Syllabus map · 30 topics · 3 HL only
IB Environmental Systems and Societies (ESS) syllabus map
IB Environmental Systems and Societies joins ecology to the human side of environmental issues, and it can count as a group 3 subject, a group 4 subject or both. The course has eight topics, held together by perspectives, systems and sustainability, plus three lenses studied only at HL. Open any subtopic below for what it covers, what you need to be able to do, the terms to use precisely and the mistake that most often costs marks.
How it's assessed
| Component | SL | HL |
|---|---|---|
| Paper 1: unseen case study | 1 h, 35 marks, 25% | 2 h, 70 marks, 30% |
| Paper 2 | 2 h, 60 marks, 50% | 2 h 30, 80 marks, 50% |
| Paper 2 Section A: short-answer and data-based | 40 marks | 40 marks |
| Paper 2 Section B: structured essays | 1 essay from a choice of 2, 20 marks | 2 essays from a choice of 3, 40 marks |
| Internal assessment: individual investigation | 30 marks, 25% (3,000 words max) | 30 marks, 20% (3,000 words max) |
This is the current course, first examined in May 2026 and the first version of ESS offered at HL. Paper 1 is built on a case study you have not seen before, and nothing is released in advance.
Tap a topic to see what it asks, what you need to be able to do, the key terms and the trap. Topic titles follow the course structure; all explanations are ours.
Topic 1 — Foundation
The three ideas that run through the whole course: perspectives, systems and sustainability.
1.1PerspectivesSL
How environmental value systems shape the way people see and respond to environmental issues. Questions ask you to identify the value system behind a viewpoint or policy and explain why stakeholders reach different conclusions from the same evidence.
You need to be able to
- Explain what makes up a perspective (assumptions, values, beliefs) and how it differs from a worldview shared across a society.
- Model an environmental value system as a system, with inputs such as culture, religion, education and media, and outputs such as decisions and actions.
- Place a statement or policy on the spectrum from ecocentric through anthropocentric to technocentric, using the reason given for it.
- Explain how particular events and movements have shifted environmental perspectives over time.
- Justify your own position on an issue while showing you understand other perspectives.
Key terms
- perspective
- worldview
- environmental value system
- ecocentric
- anthropocentric
- technocentric
- stakeholder
Common trap Labelling a policy ecocentric just because it protects nature. A national park created for tourism income is anthropocentric; classify by the reason given, not by how green the outcome looks.
Practise 1.1 in Heptavo →1.2SystemsSL
Systems thinking, the framework for the whole course: storages, flows, feedback, equilibrium and tipping points. Questions ask you to draw or interpret system diagrams and identify positive and negative feedback loops.
You need to be able to
- Distinguish open, closed and isolated systems, with an example of each that actually exists (or why it does not).
- Draw a systems diagram with storages and flows, labelling each flow as a transfer or a transformation.
- Identify positive and negative feedback loops in a described scenario and explain their effect on stability.
- Explain steady-state equilibrium, resilience and tipping points.
- Evaluate the strengths and limitations of a model.
Key terms
- system
- storage
- flow
- transfer
- transformation
- negative feedback
- positive feedback
- tipping point
Common trap Reading negative feedback as bad. Negative feedback counteracts a change and stabilises the system; positive feedback amplifies it. Trace the loop back to the original change before you label it.
Practise 1.2 in Heptavo →1.3SustainabilitySL
What it means to use the environment sustainably, and the tools used to judge it: natural capital, ecological footprints, planetary boundaries and doughnut economics. Questions ask you to apply these ideas to a case and evaluate them as models.
You need to be able to
- Explain natural capital and natural income, and what makes a yield sustainable.
- Describe the environmental, social and economic pillars of sustainability and how they can conflict.
- Interpret ecological footprint data and compare a footprint with the biocapacity available.
- Explain the planetary boundaries model and doughnut economics, and evaluate their strengths and limits.
- Distinguish strong sustainability from weak sustainability.
Key terms
- sustainability
- natural capital
- natural income
- ecological footprint
- biocapacity
- planetary boundaries
- doughnut economics
- strong sustainability
Common trap Treating a planetary boundary as the point where the Earth system collapses. Each boundary is set before the actual threshold, so crossing one means entering a zone of rising risk, not an instant tipping point.
