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

For full explanations, case-study detail and a worked answer paragraph, use the Ecosystems under stress study guide.

These notes condense section 3.3.2, Ecosystems under stress, of the OxfordAQA International AS and A-level Geography (9635) specification, Version 3.2 (content points 3.3.2.1 to 3.3.2.7). The section is Section B of Unit 3: Physical geography 2, a 1 hour 30 minute written exam where you answer all questions in each section (40 marks per section). Unit 3 counts for 20% of the International A-level.

Practise with the practice questions, and track your progress on the OxfordAQA A-level Geography hub and checklist. For how the five units fit together, see the exam preparation guide.

Key terms

Term	Meaning
Biodiversity	Variety of life: genetic, species and ecosystem diversity
Species richness	The number of species in an area (only one part of biodiversity)
Ecosystem	Living community plus its non-living environment, linked by energy and nutrient flows
Trophic level	Feeding position: producer, primary, secondary, tertiary consumer
Food web	Network of interlinked food chains
Biomass	Total mass of living organic matter per unit area
Gross primary production	Total rate at which producers fix energy by photosynthesis
Net primary production (NPP)	Gross primary production minus producers’ respiration; the new organic matter available to consumers
Seral stage	One stage in a succession
Pioneer species	First colonisers that change conditions for later species
Climatic climax	Final stable community the regional climate supports
Sub-climax	Succession halted by a natural factor (e.g. waterlogging)
Plagioclimax	Succession held back or deflected by continuing human activity (e.g. grazing, burning)
Psammosere / halosere / hydrosere / lithosere	Succession on sand / in salt water / in fresh water / on bare rock
Biome	Large-scale ecosystem with characteristic vegetation, controlled mainly by climate
Soil moisture budget	Balance of precipitation and evapotranspiration in the soil over a year (surplus, use, deficit, recharge)
Zooxanthellae	Symbiotic algae in coral tissue that supply most of the coral’s food
Coral bleaching	Corals expel their algae under stress, often heat, and turn white
Ocean acidification	Fall in seawater pH as the ocean absorbs carbon dioxide

Systems and nutrients in one view

• Inputs: sunlight, precipitation, nutrients from weathering and rain.
• Stores: biomass, litter, soil.
• Transfers: feeding, leaf fall, decomposition, uptake by roots.
• Outputs: heat, leaching, runoff, harvest.
• Energy flows (one way, out as heat). Nutrients cycle.
• Rainforest: largest store in biomass; small litter store because decay is fast.
• Savanna: more nutrients in the soil; fire returns nutrients quickly as ash.

Factors that change ecosystems

• Climate change: shifts temperature and rainfall belts, so species move or decline; warmer seas raise bleaching risk; more carbon dioxide acidifies the ocean. The carbon cycle side of this is in Section A, covered in the Water, carbon and life on Earth study guide.
• Human exploitation: clearance, grazing, burning, drainage, mining and pollution change one component, and the linked climate-vegetation-soil-topography system responds.
• Introduced species: planned (crops, ornamental plants) or unplanned (escapes, ballast water); they can outcompete natives and alter food webs.

Biomes at a glance

Feature	Tropical rainforest	Savanna grassland
Climate	Hot, wet all year	Hot; wet season and long dry season
Soil moisture	Surplus most of year; intense leaching	Surplus in wet season, deficit in dry season
Soil	Leached, red latosols	Ferruginous soils
Plant adaptations	Buttress roots, drip tips, epiphytes, lianas	Deep roots, thick bark, small or waxy leaves, grasses regrow from roots, fire tolerance
Human pressures	Logging, ranching, soy, oil palm, roads, mining	Grazing, fire, cultivation, firewood, fencing, tourism
Development issues	Frontier clearance, land conflict	More people and livestock on same land, shorter fallows

Case studies at a glance

Case study	What	Key facts	One evaluation point
Studland, Dorset (psammosere, 3.3.2.4)	Dune-to-heath succession on the Isle of Purbeck; National Trust, 631 ha	Embryo, yellow (marram), grey dunes, slacks, heath; pH falls inland; heath kept open by cattle grazing; all six native British reptiles	Space-for-time substitution assumes inland sites are older; fire and trampling can reset stages
Great Barrier Reef, Australia (3.3.2.5)	Named, located reef off Queensland	Over 2,300 km; over 2,900 reefs; Marine Park Act 1975; World Heritage 1981; mass bleaching 1998, 2002, 2016, 2017, 2020, 2022, 2024; over 90% of pollution from farm runoff; 2004 zoning raised highly protected area from 4.5% to over 33%; Reef 2050 Plan (2015)	Local management can improve water quality but cannot prevent marine heatwaves
Sungei Buloh, Singapore (3.3.2.6)	Mangrove and wetland reserve, 130 ha, National Parks Board	Nature park 1993; nature reserve 2002; ASEAN Heritage Park 2003; stopover on the East Asian-Australasian Flyway	Strong protection inside a small boundary; depends on the wider coast and overseas sites
Rondônia, Brazil (3.3.2.7 region)	Amazon frontier region	BR-364 paved in the 1980s; population about 117,000 (1970) to over 1.1 million (1991); about 70,000 km² cleared by 2003; “fishbone” clearance; cattle and soy economy; Uru-Eu-Wau-Wau Indigenous Territory	Forest Code and satellite monitoring helped cut Amazon clearance after 2004, but enforcement varies with politics

