IB DP Environmental Systems and Societies – Ecosystems, energy flow and biogeochemical cycles Revision Notes

For full explanations and worked examples, use the study guide first.

These notes cover IB Diploma Programme Environmental Systems and Societies, syllabus sections 2.1 to 2.3 (individuals, populations, communities and ecosystems; energy and biomass in ecosystems; biogeochemical cycles). 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 content applies at SL and HL; HL students study some topics in extra depth.

Links: course hub, printable checklist, practice questions for this unit, Topic 2 Ecology revision notes (the broad overview) and assessment revision notes.

2.1 Definitions to write word-perfect

Term	Definition
Species	organisms sharing common characteristics that interbreed to produce fertile offspring
Population	organisms of the same species in the same area at the same time
Habitat	the environment in which a species normally lives
Niche	the set of biotic and abiotic conditions and resources a species uses and responds to
Fundamental niche	everything a species could occupy
Realised niche	what it actually occupies, reduced by competition and other interactions
Community	populations of different species living and interacting in one area
Ecosystem	a community plus its abiotic environment, interacting
Limiting factor	the factor in shortest supply relative to need, which restricts growth
Carrying capacity (K)	the maximum population an environment can support sustainably

Population curves

• J-curve: exponential growth, no limit reached yet, often followed by a crash.
• S-curve: lag → exponential → transitional → plateau at K.
• Plateau held by negative feedback: higher density → less food per individual, more disease and predation → birth rate falls, death rate rises → numbers level off.

Interactions

Competition (intra- and interspecific), predation, herbivory, parasitism, mutualism. For each, state who gains and who loses, and give one example.

Method in steps: Lincoln index

1. Catch, count and mark M individuals; release them.
2. Leave time to mix back in.
3. Catch C individuals; count R that are marked.
4. N = (M × C) / R
5. State assumptions: closed population, marks stay on,
   marks do not affect survival, full mixing.

Method in steps: quadrats

1. Place quadrats at random (random number coordinates).
2. Count individuals (or estimate % cover) in each.
3. Mean per quadrat ÷ quadrat area = density per m².
4. Density × total area = population estimate.

2.2 Energy and biomass

Equations table

Quantity	Equation	Units
Net primary productivity	NPP = GPP − R	kJ m⁻² yr⁻¹ or g m⁻² yr⁻¹
Gross secondary productivity	GSP = food eaten − faecal loss	as above
Net secondary productivity	NSP = GSP − R	as above
Ecological efficiency	(energy into new biomass at a level ÷ energy at level below) × 100	%

Rule of thumb: about 10% passes to the next level; values vary.

Why energy is lost between levels

1. Not all biomass is eaten (roots, bones, bark).
2. Not all eaten material is digested (lost as faeces).
3. Most assimilated energy is respired and lost as heat.

This follows the second law of thermodynamics. Energy flows in one direction; matter cycles.

Worked reminder

GPP 12 000, R 7800 (kJ m⁻² yr⁻¹) → NPP = 12 000 − 7800 = 4200 kJ m⁻² yr⁻¹.

Method in steps: productivity from a data table

1. Write the equation first (NPP = GPP − R, GSP = eaten − faeces,
   NSP = GSP − R).
2. Check every value is in the same units (kJ or g, per m², per year).
3. Substitute and subtract in the right order.
4. Give the answer with units, e.g. kJ m⁻² yr⁻¹.
5. For efficiency, divide the higher trophic level by the lower one
   and multiply by 100.

A small example: producers store 3000 kJ m⁻² yr⁻¹ and herbivores store 270 kJ m⁻² yr⁻¹. Efficiency = 270 ÷ 3000 × 100 = 9.0%. If you get a value over 100%, you divided the wrong way up.

Pyramids

Pyramid	Measures	Can it be inverted?
Numbers	individuals per level	yes (one tree, many insects)
Biomass	standing stock, g m⁻²	yes (phytoplankton and zooplankton at one moment)
Productivity	flow over time, kJ m⁻² yr⁻¹	never

Biomass is measured as dry mass (heat until constant mass). Biomagnification: persistent substances (some pesticides, mercury) become more concentrated at each higher trophic level.

2.3 Biogeochemical cycles

Carbon cycle

• Storages: atmosphere, oceans, biomass, soil, fossil fuels, sedimentary rock.
• Flows: photosynthesis, respiration, feeding, decomposition, combustion, dissolving, sedimentation, fossilisation.
• Sink = takes in more than it releases. Source = releases more than it takes in.
• Human impacts: fossil-fuel combustion, deforestation, draining wetlands, ploughing. Atmospheric CO₂ about 280 ppm before industrialisation, now above 420 ppm.
• Ocean acidification: more dissolved CO₂ → carbonic acid → lower pH → less carbonate for shells and coral.
• Conversion: carbon × 44/12 = CO₂ (1 t C ≈ 3.67 t CO₂).

