Structure, energy and trophic levels

An ecosystem is a community of organisms (the biotic component) interacting with its physical environment (the abiotic component — climate, soil, water, relief, geology) as a functioning open system. Energy and matter both cross the boundary, so the systems vocabulary transfers directly: inputs, outputs, stores, flows.

• Inputs: solar radiation, precipitation, nutrients from weathering and the atmosphere, immigration of species.
• Stores: biomass, litter, soil, water.
• Flows/transfers: photosynthesis, feeding, respiration, decomposition, nutrient uptake.
• Outputs: heat, respired CO₂, run-off, leaching, emigration, harvest.

Trophic levels organise the feeding structure. Producers (autotrophs) fix solar energy. Primary consumers (herbivores) eat them; secondary and tertiary consumers follow. Decomposers and detritivores break down dead organic matter at every level and are the engine of nutrient recycling.

The crucial asymmetry: energy flows through the ecosystem in one direction and is ultimately lost as heat — it is not recycled. Nutrients cycle round and round. Getting that distinction the right way round is worth marks by itself.

Food chains are a simplification; a real ecosystem is a food web, with omnivores and multiple pathways. That redundancy matters for stress: a web with many alternative pathways is more resilient than a chain, because losing one species does not sever the flow.

Ecological pyramids. A pyramid of numbers can be inverted (one oak, thousands of insects). A pyramid of biomass can also invert in the oceans, where a small, fast-turnover standing crop of phytoplankton supports a larger mass of zooplankton at any instant. A pyramid of energy can never invert — the second law of thermodynamics forbids it. If an exam figure shows an inverted pyramid, that is the point being tested.

Gross and net primary productivity

Gross primary productivity (GPP) is the total rate at which producers fix energy — the whole photosynthetic take. But plants respire too, and that respiration is a cost paid before anything is passed on. What is left is net primary productivity (NPP): the energy (or carbon, or dry mass) actually available to build the ecosystem and to feed the consumers above.

• NPP is highest where light, warmth, water and nutrients are all abundant — hence the tropical rainforest and, in the sea, coral reefs, estuaries and upwelling zones.
• NPP collapses wherever one of those is limiting: water in a hot desert, temperature and light in tundra, nutrients across much of the open ocean.
• NPP is a rate, not a stock. A tropical rainforest has both very high NPP and a huge biomass store; a grassland can have respectable NPP but almost no standing biomass, because it is grazed and turned over each year. Never confuse productivity with biomass.

Your turn — net primary productivity

Why food chains are short

Energy transfer between trophic levels is inefficient. As a rule of thumb only about 10% of the energy available at one level ends up as production at the next — a figure that varies widely in reality, but is right in order of magnitude. The losses are not mysterious:

• Respiration — the consumer burns most of what it assimilates just staying alive, moving and (in endotherms) keeping warm. This is the biggest single loss.
• Egestion — much of what is eaten is indigestible (cellulose, bone, chitin) and passes straight through as faeces.
• Not consumed — roots, wood, bark and dead material are never eaten at all; they go to the decomposers.

Two consequences worth arguing. (1) Food chains are short — usually four or five links — because after four ×0.1 steps there is simply not enough energy left to support another level. (2) It is energetically far cheaper to eat plants than animals, which is the ecological basis of the land-use argument about diet. Both are AO2 points that examiners rarely see made properly.

Your turn — ecological efficiency

Biomass, litter and soil

Gersmehl's model reduces the nutrient cycle to three stores and a set of transfers between them, with inputs from outside and outputs lost from the system. In the classic diagram the circles are drawn proportional to the size of the store and the arrows proportional to the transfer — which is what makes it so good at comparing biomes.

• Transfers (inside the system): fallout — biomass → litter (leaf fall, death); decomposition — litter → soil; uptake — soil → biomass (via roots).
• Inputs: nutrients dissolved in precipitation, and released by the weathering of parent rock into the soil store.
• Outputs: run-off (surface loss from the litter store) and leaching (nutrients washed downwards out of the soil store beyond the reach of roots).

