S2 Interactions Within Ecosystems

Most marks lost on this topic are not lost through ignorance of the words. They are lost through picking the wrong one. Four terms sit at four different levels of organisation, and a question is always pointing at exactly one of those levels. Work out which level is being asked about and the answer follows.

Take the largest of the four first. An ecosystem is not simply a place, and it is not simply a collection of animals. It is a pairing: the organisms that live in an area together with the surroundings they live in, and the interaction between the two. Both halves must be present before the word applies. A tank of seawater that has been filtered clean of every organism is a physical environment; it becomes an ecosystem only once a community of organisms is living in it and exchanging materials with it.

The term interact is doing real work in that definition. A group of organisms standing in the same area without affecting one another is a list, not a community. Feeding on one another, competing for the same food, sheltering under one another, breaking down one another's dead bodies — those are interactions, and they are what turns an area into a working system. The term physical environment is doing equal work: the light, heat, moisture, air and mineral content of a place set the conditions under which its organisms must survive.

The four terms are easiest to hold apart when you treat them as answers to one question asked at different scales. Read a question stem and run it through the three checks below, in order:

Applied to one stretch of a freshwater stream in a nature reserve: all the river snails of one species in that stretch form a population; the snails, the dragonfly nymphs, the water hyacinth and every other kind of organism living in it form a community; and that community together with the water, current, dissolved oxygen, minerals and light forms the ecosystem. Each answer uses the same place; only the level of organisation changes.

The distinction that trips students up most often is the one between habitat and ecosystem, because both words can be used about the same patch of ground. The difference is what each word includes.

A habitat covers the non-living side only: the conditions a place provides. The living side is left out of it deliberately, because the same word has to work for a single organism as well as for a whole population — and if the other organisms were part of a habitat, no individual could be described as living in one without also dragging in everything it eats or competes with. An ecosystem, by contrast, is the whole arrangement, living and non-living together with the interactions running through it.

The physical conditions themselves are worth naming, because questions often ask you to identify which one is limiting. The ones you are expected to recognise are light intensity, temperature, availability of water, oxygen content, salinity, and the pH of soil or water. They are measurable, and they vary from place to place and from hour to hour. They are also the reason two areas that look alike can hold very different communities.

Once a community is in place, the organisms in it stop being independent. Each one's survival is tied to the others and to the conditions around it, and this is what interdependence means. Four ways the dependence shows up are worth knowing, because a question that asks you to "explain how the organisms depend on one another" is looking for one of them, stated with a named organism and a clear benefit.

Food. Energy and nutrients move from one organism to another when one feeds on another. Remove the organism being fed on and the one above it has nothing to eat; remove the feeder and the population below it may increase until something else limits it.

Shelter and support. Organisms use other organisms as places to live and to grow. A young plant may develop only under the shade of a larger one, and animals nest, burrow or attach themselves in or on the bodies of other living things.

Transport and dispersal. Animals that feed on fruit or seeds carry them away and drop them, so plants of one species appear in places the plant could not have reached by itself. In water, the same happens with organisms carried on currents or attached to moving animals.

Return of nutrients. When organisms die and their wastes are passed out, decomposers break the material down and release nutrients back into the surroundings. Those released nutrients are taken up again and used by plants. Without this step the materials needed for growth would stay locked inside dead bodies and would not be available for reuse.

A practical consequence: an ecosystem that is supplying all the resources its organisms need, and where the populations stay roughly steady over the years, is one that is holding together. That steady state is not permanent. Disturb one part of it — remove a population, alter a condition — and the effect travels through the community, because nothing in it stands alone.

Two errors are common enough to be worth checking for before you move on. The first is describing a single species as a community — a community needs more than one population. The second is describing an ecosystem without mentioning anything non-living; a definition built only from organisms is a definition of a community, however many species you list.

Everything in the rest of this topic builds on this band. A question about feeding relationships is a question about interactions inside a community; a question about energy or nutrients is a question about an ecosystem as a whole, including its physical side; and a question about survival in a changing habitat is really a question about the conditions that set the limits for the community living there.

No organism in a habitat lives as an isolated unit. Every one of them is tied to others by feeding: it eats certain organisms, and it is itself eaten by others. These links, followed species by species, are the feeding relationships of the community, and they are also the routes along which energy travels from one living thing to the next.

Producers sit at the start of every feeding sequence because only they can turn light energy into a form that living things can use. Consumers take their energy second-hand, and decomposers work on whatever dies. Fungi and bacteria are the usual decomposers; in a pond they break down fallen leaves and dead animals in the sediment, so that mineral salts and other nutrients return to the water and can be taken up again by plants. Without them, nutrients would stay locked in dead matter and the producers would eventually run short of raw materials.

Consumers are sorted by what they eat.

These labels describe an animal's diet, not a fixed rank in the community. A tilapia that grazes on algae today and swallows a mayfly nymph tomorrow is acting as a plant eater at one moment and a flesh eater at the next, so the same species can occupy more than one position in the feeding network.

