Lesson 9: Ecosystems and Energy Flow

This lesson covers Theme C — ecosystems and energy flow (SL and HL): how energy enters communities as sunlight and leaves as heat, while nutrients cycle. The IB quantitative core is energy pyramids and efficiency calculations, plus the carbon cycle and its human perturbation.

Energy flow through ecosystems

Almost all ecosystems run on sunlight captured by autotrophs (photoautotrophs, plus chemoautotrophs in special cases such as deep-sea vents). Heterotrophs obtain energy by consuming other organisms: herbivores, carnivores, detritivores (which ingest dead material — earthworms) and decomposers/saprotrophs (bacteria and fungi that digest externally and absorb the products). Energy passes along food chains as gross primary production (GPP) minus respiration (R) gives net primary production (NPP = GPP − R) — the biomass/energy actually available to consumers. At each trophic level, most energy is lost: to respiration (released as heat), to egestion (faeces) and excretion, and because not all of the prey organism is eaten. Transfer efficiencies are typically about 10% (range ~5–20%) between trophic levels. This is why food chains rarely exceed 4–5 links, why top predators need huge territories, and why eating lower on the food chain supports more people per hectare.

Worked data: an efficiency calculation

A grassland has NPP of 20,000 kJ m⁻² yr⁻¹. Herbivores consume 6,000 kJ m⁻² yr⁻¹ of it; of what they consume, 3,600 is egested, 1,800 respired, leaving 600 as new herbivore biomass (secondary production). Efficiencies: consumption efficiency = 6,000/20,000 = 30%; assimilation = (6,000−3,600)/6,000 = 40%; production efficiency = 600/2,400 ≈ 25%; overall trophic transfer = 600/20,000 = 3%. Examiners test whether you can pick the right numerator and denominator for the named efficiency — read the question's wording precisely. Pyramids of energy (always upright, units of flow per area per time) avoid the anomalies of pyramids of numbers (one tree, thousands of insects) and of biomass (a fast-turnover phytoplankton standing crop can be smaller than the zooplankton it supports).

Nutrient cycling and the carbon cycle

Unlike energy, nutrients are recycled: decomposers mineralise dead matter, returning nitrates, phosphates and CO₂ to the environment. The carbon cycle links photosynthesis (CO₂ fixed into biomass) and respiration/combustion (CO₂ released). Major stores: atmosphere (~890 Gt C as CO₂), oceans (the largest active store, ~38,000 Gt), fossil fuels, soils and terrestrial biomass, and carbonate rocks (the largest store overall, exchanged slowly). Fluxes: global photosynthesis and respiration are each ~120 Gt C yr⁻¹ on land and roughly balanced in an undisturbed system; fossil-fuel combustion adds ~9–10 Gt yr⁻¹, tipping the balance — measured directly as the rising Keeling curve of atmospheric CO₂ at Mauna Loa (from ~315 ppm in 1958 to over 420 ppm today). HL/carbon specifics: in oceans, CO₂ dissolves and marine organisms build CaCO₃ shells and skeletons; on death these may become limestone or, from organic remains under anaerobic conditions, coal, oil and gas. Peat forms in waterlogged acidic bogs where decomposition is incomplete. Methane is produced by methanogenic archaea in anaerobic conditions (wetlands, rice paddies, ruminant guts, landfill) and oxidised in the atmosphere within ~a decade; it is a far more potent greenhouse gas per molecule than CO₂ while it lasts.

Climate connections (handled fully in the HL extension and Lesson 10)

The greenhouse effect itself is natural and necessary: shortwave solar radiation warms the surface, which re-radiates longwave infrared; greenhouse gases (CO₂, methane, water vapour, nitrous oxides) absorb longwave and re-emit it in all directions, keeping Earth ~33 °C warmer than it would otherwise be. The *enhanced* greenhouse effect from anthropogenic emissions is the problem — examiners insist on this distinction and on "longwave/infrared radiation" language.

Nature of Science

The Keeling curve is a landmark of patient, standardised, long-term measurement: its year-on-year rise (with a seasonal "breathing" cycle from Northern Hemisphere forests) convinced scientists the increase was real and global. It shows why baselines and sustained monitoring matter — a theme your IA data analysis shares.

TOK link

"Ecosystem" is a human-drawn boundary: energy and nutrient budgets depend on where you draw the box and over what timescale. TOK question: how do models that simplify (food chains vs webs; pyramids that average) both reveal and distort knowledge? The 10% "law" is a rule of thumb with wide variance — treating it as exact is a category error examiners penalise.

Common mistakes (from examiner reports)

• Saying energy is recycled (it is not — it flows through and is lost as heat; nutrients recycle).
• Drawing pyramids of numbers when the question asks for energy, or omitting units/time in energy pyramids.
• Confusing detritivores with saprotrophs/decomposers.
• Writing that greenhouse gases "trap heat in a layer" or reflect sunlight back — the mechanism is absorption and re-emission of longwave radiation.

