IB DP Environmental Systems and Societies – Ecosystems, energy flow and biogeochemical cycles Study Guide

This study guide covers IB Diploma Programme Environmental Systems and Societies (ESS), syllabus sections 2.1 to 2.3 of Topic 2 Ecology: individuals, populations, communities and ecosystems; energy and biomass in ecosystems; and 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 is studied at both SL and HL; HL students study some topics in extra depth, so check the detail with your teacher.

The subject brief gives Topic 2 22 teaching hours at SL and 35 at HL. The section titles below are the syllabus subtopic names; the brief itself does not list the sub-points, so the content here is standard ecology that those titles name. For the wider picture, read the Topic 2 Ecology overview first. This guide goes further into definitions, calculations and the two named cycles.

Other pages in this unit: revision notes and practice questions. Course hub: IB DP ESS. Printable checklist: IB DP ESS checklist.

What this unit covers

Syllabus section	What you must be able to do	SL/HL
2.1 Individuals, populations, communities and ecosystems	Define species, population, habitat, niche, community and ecosystem; explain limiting factors, carrying capacity and population growth curves; describe species interactions; estimate population size	SL and HL
2.2 Energy and biomass in ecosystems	Explain energy flow through trophic levels; define and calculate GPP, NPP, GSP and NSP; calculate ecological efficiency; interpret pyramids	SL and HL
2.3 Biogeochemical cycles	Describe storages and flows in the carbon and nitrogen cycles; explain how human activity changes them	SL and HL

2.1 Individuals, populations, communities and ecosystems

Key definitions

• Species: a group of organisms that share common characteristics and can interbreed to produce fertile offspring.
• Population: a group of organisms of the same species living in the same area at the same time.
• Habitat: the environment in which a species normally lives.
• Niche: the particular set of biotic and abiotic conditions and resources to which a species responds – where it lives and what it does there.
• Fundamental niche: the full range of conditions a species could occupy. Realised niche: the part it actually occupies, usually smaller because of competition and other interactions.
• Community: all the populations of different species living and interacting in one area.
• Ecosystem: a community together with the abiotic (physical) environment it interacts with.

Abiotic factors are non-living: temperature, light, water, pH, soil type, salinity. Biotic factors are living: food supply, predators, competitors, parasites, disease. A limiting factor is the one in shortest supply relative to need. It sets the ceiling on growth or distribution.

Population growth and carrying capacity

Carrying capacity (K) is the maximum number of individuals of a species that an environment can support sustainably. Two curves describe growth:

• J-curve: exponential growth with no limit. It usually ends in a crash when resources run out.
• S-curve: lag phase, exponential phase, transitional phase, then a plateau at K.

The plateau is kept by negative feedback. As density rises, food per individual falls, and disease and predation rise. Birth rate falls and death rate rises until they are roughly equal. If the population then drops below K, pressure eases and numbers rise again.

Interactions between species

Interaction	Effect on each species	Example
Intraspecific competition	both lose	trees of one species competing for light
Interspecific competition	both lose	lions and hyenas for the same prey
Predation	predator gains, prey loses	owl eating mice
Herbivory	herbivore gains, plant loses	rabbits grazing grass
Parasitism	parasite gains, host loses	tapeworm in a mammal
Mutualism	both gain	legumes and nitrogen-fixing bacteria

Where two species compete strongly for the same resources, one may exclude the other, or each narrows into its own realised niche.

Estimating population size

For plants and slow-moving animals, use quadrats placed at random. Count individuals (density) or estimate percentage cover, then scale up.

For mobile animals, use capture-mark-release-recapture and the Lincoln index:

N = (M × C) / R

where M = number caught and marked first, C = total caught second time, R = number of marked individuals in the second catch.

Worked example 1 (Lincoln index). 40 beetles are caught, marked and released. A week later 50 are caught, of which 8 are marked.