Practise 1.3 in Heptavo →Topic 2 — Ecology
How ecosystems work: populations and niches, energy flow, nutrient cycles, biomes and succession.
2.1Individuals, populations, communities and ecosystemsSL
How ecosystems are organised, from species and populations to communities, plus niches, species interactions, population growth and the field methods used to measure them. Questions ask you to classify interactions, interpret population curves and choose sampling methods.
You need to be able to
- Define species, population, community and ecosystem, and distinguish biotic from abiotic factors.
- Distinguish a fundamental niche from a realised niche.
- Classify interactions (competition, predation, herbivory, parasitism, mutualism) by their effect on each species.
- Interpret S and J population curves using carrying capacity and limiting factors, and compare r- and K-strategists.
- Choose and justify a sampling method: random quadrats, transects, or capture–mark–release–recapture with the Lincoln index.
Key terms
- species
- population
- community
- ecosystem
- niche
- carrying capacity
- limiting factor
- keystone species
Common trap Using a transect to estimate abundance in a uniform area, or random quadrats to study a gradient. Random placement avoids bias in uniform habitat; a transect is for change along a gradient, such as up a shore.
Practise 2.1 in Heptavo →2.2Energy and biomass in ecosystemsSL
How energy flows and biomass builds up through trophic levels: food chains and webs, productivity, ecological pyramids and the fate of persistent pollutants. Questions ask you to calculate productivity and efficiency and to explain pyramid shapes.
You need to be able to
- Build food chains and webs and assign trophic levels, placing an omnivore at more than one level.
- Distinguish gross and net productivity, and calculate net productivity as gross productivity minus respiration.
- Calculate ecological efficiency between trophic levels and explain why most energy is not passed on.
- Explain why pyramids of productivity are always upright, while pyramids of numbers and biomass can be inverted.
- Explain bioaccumulation and biomagnification of persistent, fat-soluble pollutants such as DDT and mercury.
Key terms
- trophic level
- producer
- gross primary productivity (GPP)
- net primary productivity (NPP)
- ecological efficiency
- pyramid of productivity
- bioaccumulation
- biomagnification
Common trap Writing that energy is recycled. Matter cycles through an ecosystem, but energy passes through once and leaves as heat from respiration; producers cannot reuse it.
Practise 2.2 in Heptavo →2.3Biogeochemical cyclesSL
The carbon and nitrogen cycles as systems of stores and flows, and how human activity alters them. Questions ask you to label cycles, explain processes such as nitrification or sequestration, and explain eutrophication.
You need to be able to
- Draw the carbon cycle with organic and inorganic stores and named flows: photosynthesis, respiration, combustion, decomposition and ocean exchange.
- Distinguish carbon sinks from carbon sources and calculate a residence time.
- Describe nitrogen fixation, nitrification, assimilation, ammonification and denitrification, and the conditions each needs.
- Explain how fossil-fuel burning, deforestation and fertiliser use change the cycles.
- Explain the sequence from nutrient run-off to oxygen depletion in eutrophication.
Key terms
- store
- flow
- carbon sink
- carbon source
- residence time
- nitrogen fixation
- nitrification
- denitrification
Common trap Saying fertiliser run-off kills fish directly. The nutrients cause an algal bloom; when the algae die, bacteria decomposing them use up the dissolved oxygen, and the low oxygen is what kills the fish.
Practise 2.3 in Heptavo →2.4Climate and biomesSL
How climate, mainly temperature and precipitation, decides where biomes occur, and the global air circulation that sets those climates. Questions ask you to read climographs, explain biome distribution and predict how biomes will shift.
You need to be able to
- Explain how temperature and precipitation determine the main biomes, such as tropical rainforest, desert, grassland, temperate forest and tundra.
- Read a climograph and identify the biome it represents.
- Explain the tricellular model (Hadley, Ferrel and polar cells) and how it creates bands of high rainfall and desert.
- Explain why insolation varies with latitude and through the year.
- Explain how rain shadows and distance from the sea modify local climate.
Key terms
- biome
- climograph
- insolation
- tricellular model
- Hadley cell
- intertropical convergence zone (ITCZ)
- rain shadow
Common trap Explaining the hot deserts by distance from the sea alone. The main control is dry air sinking at about 30° north and south, where the Hadley cells descend.
