IB DP Environmental Systems and Societies – Climate, biomes, zonation and succession Study Guide

This study guide covers climate, biomes, zonation and succession for IB Diploma Programme Environmental Systems and Societies (ESS). It is aligned to the IB Diploma Programme Subject Brief, Environmental systems and societies, first assessment 2026, and covers syllabus subtopics 2.4 (Climate and biomes) and 2.5 (Zonation, succession and change in ecosystems) within Topic 2 Ecology. This is core content for both 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, so the two titles above follow the syllabus numbering used in the printable ESS checklist. For the whole course, start at the IB DP ESS course hub. The broad overview of Topic 2 is in the Topic 2 Ecology study guide; this page goes deeper into two of its subtopics.

What this unit covers

Syllabus section	What you must be able to do	SL/HL
2.4 Climate and biomes	Explain how insolation, temperature and precipitation control where biomes occur; use the tricellular model and ocean currents to explain climate patterns; describe and compare the major biomes; explain how climate change can shift biomes	SL and HL
2.5 Zonation, succession and change	Distinguish zonation from succession; describe primary and secondary succession and the changes that happen along a sere; explain productivity and P:R changes; explain disturbance, climax and plagioclimax communities	SL and HL

2.4 Climate and biomes

Weather, climate and biomes

Weather is the state of the atmosphere at a place over hours or days. Climate is the average pattern of weather over a long period, usually 30 years or more. A biome is a collection of ecosystems that share similar climatic conditions and so have similar communities, even on different continents. Biomes are often grouped into broad classes: aquatic, forest, grassland, desert and tundra.

Climate controls biomes mainly through two factors: temperature and precipitation. Both are limiting factors for photosynthesis, respiration and decomposition. Plotting mean annual temperature against annual precipitation places each terrestrial biome in its own region of the graph (often called a Whittaker diagram).

Why insolation changes with latitude

Insolation is incoming solar radiation. It is greatest near the equator and falls towards the poles, for two reasons:

1. Curvature of the Earth. At high latitudes the Sun’s rays strike at a low angle, so the same energy is spread over a larger area of ground.
2. Path through the atmosphere. Low-angle rays pass through more atmosphere, so more energy is absorbed, scattered or reflected before reaching the surface.

Ice and snow near the poles also have a high albedo, so more of the energy that arrives is reflected.

The tricellular model

Unequal heating drives atmospheric circulation. The tricellular model splits each hemisphere into three cells.

• Hadley cell (0–30°). Strong heating at the equator makes air rise, forming low pressure. Rising air cools, water vapour condenses and heavy rain falls. This zone of rising air is the Intertropical Convergence Zone (ITCZ). At high altitude the air moves poleward, cools and sinks at about 30° N and S, forming high pressure. Sinking air warms and dries, so rain is rare — this is where most hot deserts lie.
• Ferrel cell (30–60°). Surface air flows poleward from the 30° highs. At about 60° it meets cold polar air at the polar front and rises, forming low pressure and frequent rain. This supports temperate forests.
• Polar cell (60–90°). Cold, dense air sinks over the poles, giving high pressure and very little precipitation.

The ITCZ moves north and south during the year with the overhead Sun. Places at its edges get rain for only part of the year, which produces the distinct wet and dry seasons of tropical grassland (savanna).

Ocean currents also move heat. Warm currents carry heat poleward and raise coastal temperatures and rainfall; the North Atlantic Drift keeps north-west Europe milder than its latitude suggests. Cold currents cool the air above them, which reduces evaporation and rainfall and helps to form coastal deserts such as the Atacama.

The major terrestrial biomes

Biome	Climate	Vegetation and productivity
Tropical rainforest	Hot all year; high rainfall spread through the year	Dense, layered forest; highest NPP of land biomes; very high biodiversity; most nutrients held in biomass, soils thin and leached
Temperate deciduous forest	Mild; clear seasons; moderate rainfall	Trees shed leaves in winter; medium NPP; fertile soils from leaf litter
Boreal forest (taiga)	Long cold winters, short cool summers	Conifers; low NPP; slow decomposition and acidic litter
Tropical grassland (savanna)	Hot; distinct wet and dry seasons	Grasses with scattered trees; fire common; NPP varies with rainfall
Temperate grassland	Hot summers, cold winters; moderate, seasonal rainfall	Grasses; deep, organic-rich soils
Hot desert	Very low rainfall; large daily temperature range	Sparse, drought-adapted plants; very low NPP
Tundra	Very cold; short growing season; low precipitation	Mosses, lichens, low shrubs; permafrost; very low NPP; slow decomposition locks carbon in soil

The pattern links back to the limiting factors. Where it is both warm and wet, photosynthesis runs all year and NPP is high. Where water (desert) or temperature and light (tundra) limit growth, NPP is low.

Worked example 1: identifying a biome from climate data

Station K (fictional) records:

Month	J	F	M	A	M	J	J	A	S	O	N	D
Mean temp (°C)	26.4	26.8	27.1	27.3	27.2	26.9	26.5	26.6	26.8	27.0	26.9	26.6
Precipitation (mm)	260	240	290	310	270	200	180	190	220	270	300	290

Annual precipitation = 260 + 240 + ... + 290 = 3020 mm
Mean annual temperature = (sum of 12 months) / 12 = 26.8 °C
Annual temperature range = 27.3 − 26.4 = 0.9 °C
Driest month = 180 mm, so there is no dry season

High temperature all year, a tiny annual range and very high rainfall in every month match a tropical rainforest. The station is almost certainly close to the equator, under the rising air of the ITCZ.