Biodiversity trend data: the WWF Living Planet Report 2024 found an average 73% decline in monitored wildlife populations between 1970 and 2020; the 2019 IPBES assessment estimated around one million species threatened with extinction.

Must-know distinctions

• Sub-climax vs plagioclimax: natural halt vs human halt. Heath at Studland is a plagioclimax because cattle grazing stops scrub and trees taking over.
• Energy vs nutrients: energy flows and is lost; nutrients cycle and are reused.
• Biomass vs NPP: biomass is a store (how much); NPP is a rate (how fast new matter is made).
• Bleaching vs death: bleached coral can recover if stress ends soon.
• Natural vs human factors on reefs: the specification lists water temperature, acidity, salinity and algal blooms as natural, but warming and acidification are driven by human emissions. Say so in evaluation.
• Population decline vs extinction: the Living Planet Index measures average population change, not species lost.
• Region case study vs local ecosystem case study: the region (Rondônia) must link ecological change to the community’s economic, social and political character; the local ecosystem must cover properties, human impact and sustainable development.

Mini essay plan

“To what extent can the threats facing a named coral reef be managed?”

1. Introduction: define reef health; name the Great Barrier Reef; set out natural and human threats; judgement preview (local threats manageable, global ones less so).
2. Water quality: runoff from grazing and cropland carries sediment and nutrients; Reef 2050 Plan targets it. Manageable, but slow, because it depends on thousands of farms.
3. Fishing and tourism: 2004 zoning raised highly protected zones to over a third. Strongly manageable through zoning and permits.
4. Crown-of-thorns starfish: outbreaks linked to nutrient-rich water. Partly manageable by culling and better water quality.
5. Warming and acidification: repeated mass bleaching since 2016. Not manageable locally; depends on global emissions.
6. Conclusion: management reduces stress and helps recovery, but the overall outcome depends on global action. The more frequent the bleaching, the less local management can achieve.

Quick self-test

1. Name the three levels of biodiversity.
2. Why are food chains rarely longer than four or five links?
3. What is the difference between biomass and NPP?
4. Where is most of the nutrient store in a tropical rainforest?
5. Define plagioclimax.
6. Name the four seres in the specification.
7. Give two adaptations of marram grass.
8. State three conditions needed for coral reef growth.
9. What causes coral bleaching?
10. Give one natural and one human factor affecting the Great Barrier Reef.
11. What is the “fishbone” pattern in Rondônia?
12. Name a management measure at Sungei Buloh.

Answers

1. Genetic, species and ecosystem (habitat) diversity.
2. Only a small share of energy (often around 10%) passes between levels; the rest is lost in respiration and waste.
3. Biomass is a store of living matter; NPP is the rate of new organic matter made after respiration.
4. In the biomass (living vegetation).
5. A community where succession is held back or deflected by continuing human activity.
6. Lithosere, halosere, psammosere, hydrosere.
7. Grows up through fresh sand; rolled leaves cut water loss; long roots reach moisture.
8. Warm water (about 23-29 °C), clear shallow water for light, normal salinity, low sediment.
9. Stress, usually high water temperature, makes corals expel their zooxanthellae.
10. Natural: water temperature (marine heatwaves). Human: farm runoff from catchments such as the Burdekin.
11. Clearance along side roads branching off a main highway, seen on satellite images.
12. Legal protection as a nature reserve; boardwalks and hides keep visitors off the mud.

Where marks are usually lost

• Defining biodiversity as “the number of species”.
• Swapping sub-climax and plagioclimax, or calling any human-affected site “sub-climax”.
• Writing that energy is recycled in an ecosystem.
• Listing rainforest adaptations without linking them to climate, soil or the soil moisture budget.
• Treating savanna as “just grassland” and ignoring the seasonal moisture deficit and fire.
• Naming a reef but giving only general reef facts with no catchments, dates or management detail.
• Classing warming and acidification as purely natural without noting the human cause.
• Using the region case study to describe deforestation only, without the community’s economic, social and political character.
• Ending an evaluation essay with no clear judgement.

Official syllabus

These notes follow the OxfordAQA International AS and A-level Geography (9635) specification, Version 3.2, section 3.3.2 Ecosystems under stress.