Practise 2.4 in Heptavo →2.5Zonation, succession and change in ecosystemsSL
How communities change across space (zonation) and over time (succession), and how human activity can halt succession. Questions ask you to describe a sere, compare primary and secondary succession, and interpret changes in productivity and diversity.
You need to be able to
- Distinguish zonation from succession, and explain why a transect suits zonation.
- Describe a sere from pioneer community to climax, showing how each stage changes conditions for the next.
- Compare primary and secondary succession, including their speed.
- Explain how gross productivity, respiration, net productivity and diversity change during succession.
- Explain how grazing, burning or mowing holds a community as a plagioclimax.
Key terms
- zonation
- succession
- sere
- pioneer species
- climax community
- plagioclimax
- primary succession
- secondary succession
Common trap Saying diversity is always highest at the climax. It often peaks in mid-succession, when early and late species overlap; a climax community can be dominated by a few strong competitors.
Practise 2.5 in Heptavo →Topic 3 — Biodiversity and conservation
What biodiversity is, how human activity threatens it, and how it is protected and restored.
3.1Biodiversity and evolutionSL
The three levels of biodiversity, how diversity is measured, how new species arise and why some species are more vulnerable to extinction. Questions ask you to calculate Simpson's index, explain speciation and judge what makes a biodiversity hotspot.
You need to be able to
- Distinguish genetic, species and habitat diversity, and explain why genetic diversity matters for adaptation.
- Calculate Simpson's reciprocal index, D = N(N − 1) ÷ Σn(n − 1), and use it to compare similar habitats.
- Explain natural selection, and distinguish allopatric from sympatric speciation.
- Explain why a biodiversity hotspot needs both high endemism and high habitat loss.
- Describe the causes of past mass extinctions and the evidence for a sixth.
Key terms
- biodiversity
- species richness
- evenness
- Simpson's reciprocal index
- natural selection
- speciation
- endemic species
- biodiversity hotspot
Common trap Writing that organisms adapt in order to survive. Individuals do not change themselves; variation already exists, the environment selects which individuals survive and breed, and the population changes over generations.
Practise 3.1 in Heptavo →3.2Human impact on biodiversitySL
The causes of biodiversity loss and how extinction risk is assessed with the IUCN Red List. Questions ask you to explain specific threats, link species traits to vulnerability and evaluate the Red List.
You need to be able to
- Explain the main drivers of loss: habitat destruction and fragmentation, overexploitation, invasive species, pollution and climate change.
- Describe the IUCN Red List categories and the kinds of evidence used to assign them.
- Explain how traits such as a small range, slow reproduction and a narrow niche raise a species' vulnerability.
- Explain how edge effects and isolation make fragmentation more damaging than the area lost suggests.
- Explain why some introduced species become invasive.
Key terms
- habitat fragmentation
- edge effect
- overexploitation
- invasive species
- IUCN Red List
- threatened species
- Data Deficient
Common trap Treating Data Deficient as not at risk. It means there is not enough information to assess the species: a gap in knowledge, not a verdict.
Practise 3.2 in Heptavo →3.3Conservation and regenerationSL
Strategies to protect and restore biodiversity, from protected-area design to rewilding and ex situ work. Questions ask you to evaluate reserve designs, compare in situ and ex situ conservation and judge a named strategy.
You need to be able to
- Compare in situ and ex situ conservation, and the limits of each.
- Apply reserve design principles: size, shape, edge effects, corridors and buffer zones.
- Explain why flagship, keystone, umbrella and indicator species are chosen as conservation targets.
- Describe rewilding and how its success is measured.
- Evaluate the role of local communities and of marine protected areas.
Key terms
- in situ conservation
- ex situ conservation
- protected area
- wildlife corridor
- rewilding
- flagship species
- umbrella species
- biosphere reserve
Common trap Offering captive breeding as the solution. Ex situ work only helps if habitat remains for reintroduction; without habitat protection it preserves genes, not the species' role in an ecosystem.
Practise 3.3 in Heptavo →Topic 4 — Water
The water cycle, access to fresh water, food from aquatic systems and water pollution.
