1The idea in one paragraph

Energy enters almost every ecosystem as sunlight, is captured by producers as chemical energy in carbon compounds, and passes along food chains when one organism eats another. At every step most of it is lost, mainly as heat from respiration, and heat cannot be turned back into chemical energy by any organism, so energy flows through an ecosystem in one direction and must be replaced continuously by more sunlight. That loss is why energy pyramids narrow so fast and why food chains are short. Matter behaves differently: the carbon, nitrogen and every other element in living things are taken up, passed on, released by respiration and decay, and taken up again, so matter cycles. The carbon cycle is the example you must draw, and the Keeling Curve is its living record: the air's carbon dioxide rising and falling with the seasons' photosynthesis, and climbing year on year because we burn carbon that was locked away.

2Ecosystems are open systems

A system is a set of connected parts with a boundary. What crosses the boundary decides what kind of system it is, and Figure 1 shows the two kinds.

An open system exchanges both energy and matter with its surroundings. Every natural ecosystem is open. A pond receives light and loses heat, but it also gains water, silt and fallen leaves from its stream, and loses water through the outflow, insects that emerge and fly off, and fish taken by a visiting heron.

A closed system exchanges energy only. A sealed bottle garden lets light in and heat out, but no matter crosses the glass: the same carbon, water and minerals must be recycled inside it for ever. Closed systems are rare in nature; they are useful in experiments because they separate the two kinds of transfer.

3Sunlight: the energy source of most ecosystems

Sunlight is the principal source of energy that sustains most ecosystems. Producers capture it by photosynthesis, and every other organism in the ecosystem lives, directly or indirectly, on what the producers make.

There are exceptions where no light reaches.

• Deep ocean. Below about a thousand metres, no sunlight penetrates. Around hydrothermal vents on the ocean floor, whole communities of tube worms, clams and shrimps live on bacteria that get their energy by oxidising chemicals, such as hydrogen sulfide, in the hot vent water.
• Caves. Many cave communities still run on sunlight at second hand: leaves washed in, or droppings from bats that feed outside. But a few sealed caves, such as Movile Cave in Romania, support communities based on bacteria that oxidise chemicals in the groundwater, with no input from the surface at all.

Nature of science: laws and generalisations. "Sunlight sustains most ecosystems" is a generalisation: a rule of thumb that describes a pattern seen again and again in nature. Scientific laws are generalisations of this kind. Unlike a theory, a law does not explain why the pattern exists; it describes it. Like a theory, it can be used to make predictions: find a new ecosystem and you can predict its energy comes from light. A useful generalisation describes a pattern found across many cases, allows predictions, and is honest about its limits: "most", not "all", because vents and caves are known exceptions.

4Food chains and food webs: chemical energy passed on by feeding

A food chain is a sequence of organisms in which each one feeds on the one before it. Energy enters the chain when a producer converts light energy into chemical energy in carbon compounds, and passes along it as each consumer eats the organism before it and takes in that chemical energy.

In a food chain or food web, arrows point from the organism that is eaten to the organism that eats it. They show the direction in which energy and biomass move.

Real feeding relationships branch: most organisms eat more than one thing and are eaten by more than one thing. A food web joins all the food chains of a community into one diagram. Figure 2 is part of the food web of an English oak wood.

Read one chain out of it: oak tree → winter moth caterpillar → blue tit → sparrowhawk. Now follow a second: bramble → wood mouse → tawny owl. The web shows what no single chain can: that the blue tit eats caterpillars, aphids and ladybirds, and that a fall in caterpillars will hit the sparrowhawk through the blue tits.

Trophic levels. Each position in a food chain is a trophic level, and the guide gives four names.

Many organisms have a varied diet and so sit at different trophic levels in different food chains. A fox eating a rabbit is a secondary consumer; the same fox eating blackberries in September is a primary consumer (the amber arrow in Figure 2). A blue tit eating an aphid is a secondary consumer; eating a ladybird that has eaten aphids, it is a tertiary consumer. The level belongs to the food chain, not to the animal.

5Decomposers: energy from dead organic matter

Not everything a producer makes is eaten by a consumer. Leaves fall, animals die, faeces are dropped. That dead organic matter still holds chemical energy in its carbon compounds, and it is the food supply of the decomposers. The guide lists three sources:

• faeces, the undigested food egested by animals;
• dead parts of organisms, such as fallen leaves, shed skin, moulted feathers and dead roots;
• dead whole organisms, from a fallen tree to a dead mouse.

Decomposers include saprotrophs, mainly fungi and bacteria, which secrete enzymes onto dead matter, digest it outside their bodies and absorb the products; and detritivores, such as earthworms and woodlice, which eat pieces of dead matter and digest them internally. Decomposers and detritus feeders are not usually drawn as part of a food chain, but they receive energy from every level of it, and by respiring they release that energy as heat and the carbon as carbon dioxide.

6Autotrophs and heterotrophs

The first question about any organism in an ecosystem is where its carbon compounds come from. Figure 3 sets out the answers.