IB Exam Experience

Paper 1 style. 1. Which statement about energy in ecosystems is correct? A. Energy is recycled by decomposers B. About 90% is typically lost between trophic levels C. NPP = GPP + respiration D. Pyramids of energy can be inverted 2. A saprotroph differs from a detritivore because a saprotroph… A. eats only plants B. digests food externally and absorbs the products C. produces its own food D. is always a parasite 3. Which process removes carbon dioxide from the atmosphere? A. Combustion B. Respiration C. Photosynthesis D. Decomposition Answer key. 1 — B. 2 — B. 3 — C. Paper 2 style. A food chain has NPP at the producer level of 84,000 kJ m⁻² yr⁻¹. Herbivore production is 5,040 kJ m⁻² yr⁻¹ and carnivore production is 420 kJ m⁻² yr⁻¹. (a) Calculate the percentage energy transfer from producers to herbivores and from herbivores to carnivores. [2] (b) State two reasons why so little energy reaches the carnivores. [2] (c) Explain why the total biomass of carnivores a habitat can support is limited. [2] *Marking notes.* (a) 5,040/84,000 = 6%; 420/5,040 ≈ 8.3% [1+1]. (b) Any two: respiration losses as heat; egestion of undigested material; not all prey eaten; excretion [2]. (c) Each transfer loses ~90%+, so energy available at the top is a tiny fraction of NPP; carnivore biomass production is correspondingly small [2]. IA idea. Estimate NPP of a plant stand (e.g. grass plots or algal cultures) under different light intensities or nutrient levels by harvesting and drying biomass over a fixed period, with careful control of area and time.

Extended case study: the human energy budget and ecological footprints

Scale the 10% rule up to civilisation. Global terrestrial NPP is roughly 60 billion tonnes of carbon per year; humanity harvests, diverts or destroys a substantial fraction of it (estimates of human appropriation of NPP run from ~15% to ~40% depending on method) — through cropland, pasture, forestry and land conversion. A hectare of good cropland fixed as wheat can feed roughly ten times as many people if eaten directly as grain than if the same harvest is fed to cattle and eaten as beef, because the cattle trophic transfer keeps only ~10% (and beef is at the inefficient end, ~3–4% of feed energy appearing as edible tissue). This is not advocacy in IB terms — it is quantitative reasoning you can reproduce: take a feed conversion ratio, apply percentage-loss logic across respiration, egestion and inedible fractions, and state assumptions. Examiners who set "food security" data questions want exactly this chain, plus the caveats: not all pasture land can grow crops, livestock eat residues humans cannot, and protein quality differs. Holding both the arithmetic and the caveats is what a top-band answer looks like.

Isotopes and the architecture of the carbon cycle

Carbon cycle questions often arrive disguised as isotope data: plants discriminate slightly against ¹³C during photosynthesis, so fossil fuels (ancient plant carbon) are depleted in ¹³C relative to the atmosphere. The measured decline in the atmosphere's ¹³C/¹²C ratio as CO₂ rises (the Suess effect) is forensic evidence — independent of temperature records — that the extra CO₂ comes from fossil carbon, not from oceans or volcanoes (which have different signatures). Pair this with the direct Keeling measurements (Lesson 9) and the oxygen decline that matches combustion stoichiometry, and you have three independent lines of evidence — a model "evaluate the evidence" paragraph. Methane budgets show the same forensic approach: wetlands are the largest natural source, agriculture (ruminants, rice) and fossil-fuel leakage the main anthropogenic ones; methane's atmospheric lifetime of ~12 years means cuts pay back fast — the quantitative argument behind methane pledges, and good material for discussing why gases with different lifetimes and potencies (CO₂ vs CH₄) complicate single-metric policies such as GWP weighting.

One more application: biofuels and the NPP ledger

Biofuel debates are Lesson 9 arguments in policy dress. First-generation fuels ferment crop sugars/starches (maize ethanol, sugarcane ethanol) — directly diverting NPP from food chains, with energy returns that depend heavily on inputs; sugarcane in Brazil returns far more energy per unit invested than temperate maize because the plant is a high-NPP C4 crop in a long growing season and the bagasse fuels the distillery. Second-generation fuels target cellulose (Lesson 5's polymer) in residues and grasses, avoiding the food-versus-fuel clash but fighting cellulose's recalcitrance — the β-linkages that make it strong make it hard to hydrolyse. An evaluate answer should run the ledger openly: solar energy fixed, trophic and processing losses, fossil inputs (fertiliser, transport), land-use change (the carbon debt if forest is cleared) — conclusions differ because the boundary drawn around the system differs, which is precisely the TOK point about models from the main lesson.

Examiner's lens: final drills (ecosystems)

• Efficiency vocabulary, precisely: assimilation = consumption − egestion; production = assimilation − respiration; and "transfer efficiency" between trophic levels = production at level n+1 ÷ production at level n. Read which pair the question names before dividing.
• Pyramid choice: energy (flow, per area per time) is the only pyramid that is always upright and the only one that shows why top predators are rare; biomass pyramids can invert in aquatic systems with fast producer turnover.
• Carbon numbers to anchor essays: atmospheric CO₂ up from ~280 ppm (pre-industrial) to >420 ppm; oceans absorb roughly a quarter to a third of annual anthropogenic emissions; residence and store sizes (Lesson 9) beat vague "lots of carbon" statements.
• Self-test: why can a food chain of six links exist in the ocean more readily than on land? (Ectotherm-dominated chains waste less energy on thermoregulation, so transfer efficiencies run higher.)