IB DP Environmental Systems and Societies – Climate, biomes, zonation and succession Revision Notes

For full explanations and worked examples, use the climate, biomes, zonation and succession study guide. These notes condense it for the final weeks before your exams.

They cover IB Diploma Programme Environmental Systems and Societies, aligned to the IB Diploma Programme Subject Brief, Environmental systems and societies, first assessment 2026: syllabus subtopics 2.4 (Climate and biomes) and 2.5 (Zonation, succession and change in ecosystems) in Topic 2 Ecology. The content is core for SL and HL; HL students study some topics in extra depth. It follows the IB ESS subject brief for first assessment 2026 — the course examined in the May and November 2026, 2027 and 2028 sessions.

The brief gives Topic 2 Ecology 22 teaching hours at SL and 35 at HL. It does not list the subtopics or learning outcomes for this topic, so the numbered subtopics and outcomes on this page follow the syllabus numbering used in the printable ESS checklist, not the brief itself.

See where the unit sits on the IB DP ESS course hub and tick it off on the printable ESS checklist. When you are ready, try the practice questions. For the wider topic, the Topic 2 Ecology revision notes cover energy flow and cycling, and the ESS assessment revision notes cover the papers.

Key definitions

• Weather — atmospheric conditions over hours or days.
• Climate — the average weather pattern over a long period (usually 30+ years).
• Biome — a collection of ecosystems with similar climate and so similar communities, found in different parts of the world.
• Insolation — incoming solar radiation reaching the Earth’s surface.
• ITCZ — Intertropical Convergence Zone; the belt of rising air and heavy rain near the equator.
• Zonation — change in community along an environmental gradient in space.
• Succession — change in community at one place over time.
• Sere / seral stage — the whole sequence of succession / one stage in it.
• Pioneer species — first colonisers of bare ground; tolerate harsh conditions.
• Climax community — relatively stable end community in balance with climate and soil.
• Plagioclimax — a community held below climax by human activity.
• P:R ratio — gross productivity (GPP) divided by respiration (R).

Formulas and ratios

Quantity	How to find it	Meaning
NPP	GPP − R	Energy or biomass added to the community
P:R ratio	GPP ÷ R	Above 1: biomass accumulating. About 1: steady state (climax). Below 1: biomass falling
Annual precipitation	Sum of 12 monthly totals	Compare with biome thresholds
Annual temperature range	Warmest month mean − coldest month mean	Large range = continental or high latitude; small = equatorial
Percentage change	(new − old) ÷ old × 100	Used for transect and succession data

Climate controls in one box

Latitude ↑ → insolation per m² ↓
   because: rays spread over larger area (curvature)
            longer path through atmosphere (more absorbed/reflected)

Tricellular model (each hemisphere)
   0°    rising air, low pressure, heavy rain (ITCZ) → rainforest
   30°   sinking air warms and dries, high pressure  → hot deserts
   60°   rising air at polar front, low pressure     → temperate forest
   90°   sinking cold air, high pressure, very dry   → polar desert

Ocean currents: warm → milder, wetter coasts
                cold → cooler, drier coasts (coastal deserts)

Biome recall grid

Biome	Temperature	Precipitation	NPP
Tropical rainforest	Hot all year, tiny range	Very high, all year	Highest on land
Tropical grassland	Hot	Wet and dry seasons (ITCZ shifts)	Medium, seasonal
Hot desert	Hot days, cold nights	Very low	Very low
Temperate forest	Mild, seasonal	Moderate, all year	Medium
Temperate grassland	Hot summers, cold winters	Moderate, seasonal	Medium–low
Boreal forest	Long cold winters	Low–moderate	Low
Tundra	Very cold, short summer	Low	Very low; permafrost

Climate change: biomes shift poleward and upslope; tundra and mountain-top habitats shrink; thawing permafrost releases CO₂ and CH₄ (positive feedback).

Method in steps: identifying a biome from a climate graph

1. Add the monthly precipitation to get the annual total.
2. Find the annual temperature range (warmest − coldest month).
3. Check for a dry season (months with very little rain) or months below 0 °C.
4. Match to a biome and quote at least two figures from the data as evidence.
5. Link the pattern to its cause (ITCZ, sinking air at 30°, latitude).

Method in steps: describing a succession

1. Say whether it is primary (no soil) or secondary (soil present).
2. Name the pioneers and how they change conditions (weathering, organic matter, soil).
3. Describe the seral stages and replacement by competition.
4. Name the climax and the factor that sets it (climate, soil).
5. Describe changes in soil, biomass, diversity, P:R and species strategy (r → K).