Autotrophs make their own carbon compounds from simple inorganic substances, carbon dioxide above all, using an external energy source. Energy is needed twice: for carbon fixation, turning carbon dioxide into a sugar, and for the anabolic reactions that build sugars, amino acids and fatty acids up into macromolecules such as starch, proteins and lipids.

Autotrophs differ in where that external energy comes from.

• Photoautotrophs use light. Plants, algae and cyanobacteria are photoautotrophs, and they capture light by photosynthesis.
• Chemoautotrophs use energy released by oxidation reactions: they oxidise an inorganic chemical taken from their surroundings, and the energy released drives carbon fixation. Oxidation reactions release energy, which is why they are useful to living things (respiration is also an oxidation). The guide's example is iron-oxidising bacteria, which oxidise dissolved iron(II) ions, Fe²⁺, to iron(III), Fe³⁺. They thrive in iron-rich water draining from mines, where the Fe³⁺ they produce settles out as orange-brown deposits.

Heterotrophs obtain carbon compounds from other organisms, living or dead, and use them to build the carbon compounds they themselves need. All animals, all fungi and most bacteria are heterotrophs. The large molecules they take in, proteins, nucleic acids, polysaccharides, are not the ones they need, so they are first digested into monomers. Animals digest internally, in a gut; fungi and many bacteria digest externally, by secreting enzymes onto their food. The monomers are absorbed and then assimilated: built into the heterotroph's own proteins, nucleic acids and other molecules.

Both release energy the same way. Autotrophs and heterotrophs alike release energy from carbon compounds by oxidising them in cell respiration (C1.2). A plant respires all day and all night; photosynthesis only happens in the light.

7Energy losses between trophic levels

Only a small fraction of the energy at one trophic level reaches the next. Figure 4 follows the energy in the grass that a rabbit eats.

The causes of loss, from one level to the next:

• Not all of the organism is eaten. Roots, woody stems, bones and fur are left behind, or whole organisms die uneaten. Their energy goes to decomposers.
• Not all that is eaten is digested and absorbed. Cellulose in grass is hard to digest, so much of it passes out in faeces, and that energy goes to decomposers too.
• Energy is converted to heat in cell respiration. This is the largest loss. Most of the energy that is absorbed is used in respiration to make ATP for movement, active transport, keeping warm and building molecules.
• Only what is built into new tissue by growth and reproduction is available to be eaten by the next level. And some of that tissue dies and passes to decomposers instead.

Why respiration always produces heat. No energy transfer is 100% efficient. When glucose is oxidised in respiration, only part of the energy released is trapped in ATP; the rest becomes heat at once. Then, when ATP is used in a cell, to contract a muscle or pump an ion, the transfer is inefficient again and more heat is produced. In the end, all the energy used by an organism becomes heat. That heat is lost to the environment, and no organism can convert heat back into chemical energy. This is why energy flows through an ecosystem and cannot be recycled.

8Energy pyramids

An energy pyramid shows the energy passing through each trophic level of a food chain, in a fixed area over a fixed time, usually kJ m⁻² yr⁻¹. Each level is a horizontal bar, producers at the bottom, and the width of the bar is proportional to the energy. Figure 5 shows one built from invented grassland data.

The percentage passed on from one level to the next is a standard Paper 1B calculation:

Percentage transferred = (energy at the higher level ÷ energy at the lower level) × 100

Roughly a tenth is passed on at each step, and the rest is lost as section 7 explained. Measured ecosystems vary around this figure, but the pattern is always the same: an energy pyramid is always pyramid-shaped, never inverted, because a level can never have more energy than the level that feeds it.

The guide asks you to build pyramids from research data from specific ecosystems. Classic field studies of real ecosystems, such as H. T. Odum's measurements of Silver Springs in Florida in the 1950s, found exactly this pattern of steep losses at every level. When you use real data, draw the bars to scale if the numbers allow; if the top bars would be too thin to see, as here, label every bar with its value and say it is not to scale.

9Why food chains are short

Because only around a tenth of the energy passes on at each step, the energy available shrinks very fast up a food chain. By the fourth or fifth level there is too little energy left, per unit area, to support a viable population of another predator: it would need an impossibly large territory to find enough food. So the number of trophic levels is restricted by energy losses, and most food chains have four or five levels at most.

At each successive level there are fewer organisms, or smaller organisms, so there is less biomass. What does not fall is the energy content per gram of tissue: a gram of fox contains at least as much energy as a gram of rabbit. The energy per level falls because there is less living tissue at that level, not because the tissue is poorer.

10Primary and secondary production

Primary production is the accumulation of carbon compounds in biomass by autotrophs. It is measured as a mass of carbon per unit area per unit time, usually g m⁻² yr⁻¹. Biomass accumulates when organisms grow or reproduce, and primary production is the base on which every consumer depends.