4.1Water systemsSL
The hydrological cycle as a system, how the Earth's water is distributed, ocean circulation and the stratification of water bodies. Questions ask you to label stores and flows, interpret hydrographs and explain how human activity alters the cycle.
You need to be able to
- Name the stores and flows of the hydrological cycle, and classify each flow as a transfer or a transformation.
- Describe how the Earth's water is distributed between oceans, ice, groundwater, and lakes and rivers.
- Explain how urbanisation and deforestation change infiltration, surface run-off and the shape of a storm hydrograph.
- Describe thermohaline circulation and how fresh meltwater can weaken it.
- Explain thermal stratification in lakes.
Key terms
- hydrological cycle
- evapotranspiration
- infiltration
- surface run-off
- aquifer
- lag time
- thermohaline circulation
- stratification
Common trap Calling evaporation and condensation transfers. They change the state of the water, so they are transformations; advection and run-off move water without changing it, so they are transfers.
Practise 4.1 in Heptavo →4.2Water access, use and securitySL
Who has access to safe fresh water, why scarcity arises and how supply and demand can be managed. Questions ask you to distinguish types of scarcity, interpret water footprint data and evaluate options such as desalination or dams.
You need to be able to
- Distinguish physical water scarcity from economic water scarcity.
- Explain green, blue and grey water footprints and the idea of virtual water.
- Evaluate ways to increase supply (desalination, dams, rainwater harvesting, reclaimed water) and to reduce demand.
- Explain the effects of pumping groundwater faster than it recharges, including salt-water intrusion.
- Discuss how water shared across borders can lead to conflict or to cooperation.
Key terms
- water security
- physical water scarcity
- economic water scarcity
- water footprint
- virtual water
- desalination
- groundwater recharge
Common trap Assuming scarcity means there is not enough water. Much scarcity is economic: the water exists but the pipes, treatment or governance to deliver it are missing, so the fix is investment, not conservation.
Practise 4.2 in Heptavo →4.3Aquatic food production systemsSL
How fish and other aquatic foods are harvested and farmed, and how those systems can be managed sustainably. Questions ask you to interpret catch data, explain maximum sustainable yield and evaluate aquaculture and fishing controls.
You need to be able to
- Explain maximum sustainable yield and why quotas should be set below it.
- Explain why harvesting lower in the food chain is more energy-efficient.
- Use catch per unit effort to judge the health of a fish stock.
- Evaluate aquaculture: pollution, disease, escapes and the use of wild-caught feed.
- Evaluate fishery controls: quotas, mesh size, bycatch reduction and marine protected areas.
Key terms
- maximum sustainable yield (MSY)
- catch per unit effort
- overfishing
- bycatch
- aquaculture
- quota
- marine protected area
Common trap Judging a fishery healthy because landings are rising. Landings can rise on ever more effort while the stock shrinks; catch per unit effort is the measure that falls first.
Practise 4.3 in Heptavo →4.4Water pollutionSL
Sources and types of water pollution, how pollution is measured and how it can be managed. Questions ask you to interpret BOD and indicator-species data, explain eutrophication and evaluate management at different stages.
You need to be able to
- Distinguish point sources from non-point sources and explain why non-point sources are harder to control.
- Explain biochemical oxygen demand and interpret the oxygen sag downstream of a sewage outfall.
- Use indicator species and a biotic index to assess water quality.
- Explain eutrophication, including the positive feedback that sustains it.
- Apply the pollution management model: change the activity, control the release, or clean up and restore.
Key terms
- point source
- non-point source
- biochemical oxygen demand (BOD)
- indicator species
- biotic index
- eutrophication
- dead zone
Common trap Reading a high BOD as plenty of oxygen. BOD is how much oxygen micro-organisms need to break down the organic matter present, so a high BOD means heavy organic pollution and low dissolved oxygen.
Practise 4.4 in Heptavo →Topic 5 — Land
Soil as a living system, and the environmental costs and management of food production.
5.1SoilSL
Soil as a system: its structure, texture, profile and fertility, and how it is degraded and conserved. Questions ask you to use the soil texture triangle, describe horizons and explain processes such as salinisation and desertification.
You need to be able to
- Describe soil as a system with inputs, outputs, storages, transfers and transformations.
- Name the soil horizons (O, A, B, C) and describe what characterises each.
