 IPBES global assessment estimated that around one million animal and plant species are threatened with extinction.

The index tracks the average change in monitored vertebrate populations. It is not a count of species lost, and monitoring is uneven. Local trends can run the other way: a restored wetland can gain species while the global picture worsens.

Causes fit a simple framework: habitat change (clearance, drainage, fragmentation), overexploitation (overfishing, hunting), pollution, invasive species and climate change. Impacts include loss of ecosystem services (pollination, clean water, fisheries, coastal protection), lower resilience to shocks, and lost genetic resources for crops and medicines.

Ecosystems supply food, fuel, timber, water and space, and demand rises with population and economic development. Yet traditional fire use and grazing have shaped many ecosystems for centuries. Sustainability means using ecosystems while keeping their functions intact.

Ecosystem structure, energy and nutrients (3.3.2.2)

An ecosystem is a community of living things interacting with each other and with the non-living environment. Trophic levels are feeding positions: producers, primary consumers (herbivores), secondary and tertiary consumers. A food web shows the many linked food chains in a real ecosystem.

Energy enters as sunlight and leaves as heat. Often only around 10% passes from one trophic level to the next; the rest is lost in respiration and undigested waste. So food chains rarely exceed four or five levels.

As a system, an ecosystem has:

• inputs: solar energy, precipitation, nutrients from weathering and rain
• stores: biomass, litter, soil
• transfers: feeding, leaf fall, decomposition, nutrient uptake
• outputs: heat, leaching, runoff, harvested crops or timber

Biomass is the total mass of living organic matter, usually per unit area. Net primary production (NPP) is the rate at which producers make new organic matter, after their own respiration is subtracted. Tropical rainforest has some of the highest NPP of any land biome. Deserts and tundra have the lowest.

Mineral nutrient cycling is often shown with a three-store model (biomass, litter, soil) where circle size shows the size of the store. In a tropical rainforest the biomass store is largest; litter decomposes fast in hot, wet conditions, so the litter store is small. In a savanna more nutrients sit in the soil.

Terrestrial ecosystems result from links between climate, vegetation, soil and topography. Climate sets temperature and moisture; vegetation supplies litter that forms soil; topography alters drainage and exposure. Change one part and the others respond: drier conditions favour drought-tolerant species and slow decomposition.

Factors that change ecosystems include climate change (shifting temperature and rainfall belts) and human exploitation (clearance, grazing, fire, mining, pollution).

Succession (3.3.2.2 and 3.3.2.4)

Succession is the sequence of plant communities that develops over time on a site; each stage is a seral stage. Pioneers alter conditions, adding organic matter and shelter, so other species can arrive.

• Climatic climax: the final, stable community the regional climate allows.
• Sub-climax: succession is halted before the climax by a natural factor such as poor drainage or a regular disturbance.
• Plagioclimax: succession is held back or deflected by continuing human activity such as grazing, burning or trampling.

The four seres in the specification are the lithosere (bare rock), halosere (salt water), psammosere (sand) and hydrosere (fresh water). You need to study one in detail.

Example: a psammosere at Studland, Dorset

Studland and Godlingston Heath, on the Isle of Purbeck in Dorset, is a 631-hectare National Nature Reserve owned and managed by the National Trust. It contains sand dunes, lowland heath, wetland and a freshwater lagoon, Little Sea.

A psammosere runs inland from the beach:

1. Embryo dunes: salt-tolerant pioneers trap sand. The sand is dry and nutrient-poor, and slightly alkaline where shell fragments are present.
2. Fore dunes and yellow dunes: marram grass dominates. It grows up through fresh sand, has rolled leaves that cut water loss and long roots.
3. Grey dunes: more humus darkens the surface. More species arrive, including mosses, lichens and flowering plants. pH falls as calcium carbonate is leached.
4. Dune slacks: damp hollows where the water table is near the surface.
5. Heath and scrub: acidic soils carry heather and gorse, then birch and pine.