Biomes vary greatly in their capacity to accumulate biomass. Tropical rainforests are the most productive land biomes, because they have high light intensity, warmth and water all year round. Deserts and tundra are among the least productive: plant growth is held back by lack of water in one and by cold and a short growing season in the other. A number helps you picture the unit. If a hectare of grassland (10,000 m²) has a primary production of 450 g m⁻² yr⁻¹ of carbon, it adds 450 × 10,000 = 4,500,000 g, or 4.5 tonnes of carbon, to its biomass every year.

Secondary production is the accumulation of carbon compounds in biomass by heterotrophs. It is always lower than primary production in the same ecosystem. The reason is the one that runs through this whole subtopic: when heterotrophs respire, they convert carbon compounds into carbon dioxide and water, which leave the body. That biomass is lost, so only a fraction of what consumers eat ends up as consumer biomass.

11The carbon cycle

Energy flows through an ecosystem, but carbon goes round. Every carbon atom in your body has been in the atmosphere as carbon dioxide, and will be again. Figure 6 is the cycle the guide asks you to draw.

Three processes run the cycle.

1. Photosynthesis takes carbon dioxide from the atmosphere (or dissolved in water) and fixes it into carbon compounds in producers.
2. Feeding passes those carbon compounds from producers to consumers, and from dead matter to decomposers.
3. Cell respiration by producers, consumers and decomposers oxidises carbon compounds and releases carbon dioxide back to the atmosphere.

Carbon sinks and carbon sources. An ecosystem is a carbon sink if it takes up more carbon dioxide than it releases: photosynthesis exceeds respiration, so carbon accumulates in biomass and soil. A young, growing forest is a sink. An ecosystem is a carbon source if it releases more carbon dioxide than it takes up: respiration exceeds photosynthesis. A peat bog that has been drained is a source, because air reaches the peat and decomposers start to respire it away.

Combustion. Where decay is slow, dead organic matter can build up over long periods and become peat, and over millions of years coal, oil and natural gas. These are carbon stores, and they formed at very different times.

Combustion of any of these oxidises the carbon and releases it as carbon dioxide. Burning happens naturally, when lightning sets a forest or grassland alight, but human activities have enormously increased the rate: we burn fossil fuels for energy and transport, burn forests to clear land, and dry and burn peat. Carbon locked away for millions of years is being returned to the atmosphere within a few human lifetimes.

12The Keeling Curve

In 1958 the scientist Charles David Keeling began measuring the concentration of carbon dioxide in the air at an observatory on Mauna Loa, a volcano in Hawaii, far from local sources of pollution. The measurements have continued ever since, and the graph of them is called the Keeling Curve. Figure 7 shows its shape.

Two patterns, two explanations.

The annual fluctuation. Every year the concentration rises and falls by about 6 ppm (parts per million). It is highest in about May and lowest in about September or October. Most of the world's land, and so most of its plants, is in the Northern Hemisphere. In the northern spring and summer, plants photosynthesise fast; photosynthesis exceeds respiration worldwide, so carbon dioxide is drawn down from May to the autumn. In the northern autumn and winter, leaves fall, photosynthesis slows, and respiration by plants, animals and decomposers continues, so respiration exceeds photosynthesis and the concentration climbs back up to its May peak.

The long-term trend. Under the zigzag, the concentration has risen every single year: from about 315 ppm in 1958 to above 420 ppm in the 2020s. The rise is also getting faster, since the curve is steepening. The main cause is combustion of fossil fuels, with deforestation adding to it, releasing carbon dioxide faster than photosynthesis and the oceans can absorb it.

When you analyse a Keeling Curve in the exam, say both things: the seasonal cycle is photosynthesis versus respiration, the upward trend is combustion.

13Autotrophs and heterotrophs need each other: oxygen and carbon dioxide

Aerobic respiration in every organism depends on oxygen in the atmosphere, and that oxygen was produced, and is continuously replaced, by photosynthesis. Photosynthesis in turn depends on carbon dioxide in the atmosphere, which is continuously replaced by respiration (and, now, combustion). Each process uses up what the other produces. The quantities exchanged in a year are enormous, so this is one of the largest interactions between autotrophs and heterotrophs on the planet, and it is why a change in one, a large loss of forest for instance, affects the other.

14All the elements are recycled

Carbon is the example you draw, but it is not special. Every element that living organisms use, nitrogen, phosphorus, sulfur, potassium, calcium, iron and the rest, is recycled in ecosystems. Producers take the elements up as simple inorganic ions from the soil or water, build them into their compounds, and pass them along food chains by feeding. When organisms die or egest waste, decomposers break down the organic compounds and release the elements again as simple inorganic ions, which producers can reuse. Without decomposers, elements would stay locked in dead matter and the soil would run out of nutrients. You do not need the details of the nitrogen cycle or any other nutrient cycle, only the principle.

This is the answer to the question at the top of the page. Matter is recycled because decomposers and respiration return it to forms that producers can take up again. Energy is not recycled because it leaves each organism as heat, and nothing can take heat back in and turn it into chemical energy. So every ecosystem needs a continuous input of energy, from sunlight or, rarely, chemical oxidation, to replace what is lost at every step.