- Use the soil texture triangle to classify a soil, and compare sand, clay and loam for drainage and nutrient holding.
- Explain the causes of soil degradation: erosion, salinisation, nutrient depletion, compaction and desertification.
- Evaluate conservation measures such as terracing, windbreaks, cover crops and contour ploughing.
Key terms
- soil profile
- horizon
- humus
- soil texture
- loam
- leaching
- salinisation
- desertification
Common trap Recommending more irrigation to fix salinisation. Evaporating irrigation water is what leaves the salt behind; the remedies are less evaporation (drip irrigation), better drainage and flushing salt out of the root zone.
Practise 5.1 in Heptavo →5.2Agriculture and foodSL
How food production systems differ, their environmental impacts and how they can be made more sustainable. Questions ask you to compare farming systems, explain the land and energy cost of different diets and evaluate sustainable practices.
You need to be able to
- Compare intensive and extensive, and commercial and subsistence, farming in terms of inputs, outputs and yield.
- Explain why eating lower in the food chain needs less land per person fed.
- Explain how food loss and waste differ between low-income and high-income countries.
- Evaluate sustainable practices: crop rotation, integrated pest management, reduced tillage, agroforestry.
- Evaluate the gains and costs of the green revolution.
Key terms
- intensive farming
- extensive farming
- subsistence farming
- commercial farming
- monoculture
- food security
- food waste
- integrated pest management
Common trap Calling intensive farming more efficient without saying per what. It gives more output per hectare but often uses more energy and fertiliser per tonne; name the measure you are using.
Practise 5.2 in Heptavo →Topic 6 — Atmosphere and climate change
The atmosphere, the causes and impacts of climate change, responses to it, and the ozone layer.
6.1Introduction to the atmosphereSL
The structure and composition of the atmosphere, the natural greenhouse effect and albedo. Questions ask you to describe the atmospheric layers, explain how the greenhouse effect works and apply feedback loops.
You need to be able to
- Describe the composition of dry air and the role of trace greenhouse gases.
- Describe the troposphere and stratosphere, and explain why temperature changes with height differently in each.
- Explain the natural greenhouse effect using incoming shortwave and outgoing longwave radiation.
- Explain albedo and how changes in surface cover alter the energy absorbed.
- Explain positive feedbacks such as the ice–albedo feedback and permafrost thaw.
Key terms
- troposphere
- stratosphere
- greenhouse effect
- greenhouse gas
- shortwave radiation
- longwave radiation
- albedo
Common trap Saying greenhouse gases trap incoming sunlight, or that the ozone layer causes the greenhouse effect. Greenhouse gases absorb and re-emit outgoing longwave (infrared) radiation from the surface; stratospheric ozone absorbs incoming UV.
Practise 6.1 in Heptavo →6.2Climate change — causes and impactsSL
How human activity enhances the greenhouse effect, the evidence for it and the impacts on ecosystems and societies. Questions ask you to interpret climate data, explain feedbacks and evaluate who bears the impacts.
You need to be able to
- Explain how rising greenhouse gas concentrations cause warming, comparing carbon dioxide and methane.
- Interpret evidence such as temperature records, ice cores, CO₂ measurements and attribution studies.
- Explain impacts including sea level rise, extreme weather, shifting species ranges and ocean acidification.
- Explain positive and negative feedbacks in the climate system.
- Discuss why the impacts fall unequally between and within countries.
Key terms
- enhanced greenhouse effect
- global warming potential
- thermal expansion
- sea level rise
- ocean acidification
- tipping point
- climate justice
Common trap Blaming ocean acidification on warming. It is caused by CO₂ dissolving in seawater to form carbonic acid, and would happen even with no rise in temperature.
Practise 6.2 in Heptavo →6.3Climate change — mitigation and adaptationSL
Strategies that reduce greenhouse gas concentrations (mitigation) or reduce the harm they cause (adaptation), and the international agreements behind them. Questions ask you to classify and evaluate strategies.
You need to be able to
- Classify a strategy as mitigation or adaptation according to what it changes.
- Evaluate mitigation strategies: renewable energy, carbon pricing, carbon capture and storage, reforestation.
- Evaluate adaptation strategies: sea defences, drought-resistant crops, changes in building and land use.