The rainforest paradox — a top-tier point. The most luxuriant vegetation on Earth stands on some of the poorest soils. Warmth and moisture drive extremely rapid decomposition and equally rapid uptake, so nutrients spend almost no time in the soil: they are held in the biomass. Remove the biomass and you remove the nutrient capital — heavy rainfall then leaches what little the soil holds, which is exactly why cleared rainforest land loses fertility within a few seasons and why the stress is so hard to reverse.

Store, transfer, or input/output?

Tap a term, then tap the column it belongs to. Gersmehl only works if you can tell the three apart instantly.

From pioneer to climax

Succession is the directional change in an ecosystem's plant and animal community over time. Primary succession begins on a surface that has never carried vegetation — bare rock, new lava, sand, open water. Secondary succession begins where vegetation has been removed but a soil and seed bank remain (after fire or clearance), and is therefore much faster.

The named primary seres are examinable:

• Lithosere — succession on bare rock.
• Psammosere — succession on sand (a dune system).
• Hydrosere — succession in fresh water (a pond infilling).
• (A halosere is the saltmarsh equivalent.)

The end point of undisturbed succession is the climatic climax community: the vegetation in equilibrium with the regional climate (in lowland Britain, deciduous oak woodland). Along the way, biomass, soil depth, biodiversity, nutrient stores and stability all rise.

The plagioclimax. If an arresting factor halts succession before the climatic climax, the community that persists is a plagioclimax (a "deflected" climax). Arresting factors are usually human: grazing (sheep on chalk downland or moorland), burning (managed heather moor), mowing, trampling, deforestation. Two consequences: (1) many landscapes we call "natural" are nothing of the kind; (2) some of Britain's richest habitats are plagioclimaxes — chalk grassland is species-rich because grazing suppresses the coarse grasses and scrub that would shade the orchids out. Stop the management and you lose biodiversity. That paradox is a superb evaluative card to play.

Why manage a "natural" grassland?

Biomes and their global distribution

A biome is a global-scale ecosystem type, defined by its dominant vegetation, whose distribution is controlled overwhelmingly by climate — specifically the combination of temperature and precipitation (and their seasonality). That is why biomes broadly form latitudinal belts, modified by continentality, ocean currents and — locally — by altitude, which reproduces the latitudinal sequence up a mountainside.

• Tropical rainforest — hot and wet all year; highest NPP and biodiversity of any terrestrial biome; nutrients in the biomass.
• Savanna / tropical grassland — marked wet and dry seasons; fire and grazing are integral, not accidental.
• Hot desert — water is the limiting factor; very low NPP, specialist adaptations.
• Temperate deciduous forest — seasonal, moderate; the climatic climax across much of NW Europe, now largely cleared.
• Boreal forest (taiga) — cold slows decomposition, so nutrients accumulate in a thick litter layer; acidic, podsolised soils.
• Tundra — temperature-limited, permafrost, extremely low NPP, very slow recovery from disturbance.

Zonal, azonal, local. A zonal soil/ecosystem reflects the regional climate. An azonal one (a sand dune, a saltmarsh, a scree slope) is dominated by local parent material or drainage instead — which is why AQA insists you can study ecosystems at local scale as well as global. A pond, a hedgerow, an urban brownfield site and a road verge are all legitimate ecosystems for a local case study.

Where the nutrients sit

How much can a system absorb?

Stress is any pressure — natural or human — that pushes an ecosystem away from its equilibrium. What happens next depends on three properties AQA wants you to distinguish carefully:

• Resistance — the ability to withstand a disturbance without changing much.
• Resilience — the ability to recover to the previous state after disturbance. High resilience usually comes from a fast growth rate, a large seed bank, a redundant food web, and short-lived species.
• Fragility — vulnerability to permanent damage. Fragile systems are typically those with extreme limiting factors (tundra: cold; desert: water), slow growth, specialist species, or nutrient capital held somewhere easily removed (rainforest biomass; coral's symbiotic algae).

Thresholds and tipping points. Systems often absorb stress with little visible change — and then flip abruptly to a new, self-reinforcing state that is very hard to reverse (a hysteresis effect): forest → savanna, coral reef → algae-dominated rubble, grassland → desert. The danger is that the warning signs are weak right up to the threshold.