N = (40 × 50) / 8
  = 2000 / 8
  = 250 beetles

The method assumes: no migration, births or deaths between samples; marks do not come off; marks do not change survival; marked animals mix fully back into the population.

Worked example 2 (quadrats). Eight 0.5 m² quadrats in a 350 m² meadow contain 6, 9, 4, 7, 5, 8, 3 and 6 orchids.

Mean per quadrat = 48 / 8 = 6.0
Density          = 6.0 / 0.5 = 12 orchids per m²
Estimate         = 12 × 350 = 4200 orchids

2.2 Energy and biomass in ecosystems

Energy flows, it does not cycle

Sunlight is converted into chemical energy by photosynthesis. This energy passes along food chains as biomass. At every step some is lost as heat from respiration. This follows the second law of thermodynamics: every energy transformation increases entropy, so less energy is available for work. Energy therefore flows one way. Matter, by contrast, is recycled (section 2.3).

Trophic levels: producers → primary consumers → secondary consumers → tertiary consumers. Decomposers and detritivores feed on dead matter from every level.

Productivity terms

Productivity is a rate: energy or biomass gained per unit area per unit time, for example kJ m⁻² yr⁻¹ or g m⁻² yr⁻¹.

Term	Meaning	Equation
Gross primary productivity (GPP)	total energy fixed by producers in photosynthesis	–
Net primary productivity (NPP)	energy left after producers’ own respiration; available to consumers	NPP = GPP − R
Gross secondary productivity (GSP)	energy assimilated by consumers	GSP = food eaten − faecal loss
Net secondary productivity (NSP)	energy stored in new consumer biomass	NSP = GSP − R

Worked example 3. A grassland has GPP = 18 500 kJ m⁻² yr⁻¹ and producer respiration = 11 100 kJ m⁻² yr⁻¹.

NPP = 18 500 − 11 100 = 7400 kJ m⁻² yr⁻¹   (40.0% of GPP)

Grasshoppers in the grassland eat 1850 kJ m⁻² yr⁻¹, lose 1110 in faeces and 592 in respiration.

GSP = 1850 − 1110 = 740 kJ m⁻² yr⁻¹
NSP = 740 − 592   = 148 kJ m⁻² yr⁻¹

Ecological efficiency

ecological efficiency (%) = (energy used for new biomass at one level ÷ energy at the level below) × 100

Values are usually around 10%, though they vary. So 10 000 units at the producer level give roughly 1000, then 100, then 10 units. Energy is lost between levels because:

• not all of the lower level is eaten (roots, bark, bones);
• not all that is eaten is digested – it leaves as faeces;
• much of what is assimilated is used in respiration and lost as heat.

Worked example 4. In a pond, producer NPP is 7400 kJ m⁻² yr⁻¹ and primary consumers store 666 kJ m⁻² yr⁻¹.

efficiency = 666 / 7400 × 100 = 9.0%

This is why food chains are short (rarely more than four or five levels), and why a diet from lower trophic levels can feed more people from the same area of land.

Measuring biomass

Biomass is measured as dry mass: samples are dried in an oven at low heat until the mass stops changing, then weighed. Samples from a known area are scaled up. The method is destructive, so small samples are used. Energy content can be found by burning dry samples in a calorimeter.

Ecological pyramids

• Pyramid of numbers: individuals at each level. Can be inverted (one oak tree supporting thousands of caterpillars).
• Pyramid of biomass: standing stock at one moment (g m⁻²). Can be inverted in water, where fast- reproducing phytoplankton support a larger mass of zooplankton at a given time.
• Pyramid of productivity: energy flow over time (kJ m⁻² yr⁻¹). Never inverted, because each level receives only a fraction of the energy of the one below.

Pyramid shape also explains biomagnification: substances that are not broken down or excreted, such as some pesticides and mercury, become more concentrated at each higher trophic level. Top predators receive the highest doses.

2.3 Biogeochemical cycles

A biogeochemical cycle moves an element between living (biotic) and non-living (abiotic) storages. You describe it with systems language: storages (stocks) and flows (transfers and transformations).