- Explain the roles of the IPCC, the UNFCCC, the Kyoto Protocol and the Paris Agreement.
- Explain adaptive capacity and why it differs between communities.
Key terms
- mitigation
- adaptation
- carbon sequestration
- carbon capture and storage
- carbon tax
- cap and trade
- adaptive capacity
- nationally determined contribution
Common trap Classifying sea walls or air conditioning as mitigation. They reduce harm without lowering greenhouse gas concentrations, so they are adaptation.
Practise 6.3 in Heptavo →6.4Stratospheric ozoneSL
How the ozone layer forms and protects life, how ozone-depleting substances destroy it and how the Montreal Protocol responded. Questions ask you to explain ozone depletion and evaluate the Protocol.
You need to be able to
- Explain how ozone is formed and broken down in a steady state that absorbs UV.
- Explain how chlorine released from CFCs destroys ozone catalytically.
- Explain the springtime Antarctic ozone hole and the role of polar stratospheric clouds.
- Describe the effects of increased UV-B on human health and ecosystems.
- Evaluate the Montreal Protocol and explain why recovery takes decades.
Key terms
- stratospheric ozone
- ultraviolet radiation
- ozone-depleting substance
- CFC
- ozone hole
- Montreal Protocol
- polar stratospheric clouds
Common trap Mixing up ozone depletion and global warming. The ozone layer filters incoming UV; thinning it raises UV exposure at the surface, not air temperature.
Practise 6.4 in Heptavo →Topic 7 — Natural resources
How natural resources, energy sources and solid waste are used and managed.
7.1Natural resources — uses and managementSL
How natural resources are classified and managed, including the tragedy of the commons and the circular economy. Questions ask you to classify resources, explain overuse of shared resources and evaluate management approaches.
You need to be able to
- Classify resources as renewable, replenishable or non-renewable by comparing their rate of replacement with their rate of use.
- Distinguish natural capital from natural income, and goods from ecosystem services.
- Explain the tragedy of the commons and how community rules, regulation or ownership can prevent it.
- Distinguish a resource from a reserve and explain why reserve estimates change.
- Compare a circular economy with a linear one.
Key terms
- renewable resource
- non-renewable resource
- natural capital
- ecosystem services
- tragedy of the commons
- reserve
- circular economy
Common trap Stating that a resource will run out in a fixed number of years. Reserve figures depend on today's prices and technology, and demand changes, so years of supply is an economic estimate, not a physical countdown.
Practise 7.1 in Heptavo →7.2Energy sources — uses and managementSL
Renewable and non-renewable energy sources, energy security and the factors behind a country's energy choices. Questions ask you to evaluate energy sources and explain a country's energy mix.
You need to be able to
- Classify energy sources as renewable or non-renewable and, separately, as high- or low-carbon.
- Evaluate the advantages and disadvantages of fossil fuels, nuclear, solar, wind, hydroelectric and biomass.
- Explain energy security and how a country can increase it.
- Explain the factors that shape an energy mix: resources, cost, existing infrastructure, technology and politics.
- Explain intermittency and the role of storage and grid connections.
Key terms
- energy security
- energy mix
- renewable energy
- non-renewable energy
- low-carbon
- intermittency
- energy efficiency
Common trap Treating renewable and low-carbon as the same thing. Nuclear power is low-carbon but not renewable; biomass is renewable but releases CO₂ when it is burned.
Practise 7.2 in Heptavo →7.3Solid wasteSL
Types and sources of solid domestic waste, ways of disposing of it and strategies for reducing it. Questions ask you to evaluate disposal options and apply the waste hierarchy.
You need to be able to
- Describe the composition of solid domestic waste and how it varies with income.
- Evaluate landfill, incineration, composting and anaerobic digestion.
- Apply the waste hierarchy, from prevention and reuse down to recycling, recovery and disposal.
- Explain the features of a sanitary landfill and the problems of open dumps.
- Discuss the environmental justice issues raised by exporting waste.
Key terms
- solid domestic waste
- waste hierarchy
- landfill
- leachate
- incineration
- composting
- anaerobic digestion
Common trap Putting recycling at the top of the waste hierarchy. Preventing waste and reusing items save more material and energy than recycling, which still needs collection, sorting and reprocessing.