Biodiversity and how it is measured. Biodiversity is not just a species count. Species richness = how many species. Species evenness = how equally abundant they are — 100 individuals split 20/20/20/20/20 is far more diverse than 96/1/1/1/1, even though richness is identical. Indices such as Simpson's combine both. Add genetic diversity (within a species) and ecosystem/habitat diversity, and you have the three levels. Precision here separates a strong answer: "biodiversity fell" is weak; "richness held up but evenness collapsed as one invasive species came to dominate" is A-level.

Name that concept

Tap a description on the left, then the term it defines. Command of this vocabulary is the difference between describing an ecosystem and explaining it.

The tropical rainforest under stress

The rainforest is the flagship case of a system that looks indestructible and is in fact fragile — precisely because its nutrient capital is in the biomass, not the soil.

• Drivers of stress: clearance for cattle pasture and commercial cropping (notably soy and oil palm), commercial and illegal logging, mining, hydroelectric impoundment, and the road that makes all of the above possible — access is the multiplier.
• Fragmentation — the underrated mechanism. Even where forest cover remains, cutting it into patches creates edge effects: hotter, drier, windier, more fire-prone margins that penetrate hundreds of metres inward. Interior species decline, large-ranging animals lose viable territory, and the fragment slowly degrades from the outside in. A map showing the same forest area can conceal severe fragmentation.
• Soil degradation: once the biomass is removed, the nutrient store goes with it. Intense rainfall on bare ground causes leaching and erosion; laterisation can harden the surface. Fertility falls within a few seasons, driving further clearance — a positive feedback.
• Hydrological and carbon feedbacks: less evapotranspiration means less moisture recycled into local rainfall, so the remaining forest becomes drier and more flammable; carbon shifts from a sink towards a source. Researchers argue that beyond some threshold of loss parts of a basin could shift towards a drier, savanna-like state. Treat that threshold as a contested projection, not a fact — and say so; hedging accurately is rewarded, over-claiming is not.

Honesty rule. Do not invent hectares, percentages or rates. "Clearance has been driven overwhelmingly by pasture expansion, with logging roads opening the frontier" is stronger, and safer, than a half-remembered statistic. Examiners reward the mechanism.

Why is fragmentation so damaging?

Coral reefs under stress

Reef-building corals are animals living in symbiosis with photosynthetic algae (zooxanthellae) inside their tissues. The algae supply most of the coral's energy and its colour; the coral supplies shelter and nutrients. That partnership explains both the reef's extraordinary productivity in nutrient-poor tropical water — and its fragility.

• Narrow tolerances: corals need warm but not too warm water, high light (hence shallow, clear water), normal salinity, and low sediment and nutrient loading. A system living close to its thermal limit has almost no headroom.
• Bleaching — under heat stress (and high light) the coral expels its zooxanthellae, turning white. Bleaching is not death: if the stress passes quickly the algae can be reacquired. If it is prolonged or repeated, the coral starves and dies, and the reef structure erodes. Mass bleaching events driven by marine heatwaves are widely reported to have become more frequent and severe.
• Ocean acidification — the ocean absorbs a large share of anthropogenic CO₂; dissolved CO₂ forms carbonic acid, lowering pH and reducing the availability of carbonate ions. That makes it harder and more energetically costly for corals to build calcium-carbonate skeletons, and easier for existing structure to dissolve. Note this is a separate mechanism from warming, driven by the same cause.
• Overfishing — removing herbivorous fish lets macroalgae overgrow the coral, blocking light and preventing larval settlement. This is the classic route to a reef "flipping" to an algae-dominated state that resists recovery. Destructive methods (blast and cyanide fishing) destroy the structure outright.
• Sediment and nutrients — run-off from deforested catchments and agriculture smothers polyps, cuts light, and fertilises the very algae that outcompete the coral. Local stressors like these compound the global ones and are the part communities can actually control.

Why reefs matter (for the "assess the significance" question): a very large share of marine species depend on reefs; they support fisheries and tourism livelihoods, provide coastal protection by dissipating wave energy, and hold pharmaceutical potential. Their loss is simultaneously an ecological, economic and hazard-exposure problem — which is what makes reef management a genuinely hard trade-off rather than an obvious choice.