Nitrogen cycle

Process	Change	Who / conditions
Fixation	N₂ → NH₄⁺ (lightning: N₂ → nitrogen oxides → nitrate)	free-living bacteria, Rhizobium in legume nodules, Haber process, lightning
Ammonification	organic N → NH₄⁺	decomposers
Nitrification	NH₄⁺ → NO₂⁻ → NO₃⁻	Nitrosomonas, Nitrobacter; aerobic
Assimilation	NO₃⁻ / NH₄⁺ → plant protein	producers
Denitrification	NO₃⁻ → N₂	anaerobic bacteria, waterlogged soil
Leaching	NO₃⁻ washed into water	rain, bare soil

Human impacts: Haber-process fertiliser, leaching and eutrophication, planting legumes, drainage, nitrogen oxides from combustion.

Cycles in systems language

Answers are stronger when they use the systems vocabulary from Topic 1. For each cycle, be ready to say:

• which storage the element leaves and which it enters (for example, atmosphere → biomass);
• whether the flow is a transfer (moves the element, same form: feeding, leaching) or a transformation (changes its form: photosynthesis, nitrification, combustion);
• how fast it is (respiration is fast; fossilisation takes millions of years);
• which human activity speeds up, slows down or adds a flow.

Must-know distinctions

• Energy vs matter: energy flows and is lost as heat; carbon and nitrogen cycle.
• Productivity vs biomass: productivity is a rate (per year); biomass is a stock at one time.
• GPP vs NPP: NPP is what is left after producer respiration.
• GSP vs NSP: GSP is what is assimilated; NSP is what is stored as new tissue.
• Fundamental vs realised niche: could vs does.
• Nitrification vs denitrification: aerobic, makes nitrate vs anaerobic, removes nitrate.
• Sink vs store: a store holds carbon; a sink is a store that is gaining carbon.

Quick self-test

1. Define “population”.
2. What is the difference between a habitat and a niche?
3. 30 snails are marked. Later 45 are caught, 9 of them marked. Estimate the population.
4. Five 0.25 m² quadrats have a mean of 5 daisies each. Estimate the number in a 600 m² lawn.
5. GPP = 12 000 kJ m⁻² yr⁻¹, R = 7800 kJ m⁻² yr⁻¹. Find NPP.
6. A herbivore population eats 900 kJ m⁻² yr⁻¹, loses 540 in faeces and 250 in respiration. Find GSP and NSP.
7. Primary consumers store 2000 kJ m⁻² yr⁻¹; secondary consumers store 150. Find the ecological efficiency.
8. Why can a pyramid of productivity never be inverted?
9. Name the process that converts ammonium to nitrate, and one condition it needs.
10. Why do waterlogged soils lose nitrogen?
11. A peatland releases 2 t of carbon per hectare per year. Convert this to tonnes of CO₂.
12. Explain why a mature, undisturbed forest may be neither a significant sink nor a source.

Answers

1. Organisms of the same species living in the same area at the same time.
2. A habitat is where a species lives; a niche is its whole role – where it lives and how it uses resources and interacts.
3. N = (30 × 45) / 9 = 150 snails.
4. Density = 5 ÷ 0.25 = 20 per m²; 20 × 600 = 12 000 daisies.
5. NPP = 12 000 − 7800 = 4200 kJ m⁻² yr⁻¹.
6. GSP = 900 − 540 = 360 kJ m⁻² yr⁻¹; NSP = 360 − 250 = 110 kJ m⁻² yr⁻¹.
7. 150 ÷ 2000 × 100 = 7.5%.
8. Each level receives only part of the energy flowing into the level below, because energy is lost as heat, in faeces and in uneaten parts.
9. Nitrification; it needs oxygen (aerobic, well-drained soil).
10. Low oxygen favours denitrifying bacteria, which convert nitrate to nitrogen gas.
11. 2 × 44/12 = 7.33 t CO₂ per hectare per year.
12. Photosynthesis is roughly balanced by respiration of plants and decomposers, so net uptake is small.

Where marks are usually lost

• Productivity values written without “per year” (or other time unit) and “per m²”.
• For consumers, subtracting respiration from food eaten without removing faecal loss first.
• Saying energy is “recycled” in an ecosystem.
• Ecological efficiency calculated the wrong way up (larger level ÷ smaller level gives over 100%).
• Stating a single reason for energy loss when the question says “explain” and gives several marks.
• Lincoln index answers given as a decimal of animals; round to a whole number and state it is an estimate.
• Naming a cycle process without saying which storage it moves the element from and to.
• Swapping nitrification and denitrification, or omitting the oxygen condition.
• Describing a human impact (“deforestation”) without the change in flow (less photosynthesis, more combustion and decomposition).

Official syllabus

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

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.