In lowland Britain the climatic climax would be deciduous woodland dominated by oak. At Studland much of the land is open heath. Heath is usually treated as a plagioclimax, kept open by management. The National Trust grazes it with Red Devon cattle; without grazing, birch and pine would invade. The heath supports all six native British reptiles, including the sand lizard and smooth snake. Studland is also prone to summer fires, which can reset succession.

Evaluation. The sequence is a space-for-time substitution: we assume sites further inland are older. Blowouts, trampling and fire can skip or reverse stages.

Worked example paragraph

Question: “Explain how human activity can produce a plagioclimax.”

At Studland, the heath behind the dunes would, if left alone, succeed to birch and pine scrub and then towards oak woodland, the climatic climax for lowland Britain (knowledge). The National Trust keeps it open by grazing it with Red Devon cattle, so the community is held at heath (application). This means the vegetation reflects management, not climate, which is the defining feature of a plagioclimax (analysis). However, fires at Studland also hold back succession, so heath is partly maintained by a mix of human and natural disturbance (evaluation).

Biomes (3.3.2.3)

A biome is a large-scale ecosystem with a characteristic vegetation type, controlled mainly by climate. Rainforest lies near the equator, savanna on its poleward margins and deserts near the tropics.

Tropical rainforest

• Climate: hot all year, often 26-28 °C, with heavy rainfall and no real dry season.
• Soils: deep, red, leached and nutrient-poor (latosols). The soil moisture budget is in surplus for most of the year, so leaching is intense.
• Adaptations: buttress roots support tall trees in shallow soil; drip-tip leaves shed water; epiphytes and lianas reach light; many animals live in the canopy.
• Nutrients: most are stored in the biomass. Clearance removes the main store, and rain then leaches the soil.
• Human activity: logging, ranching, soy, oil palm, mining, roads and dams.

Savanna grassland

• Climate: hot all year with a distinct wet season and a long dry season.
• Soils: ferruginous soils; the moisture budget swings between surplus and deficit.
• Adaptations: grasses die back above ground in the dry season and regrow from roots; trees such as acacias and baobabs have deep roots, thick bark and small or waxy leaves; many trees resist fire. Large herbivores migrate with the rains.
• Human activity: pastoralism, fire, cultivation, firewood, wildlife tourism and protected areas.

Development issues in both biomes

In rainforest the main issue is usually frontier clearance for ranching and crops. In savanna it is often more people and livestock on the same land, shorter fallows, and fencing that blocks herd movement. Both cause soil erosion and species loss. Both also have better options, such as agroforestry, rotational grazing and community-run conservation.

Coral reefs (3.3.2.5)

Coral reefs are built by coral polyps that host symbiotic algae (zooxanthellae), which photosynthesise and provide most of the coral’s food. Reefs lie mainly between about 30° N and 30° S, especially on the eastern sides of continents where warm currents flow.

Conditions for reef development: warm water (roughly 23-29 °C), clear, shallow water so light reaches the algae, normal sea salinity, low sediment, and wave action that supplies oxygen and nutrients.

Named reef: the Great Barrier Reef, Australia

The Great Barrier Reef, off Queensland, is over 2,300 km long. It has over 2,900 individual reefs and about 900 islands. The Great Barrier Reef Marine Park Act dates from 1975, and the reef became a World Heritage Site in 1981.

Natural factors

• Water temperature: marine heatwaves cause bleaching, where corals expel their algae. Mass bleaching occurred in 1998 and 2002 and repeatedly from 2016, including 2016, 2017, 2020, 2022 and 2024.
• Acidity: rising carbon dioxide lowers ocean pH, which makes it harder for corals to build skeletons.
• Salinity: floodwater after cyclones and heavy rain lowers salinity near the coast.
• Algal blooms: nutrient-rich water feeds blooms of phytoplankton (microscopic algae). Crown-of-thorns starfish larvae feed on phytoplankton, so more survive, and the adult starfish eat coral.