Practise 7.3 in Heptavo →Topic 8 — Human populations and urban systems
How human populations change, how cities work as systems, and urban air pollution.
8.1Human populationsSL
How human populations grow and change, the models used to describe them and the factors that affect fertility and mortality. Questions ask you to calculate growth rates and doubling times, interpret age–sex pyramids and apply the demographic transition model.
You need to be able to
- Calculate crude birth and death rates, natural increase and doubling time (70 ÷ growth rate in %).
- Interpret age–sex pyramids, including what a notch or a narrow base tells you.
- Apply the demographic transition model and explain its limitations.
- Explain the factors that lower fertility, such as girls' education, falling child mortality and access to contraception.
- Compare Malthusian and Boserupian views of population and resources.
Key terms
- crude birth rate
- crude death rate
- natural increase
- doubling time
- total fertility rate
- replacement fertility
- demographic transition model
- age–sex pyramid
Common trap Including migration in natural increase. Natural increase is births minus deaths only; add net migration to get the overall rate of population change.
Practise 8.1 in Heptavo →8.2Urban systems and urban planningSL
Cities as systems, the causes and impacts of urbanisation, and how planning can make cities more sustainable. Questions ask you to model a city as a system and evaluate planning strategies.
You need to be able to
- Model a city as an open system with inputs, outputs, storages and flows.
- Explain urbanisation and urban sprawl and their environmental impacts.
- Explain the urban heat island effect and strategies that reduce it.
- Evaluate features of sustainable cities: compact form, public transport, green space and sustainable drainage.
- Explain why a city's ecological footprint is far larger than its built-up area.
Key terms
- urbanisation
- urban sprawl
- urban heat island
- megacity
- mixed land use
- green infrastructure
- sustainable urban drainage systems (SUDS)
Common trap Suggesting air conditioning to reduce the urban heat island. It pumps heat from inside to the street and adds waste heat; measures that tackle the causes are trees, green roofs and reflective surfaces.
Practise 8.2 in Heptavo →8.3Urban air pollutionSL
Primary and secondary air pollutants, photochemical smog, temperature inversions and how urban air pollution is managed. Questions ask you to explain how smog forms, link pollutants to health effects and evaluate management strategies.
You need to be able to
- Distinguish primary pollutants (NOₓ, SO₂, CO, particulates, VOCs) from secondary pollutants (tropospheric ozone, PANs).
- Explain how photochemical smog forms in sunlight.
- Explain how a temperature inversion traps pollutants near the ground.
- Explain why fine particulates (PM2.5) are more harmful than coarse ones.
- Evaluate management at each level: changing the activity, controlling the release, cleaning up afterwards.
Key terms
- primary pollutant
- secondary pollutant
- photochemical smog
- tropospheric ozone
- temperature inversion
- particulate matter (PM2.5)
- nitrogen oxides
Common trap Listing tropospheric ozone as a primary pollutant. It is not emitted directly but forms in sunlight from nitrogen oxides and volatile organic compounds, which is why it peaks in the afternoon and downwind of the source.
Practise 8.3 in Heptavo →HL lenses — Environmental law, economics and ethics
Three HL-only lenses, law, economics and ethics, that HL students apply across the rest of the course.
HL.aEnvironmental lawHL
How environmental law works at international, national and local levels, the principles it rests on and why it is hard to enforce between states. Questions ask you to apply legal principles to a case and evaluate a treaty's effectiveness.
You need to be able to
- Distinguish hard law from soft law, and international, national and local levels of law.
- Explain the polluter pays principle, the precautionary principle, common but differentiated responsibilities and intergenerational equity.
- Distinguish signing from ratifying a treaty, and a framework convention from a protocol.
- Match major agreements (Montreal, Paris, CITES, Ramsar, Basel, the Convention on Biological Diversity) to the issue each addresses.
- Evaluate why some agreements achieve their aims and others do not.
Key terms
- hard law
- soft law
- ratification
- framework convention
- protocol
- polluter pays principle
- precautionary principle
- sovereignty
Common trap Saying that signing a treaty makes it legally binding on a state. Ratification creates the obligations; signing only signals intent.