Bleaching vs acidification

Preserve it, or use it sustainably?

The whole management half of this unit turns on one argument: preservation (keep people out; protect the ecosystem for its own sake and its intrinsic value) versus sustainable exploitation (allow use that does not degrade the resource, so that local people gain a stake in keeping it). Neither wins outright, and saying which wins under what conditions is how you get the top band.

• Protected areas — national parks, nature reserves, marine protected areas, biosphere reserves with a zoned core/buffer/transition structure. Strength: legal protection of critical habitat. Weakness: a "paper park" with no funding or enforcement protects nothing; exclusion can displace and impoverish the people who lived there, breeding resentment and poaching.
• Ecotourism — low-impact tourism that funds conservation and gives the ecosystem a standing economic value. Strength: aligns incentives; can outcompete logging or fishing financially. Weakness: vulnerable to trampling, waste, disturbance and greenwashing; volatile (a shock to travel collapses the funding); and leakage means much of the revenue may never reach the community.
• Community-led conservation — local ownership and management of resources, with revenue-sharing. Generally the most durable arrangement, because enforcement becomes self-interest rather than an external imposition, but it needs secure land tenure and genuine devolution of power.
• Rewilding — restoring natural processes, often by reintroducing keystone species, and then stepping back. Strength: cheap in the long run, restores function rather than a snapshot. Weakness: outcomes are uncertain by design; conflicts with farming; and it sits awkwardly with plagioclimax habitats whose value depends on continued management.
• Environmental Impact Assessment (EIA) — a formal, usually statutory assessment of a proposed development's environmental effects, including a baseline survey, prediction of impacts, mitigation measures and monitoring. Strength: forces the ecology onto the decision table before consent. Weakness: only as good as its enforcement, its baseline data and its independence; it can become a box-ticking exercise that legitimises the scheme it assesses.

Sustainable development here means meeting present needs without compromising future generations' ability to meet theirs — and in an ecosystem context that means not drawing down the natural capital: harvest below the rate of regeneration, protect the processes (pollination, decomposition, nutrient cycling) as well as the species, and keep the system on the safe side of its thresholds.

Evaluating a strategy

Writing the evaluative essay

Essays here are nearly always a trade-off question — conservation vs development, local vs global, preservation vs sustainable use. The mark scheme rewards a line of argument that is sustained and a conclusion that judges.

• Unpack the key term. "Sustainable" for whom, over what timescale? "Ecosystem stress" — measured as biodiversity loss, productivity loss, or loss of function? Different answers point in different directions; say so.
• Use the models as tools of argument. Gersmehl explains why rainforest soils fail after clearance. Succession and the plagioclimax explain why "leaving it alone" can reduce biodiversity. Resilience and thresholds explain why a reef can look fine and then flip. Deploy them; don't define them and move on.
• Work the scales. A strategy can succeed locally and be irrelevant globally (a reef reserve cannot stop bleaching), or vice versa. That mismatch is often the heart of the question.
• Argue in blocks: claim → mechanism → evidence → counterweight. Every paragraph needs its "but".
• Conclude conditionally. Name the condition on which your judgement depends.

Try it: "To what extent can ecosystems under stress be managed sustainably?" Three blocks — (1) local stressors can be managed: overfishing, sediment run-off, trampling and land clearance all respond to community-led management, protected areas with real enforcement, and EIA; (2) but global stressors cannot be managed locally: warming, bleaching and acidification are driven by emissions outside the manager's jurisdiction, so even a perfectly managed reef can be destroyed by a marine heatwave; (3) and management can be self-defeating: preservation that excludes people breeds non-compliance, while rewilding a plagioclimax can destroy the biodiversity it was meant to protect. Judgement: sustainable management is achievable for local, direct stressors and can buy real resilience — but it is necessary and not sufficient, because the binding constraint on many ecosystems is now global and can only be addressed at that scale.

Sharpening the judgement

The big ideas to know

That is the core of AQA 3.1.6. Press Finish to see your score.