Human activity

• Major drainage basin schemes, onshore development and pollution: large dams such as the Burdekin Falls Dam alter the freshwater and sediment reaching the coast. Catchments such as the Burdekin and Fitzroy drain grazing land and cropland. Over 90% of the pollution reaching the reef is from farm runoff, carrying sediment, fertiliser and pesticide. Plans to dump dredged seabed near Abbot Point drew strong opposition.
• Desalination: minor here; it matters more on arid coasts such as the Gulf, where brine raises salinity.
• Tourism: around two million visitors a year bring income, but also anchor damage and trampling.
• Fishing: removes predators and grazers.

Management and future prospects. The 2004 zoning plan raised highly protected zones from 4.5% to over 33% of the marine park. The Reef 2050 Long-Term Sustainability Plan (2015) targets water quality. Local management can cut runoff and fishing pressure, but not marine heatwaves, which depend on global emissions. Reefs can recover between events, but recovery takes years; if bleaching returns every year or two, there is not enough time.

Local ecosystems (3.3.2.6)

The specification suggests forest conservation, mangrove conservation, managed parkland, a pond or a dune system. Your teacher will confirm which local example your class studies. One option is below.

Example: Sungei Buloh Wetland Reserve, Singapore (mangrove)

Sungei Buloh is a 130-hectare mangrove and wetland reserve in north-west Singapore, managed by the National Parks Board. It opened as a nature park in 1993, was gazetted as a nature reserve in 2002 and became an ASEAN Heritage Park in 2003. It is a stopover for migratory shorebirds on the East Asian-Australasian Flyway, such as the common greenshank and Pacific golden plover.

Adaptations. Mangroves live in waterlogged, salty, low-oxygen mud. Some species have prop roots that brace them in soft mud. Others put up aerial roots (pneumatophores) to take in oxygen. Many exclude or excrete salt. Some release seedlings that have already germinated on the parent tree, so they can root quickly between tides.

Local factors in change. Urban mangroves face land reclamation, coastal building, pollution and introduced species, planned (ornamental planting) or unplanned (escaped pets, ballast-water organisms).

Management. Legal protection, boardwalks and hides that keep visitors off the mud, and education. Evaluation: protection is strong inside the boundary, but the reserve is small and depends on the wider coast and on overseas sites the birds use.

Required case studies (3.3.2.7)

Region experiencing ecological change: Rondônia, Brazil

Rondônia, in the western Brazilian Amazon, once held over 200,000 km² of rainforest. By 2003, around 70,000 km² had been cleared. Highway BR-364 was paved in the 1980s. Tax incentives, federal investment and road building drew settlers. The population rose from about 117,000 in 1970 to over 1.1 million in 1991. Clearing along side roads off the highway created the well-known “fishbone” pattern seen on satellite images.

How the community reflects its ecological setting. The economy rests on cattle (over 14 million head by 2017) and soybeans. Land conflict is a political issue: Indigenous territories such as the Uru-Eu-Wau-Wau Indigenous Territory (over 1.8 million hectares) sit beside ranches.

Responses. Brazil’s Forest Code requires Amazon landowners to keep a share of their land under native vegetation. Satellite monitoring (PRODES and DETER) tracks clearance. Across the Brazilian Amazon, annual deforestation was over 27,000 km² in 2004 and then fell sharply.

Evaluation. Enforcement has varied with national politics, and monitoring shows clearance but does not stop it.

Ecosystem at a local scale

Use Studland or Sungei Buloh above. Cover the ecosystem’s properties, human impact, and the challenges and opportunities of sustainable development.

Common errors

• Writing “biodiversity is the number of species”. That is species richness, only one part of it.
• Mixing up sub-climax (natural halt) and plagioclimax (human halt).
• Saying energy is “recycled”. Nutrients cycle; energy flows through and leaves as heat.
• Describing coral bleaching as coral death. Bleached coral can recover if conditions improve quickly.
• Giving rainforest adaptations without linking them to climate, soil or the soil moisture budget.

Where to go next

Test yourself with the practice questions. Review systems concepts in the Physical Geography 1 guide and paper structure in the exam preparation guide.

Official syllabus

This guide is based on the OxfordAQA International AS and A-level Geography (9635) specification, Version 3.2, section 3.3.2 Ecosystems under stress.