Practise HL.a in Heptavo →HL.bEnvironmental and ecological economicsHL
How economic thinking treats the environment, from pricing external costs to questioning growth itself. Questions ask you to explain market failure, compare environmental and ecological economics and evaluate ways of valuing nature.
You need to be able to
- Explain externalities and why an unregulated market overproduces polluting goods.
- Compare a carbon tax with cap and trade as ways of internalising external costs.
- Distinguish environmental economics (weak sustainability) from ecological economics (strong sustainability).
- Evaluate methods of valuing nature, such as contingent valuation, travel cost and hedonic pricing.
- Explain why GDP is a poor measure of sustainable progress, and alternatives such as natural capital accounting.
Key terms
- externality
- market failure
- internalisation
- cost–benefit analysis
- contingent valuation
- decoupling
- public good
- common-pool resource
Common trap Treating a carbon tax and cap and trade as the same tool. A tax fixes the price and lets emissions vary; cap and trade fixes the quantity of emissions and lets the price vary.
Practise HL.b in Heptavo →HL.cEnvironmental ethicsHL
The ethical frameworks used to judge environmental decisions, and how far moral concern should extend. Questions ask you to apply ethical theories to a case and evaluate competing arguments.
You need to be able to
- Distinguish instrumental value from intrinsic value.
- Explain anthropocentric, sentientist, biocentric and ecocentric views of who or what has moral standing.
- Apply virtue ethics, consequentialist ethics and rights-based ethics to an environmental decision.
- Distinguish distributive, procedural and intergenerational justice.
- Explain how the same policy can be supported for different ethical reasons.
Key terms
- intrinsic value
- instrumental value
- anthropocentrism
- biocentrism
- ecocentrism
- consequentialism
- rights-based ethics
- environmental justice
Common trap Assuming a protective policy must be ecocentric. Protecting a forest for its timber, water or tourism value is still anthropocentric; the ethical label depends on the reason, not the result.
Practise HL.c in Heptavo →Try 4 Heptavo questions
Real questions from the app, with the explanation after each answer.
1.2 · Systems
A system in steady-state equilibrium is one in which
- the system never departs from a fixed value
- no inputs or outputs occur at all
- any disturbance causes the system to move to a new state
- conditions fluctuate around a constant average
Show the answer and why
Answer: D
Steady state is dynamic: matter and energy move through continuously and the overall condition holds. A static equilibrium is the one with no flows, and confusing the two makes a living ecosystem look like a jar on a shelf.
2.2 · Energy and biomass in ecosystems
A red fox eats blackberries in autumn and rabbits for the rest of the year. In a food web of its habitat the fox should be placed
- at both the second and third trophic levels
- at the fourth trophic level, as an omnivore is always one level above a carnivore
- at the second trophic level, because berries are eaten directly from a producer
- at the third trophic level only, since rabbits form most of its diet
Show the answer and why
Answer: A
Trophic level is a property of a feeding relationship, not of a species, so an animal that eats producers and herbivores occupies two levels at once and a food web draws both arrows. Assigning it to the third level only because rabbits dominate the diet is a reasonable simplification for a food chain but loses the information a web exists to show; the fourth-level reading invents a rule that does not exist.
4.3 · Aquatic food production systems
A coastal district clears most of its mangroves for shrimp ponds. Within a decade the local wild fish catch falls sharply, most likely because
- shrimp escaping from the ponds compete with the wild fish for food and eat their eggs
- the ponds release fresh water that lowers the salinity of the bay
- the mangroves were the nursery in which young fish of many species grew up
- the shrimp ponds draw the fish into them, where they are harvested
Show the answer and why
Answer: C
The roots of a mangrove forest shelter juveniles of reef and offshore species from predators, so clearing the forest removes a generation of recruits that were never counted in the district's catch. The paradox is that one aquatic food system destroys the foundation of another; blaming escaped shrimp assumes a competitive effect far smaller than the loss of habitat.
5.1 · Soil
Windbreaks reduce erosion in an arid farming region by
- preventing livestock from entering fields
- lowering wind speed at the soil surface
- increasing rainfall over the fields
- raising the humidity of the soil
Show the answer and why
Answer: B
Detachment by wind depends on velocity at the surface, so a barrier that slows the wind protects a strip many times its own height. Shelterbelts also reduce transpiration losses in the crop, which is a co-benefit worth mentioning.