Zonation in three settings

Setting	Main gradient	What changes along it
Mountain	Altitude	Temperature falls and growing season shortens: forest → shrubs → alpine tundra
Rocky shore	Height above low tide	Time exposed to air, drying and temperature swings
Sand dunes	Distance inland	Sand stability, organic matter, pH and water retention

Method in steps: measuring zonation. Lay a line or belt transect along the gradient; place quadrats at regular intervals; record species (percentage cover or counts) and abiotic factors at each quadrat; plot each against distance or height; describe the trend with figures, then explain it using the abiotic change.

Worked reminder: marram grass cover falls from 60% to 15% between two quadrats. Percentage change = (15 − 60) ÷ 60 × 100 = −75%, a 75% decrease.

Must-know distinctions

• Zonation vs succession — space vs time. A dune transect shows zonation that reflects succession, because inland dunes are older.
• Primary vs secondary — bare substrate vs soil already present. Secondary is faster.
• GPP vs NPP — GPP keeps rising towards climax; NPP peaks mid-succession then falls.
• r vs K strategists — r: many small offspring, fast growth, good dispersal (pioneers). K: few large offspring, long-lived, strong competitors (climax).
• Climax vs plagioclimax — natural end point vs human-maintained stage (grazing, burning, mowing).
• Weather vs climate — short-term state vs long-term average.

Small worked reminders

A community has GPP = 12,000 and R = 7,500 kJ m⁻² yr⁻¹.

NPP = 12000 − 7500 = 4500 kJ m⁻² yr⁻¹
P:R = 12000 ÷ 7500 = 1.6  → above 1, so biomass is still accumulating

A station has monthly means from −25 °C to 9 °C and 182 mm precipitation a year: range = 9 − (−25) = 34 °C; cold and dry → tundra.

Quick self-test

1. Give two reasons why insolation per unit area is lower at high latitudes.
2. Why is there heavy rainfall near the equator?
3. Why are many hot deserts found at about 30° N and S?
4. How does a cold ocean current affect the climate of the nearby coast?
5. A station’s monthly precipitation totals are 120, 110, 100, 80, 60, 40, 30, 35, 60, 90, 110 and 125 mm. Calculate the annual total.
6. State the difference between zonation and succession.
7. Classify: succession on a lava flow; succession on a field abandoned after farming.
8. Give two features of pioneer species.
9. GPP = 12,000 and R = 7,500 kJ m⁻² yr⁻¹. Calculate NPP and the P:R ratio.
10. At which stage of succession is NPP usually highest, and why does it fall later?
11. Define plagioclimax and give one example.
12. Explain one positive feedback linking climate change and the tundra biome.

Answers

1. Rays hit at a low angle so the energy spreads over a larger area; rays pass through more atmosphere, so more is absorbed, scattered or reflected.
2. Strong heating makes air rise; it cools, water vapour condenses and convectional rain falls (the ITCZ).
3. Air from the Hadley cell sinks there, warming and drying as it descends, so high pressure prevents cloud and rain.
4. It cools the air above it, reducing evaporation and rainfall, so the coast is cooler and drier.
5. 960 mm.
6. Zonation is change in community along a gradient in space; succession is change in community over time at one place.
7. Lava flow: primary (no soil). Abandoned field: secondary (soil present).
8. Any two: tolerate extreme conditions; grow fast; produce many small, well-dispersed seeds or spores; add organic matter and help build soil; r-strategists.
9. NPP = 4,500 kJ m⁻² yr⁻¹; P:R = 1.6.
10. Mid-succession. Later, respiration by the large standing biomass rises to nearly match GPP, so NPP falls towards zero.
11. A community held below climax by human activity; e.g. grazed pasture, a mown lawn, or moorland burned in rotation.
12. Warming thaws permafrost; decomposition releases CO₂ and CH₄; these greenhouse gases cause more warming and more thaw.

Where marks are usually lost

• Stating that insolation falls with latitude but giving no mechanism.
• Explaining desert location with “hot air” instead of sinking air warming and drying under high pressure.
• Naming a biome from a climate graph without quoting figures from the data.
• Forgetting units (mm, °C, kJ m⁻² yr⁻¹) in calculated answers.
• Swapping zonation and succession, or calling a dune transect “just zonation” when asked about succession.
• Calling secondary succession primary because the site “looks bare”.
• Saying NPP is highest at climax; confusing a high GPP with a high NPP.
• Describing r- and K-strategists without linking them to early and late seral stages.
• Treating a plagioclimax as natural, or forgetting to name the human activity that maintains it.
• Writing that the climax is fixed, instead of noting it depends on climate, soil and disturbance.

Official syllabus

International Baccalaureate Organization, Diploma Programme Subject Brief, Environmental systems and societies, first assessment 2026. The brief lists subtopics for Topic 1 only; the subtopic numbering on this page follows the printable ESS checklist.