The carbon cycle

Storages	Flows
atmosphere (CO₂, CH₄)	photosynthesis (atmosphere → producers)
oceans (dissolved CO₂, carbonate)	respiration (organisms → atmosphere)
organisms (biomass)	feeding (producers → consumers)
soil organic matter	decomposition (dead matter → soil and atmosphere)
fossil fuels (coal, oil, gas)	combustion (biomass and fossil fuels → atmosphere)
sedimentary rock (limestone)	dissolving and release at the ocean surface
	sedimentation and fossilisation (slow, geological)

A carbon sink takes up more carbon than it releases (a growing forest, the ocean surface). A carbon source releases more than it takes up (a burning forest, a drained peatland).

Human impacts. Burning fossil fuels moves carbon from long-term storage to the atmosphere far faster than natural processes return it. Deforestation removes a sink, and burning or decay of the wood adds a source. Draining wetlands and ploughing soils speed up decomposition. Atmospheric CO₂ was about 280 ppm before industrialisation and is now above 420 ppm. More CO₂ dissolving in seawater forms carbonic acid and lowers ocean pH (ocean acidification). This reduces the carbonate available to corals and shelled organisms.

Tip: 1 tonne of carbon corresponds to 44/12 ≈ 3.67 tonnes of CO₂. Check which one a data table uses.

The nitrogen cycle

Nitrogen gas (N₂) makes up most of the atmosphere, but most organisms cannot use it directly.

Process	What happens	Organisms / conditions
Nitrogen fixation	N₂ → ammonium (NH₄⁺)	free-living bacteria; Rhizobium in legume root nodules; the industrial Haber process (lightning also fixes N₂, forming nitrogen oxides and then nitrate)
Ammonification (decomposition)	proteins and urea in dead matter and waste → NH₄⁺	decomposers (bacteria, fungi)
Nitrification	NH₄⁺ → nitrite (NO₂⁻) → nitrate (NO₃⁻)	nitrifying bacteria, e.g. Nitrosomonas, Nitrobacter; need oxygen
Assimilation	nitrate and ammonium taken up by plants to make proteins	producers, then consumers by feeding
Denitrification	NO₃⁻ → N₂	denitrifying bacteria in waterlogged, anaerobic soil
Leaching	nitrate washed out of soil into water	heavy rain, bare or over-fertilised soil

Human impacts. The Haber process fixes very large amounts of nitrogen for fertiliser. Excess nitrate leaches into rivers and lakes and can cause eutrophication. Growing legumes adds fixed nitrogen. Draining land raises oxygen in soil and reduces denitrification. Burning fuels releases nitrogen oxides, which contribute to acid deposition.

Exam technique

Paper 1 gives you data from an unseen case study; Paper 2 has short-answer and data-based questions (Section A) and structured essays (Section B). So you must both calculate from data and write extended explanations. Always:

• give units with productivity values (kJ m⁻² yr⁻¹ or g m⁻² yr⁻¹);
• show the equation you use before substituting;
• name the storage and the flow when describing a cycle;
• link a human impact to a specific change in a flow.

The exam preparation guide covers paper structure in more detail.

Common errors

• Writing that energy is “used up” or “recycled”. It is transformed and lost as heat.
• Confusing GPP with NPP, or forgetting to subtract faeces before respiration for consumers.
• Giving productivity without a time unit. Productivity is a rate; biomass is a stock.
• Saying all pyramids can be inverted. Pyramids of productivity cannot.
• Mixing up nitrification (needs oxygen) and denitrification (anaerobic).
• In the Lincoln index, putting R on top instead of on the bottom.
• Calling a forest a carbon sink without saying whether it is growing or mature.

Next steps

Condense this with the revision notes, then test yourself with the practice questions. Revisit systems vocabulary in the Topic 1 Foundations guide and the course structure in the syllabus guide.

Official syllabus

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