Climate change and biomes

As the climate warms, the conditions each biome needs move poleward and upslope. Boreal forest can spread north into tundra, and tree lines on mountains rise. Species that disperse slowly, or already live on mountain tops or in polar regions, may have nowhere left to go. Changes in rainfall patterns can also push a biome towards a drier type. Thawing permafrost releases carbon dioxide and methane, which adds to warming: a positive feedback. Link this to the tipping points and feedback ideas in the Topic 1 Foundations study guide.

2.5 Zonation, succession and change in ecosystems

Zonation versus succession

Zonation is the change in community along an environmental gradient in space — for example, with altitude up a mountain, with height on a rocky shore, or with distance inland across sand dunes. Abiotic factors (temperature, exposure, salinity, soil depth) change along the gradient, and biotic factors such as competition decide which species dominate each zone. You measure zonation with a line or belt transect and quadrats.

Succession is the change in community over time at one place. A sand dune system shows both at once: the dunes further inland are older, so walking inland along a transect shows zonation in space that reflects succession in time.

Primary and secondary succession

• Primary succession starts on bare ground with no soil: new volcanic rock, rock exposed by a retreating glacier, newly deposited sand.
• Secondary succession starts where soil already exists but the community has been removed: after a fire, a flood, or on abandoned farmland. It is faster, because soil, seeds and nutrients are already present.

Each stage is a seral stage; the whole sequence is a sere. Early colonisers are pioneer species such as lichens and mosses. They tolerate extreme conditions and help weather rock and add organic matter. This changes the conditions so other species can establish, and those species then outcompete the pioneers. The process ends in a relatively stable climax community in balance with the local climate and soil.

What changes during succession

Feature	Early stages	Climax
Soil	Thin, little organic matter, poor water retention	Deep, organic-rich, holds water and nutrients
Species type	r-strategists: many small offspring, rapid growth, good dispersal	K-strategists: fewer, larger offspring, long-lived, strong competitors
Biomass	Low but rising	High
Diversity and food webs	Low; simple food chains	Higher; complex food webs (diversity may dip slightly at climax as dominant species outcompete others)
Nutrient cycling	Open; nutrients easily lost	More closed; much held in biomass
GPP and respiration	Both low	Both high
P:R ratio	Greater than 1; biomass accumulates	Close to 1; little net gain

The P:R ratio compares gross productivity (P, the GPP) with respiration (R). NPP = GPP − R. When P:R is above 1, NPP is positive and biomass builds up. At climax, respiration by the large standing biomass nearly matches GPP, so P:R ≈ 1.

Worked example 2: productivity during succession

Fictional data (kJ m⁻² yr⁻¹):

Stage	GPP	R
Pioneer	3,000	1,200
Early	9,000	4,500
Mid	15,000	10,000
Climax	20,000	19,000

NPP = GPP − R            P:R = GPP ÷ R
Pioneer: 3000 − 1200 = 1800      3000 ÷ 1200 = 2.5
Early:   9000 − 4500 = 4500      9000 ÷ 4500 = 2.0
Mid:    15000 − 10000 = 5000     15000 ÷ 10000 = 1.5
Climax: 20000 − 19000 = 1000     20000 ÷ 19000 = 1.05 (3 s.f.)

GPP rises throughout as leaf area grows. NPP peaks mid-succession and then falls, because respiration by the growing biomass catches up. The P:R ratio falls towards 1 as the community approaches climax.

Disturbance, plagioclimax and change

Disturbance is any event that removes biomass and resets succession. Natural disturbances include fire, storms, floods, landslides and volcanic eruptions. Human disturbances include clearing, ploughing, grazing and burning. Moderate, occasional disturbance can raise diversity by opening space for pioneers alongside later species.

When human activity stops succession before climax and holds it there, the result is a plagioclimax. Examples are grazed pasture, lawns and heather moorland that is burned in rotation. Stop the grazing or burning and secondary succession resumes.

The climax is not a fixed, single end point. It depends on climate and soil, and disturbance or climate change can push an ecosystem past a tipping point into a different stable state. How far a system can absorb disturbance and return depends on its resilience — linked to diversity, the size of its storages and how fast its species reproduce.

Common errors

• Describing insolation falling with latitude without giving the reason (curvature spreading energy; longer atmospheric path).
• Saying deserts form at 30° because it is “hot” rather than because sinking, warming air stops cloud and rain forming.
• Mixing up zonation (space) and succession (time).
• Calling succession on abandoned farmland “primary” — soil is present, so it is secondary.
• Claiming NPP is highest at climax. GPP is highest; NPP is usually highest mid-succession.
• Writing P:R = 1 as “no productivity”. GPP is high; it is the net gain that is near zero.
• Treating a plagioclimax as natural; it is maintained by human activity.

Where to go next

Condense this into recall with the revision notes, then test yourself with the practice questions. For how Paper 1 and Paper 2 are set out, read the ESS exam preparation guide and the syllabus guide.

Official syllabus

International Baccalaureate Organization, Diploma Programme Subject Brief, Environmental systems and societies, first assessment 2026.