Predator and prey name the two sides of a hunting link: the predator is the animal that hunts and kills, and the prey is the animal that is hunted and eaten. A predator is often equipped for the chase — sharp senses to locate the prey, claws, jaws or a fast turn of speed to take it — while the prey usually survives by escaping rather than fighting: camouflage, speed, a hard shell, or living in groups. Bear in mind that the roles can swap with size and circumstance; a dragonfly nymph eats tadpoles, but a large frog will eat a dragonfly nymph.

Competition is what happens when two or more organisms need the same resource and there is not enough of it to go round. The resource may be food, water, shelter, nesting space, mates, or, for plants, light and mineral salts. Competition can be between members of the same species or between different species, and it limits how large a population can grow even where no predator is involved. Predator and prey are also competitors in a sense: the dragonfly nymph and the tilapia both chase mayfly nymphs, so whichever forages more efficiently leaves less food for the other.

Take the four members of a pond community in order of who eats whom:

hydrilla (a submerged pond plant) → mayfly nymph → dragonfly nymph → pond heron

Read the arrows as a statement about energy, not about appetite: each arrow begins at the organism that is eaten and points to the organism that eats it, so energy is shown leaving the one and arriving in the other. Hydrilla is the producer, the mayfly nymph feeds on it, the dragonfly nymph catches the mayfly nymph, and the pond heron takes the dragonfly nymph. Every position in that sequence is a trophic level.

Feeding is a wasteful way of moving energy onward. Only a small fraction — about 10% — of the energy held in one trophic level becomes body material in the level above; the rest has already left the food as heat from respiration, through movement, and in material that is passed out or excreted. Because so much is lost at every step, the fourth level receives only a thousandth of what the producer captured. That is why chains are short, rarely more than four or five levels: a further level would have too little energy left to support a viable population.

Few animals are limited to a single kind of food, so a single chain tells only part of the story. The table below sets out the feeding links of a freshwater pond community; together, these links form its food web.

To read a web, work through it in this order. First find the producers: they are the organisms with no arrow pointing at them as food, because they make their own food rather than eating anything. Next follow the arrows outward and note that an organism with several arrows coming in from its prey is a consumer with a varied diet. Then check that an organism can appear at more than one trophic level: the tilapia is a primary consumer when it grazes on algae and a secondary consumer when it takes a mayfly nymph. Finally, trace the longest route from a producer to a top carnivore, and count the trophic levels on it. In this web the longest route runs duckweed → tadpole → dragonfly nymph → monitor lizard, four levels long.

Using the web to predict. A question in this form asks what happens to a named population when something else in the web changes. Suppose disease removes most of the dragonfly nymphs from the pond. Follow every arrow that touches them.

The mayfly nymphs and tadpoles lose one of their predators. Fewer of them are eaten, so both populations are likely to rise. With more tadpoles grazing, the duckweed and algae are cropped harder, so the producer population may fall — which in turn means less food for the surviving tadpoles and mayflies. Both the heron and the monitor lizard lose the dragonfly nymph as prey, so they must turn to what is left: the heron hunts more tilapia, tadpoles and mayflies, adding to the pressure on those populations, and the monitor lizard takes more tilapia and heron eggs and chicks. The tilapia also loses a competitor for mayfly nymphs, so it may feed more freely on them, offsetting the heron's extra hunting. Given the same change, the numbers of mayflies and tadpoles could still fall overall if the heron's increased hunting outweighs the loss of the dragonfly nymph.

Every living thing needs energy to stay alive, grow and reproduce. In almost every ecosystem that supply begins as light from the Sun. Green plants, algae and phytoplankton capture it during photosynthesis and store it as chemical potential energy inside the food they make. From then on the energy does not stay still — it is handed from one organism to the next whenever something is eaten, and at every handover a large slice of it is lost.

Organisms at different trophic levels never hold the same amount of energy. Only a small fraction — around a tenth — of what one level contains is built into the bodies of the next, so each transfer is wasteful and short chains are the norm. Chains of five or six levels are rarely found because too little energy would remain at the top to keep those animals alive.

Here is a pond chain using our own figures, for one square metre of pond over a year:

Drawn to scale, these four bars form a pyramid of energy that narrows sharply towards the top. A pyramid of energy is the most reliable of the three pyramids, because energy can only decrease as it moves upward; a higher level can never hold more than the level below it.

Energy leaves the chain through ordinary living. It is spent and released as heat during respiration; it is carried away in food that is swallowed but never absorbed, leaving the body as faeces; it goes out in the nitrogenous waste excreted by the kidneys; it leaks away as body heat radiated to the surroundings; and whatever is left when an organism dies is released by the decomposers that break the body down. None of it can be gathered up and used again.

Energy and nutrients behave in opposite ways inside an ecosystem, and the contrast is worth remembering as one idea.

Decomposers — chiefly bacteria and fungi — do the recycling. They feed saprophytically: they release digestive enzymes onto dead plants, dead animals, fallen leaves and animal waste, digest the material outside their own bodies, then absorb the soluble products. This feeds them, and it unlocks the mineral salts tied up in that dead tissue. The salts pass into the soil or water, where plant roots absorb them and build them into new proteins, chlorophyll and vitamins. Animals gain those nutrients by eating the plants. A fallen leaf on a forest floor is therefore not waste but the start of a fresh nutrient supply for the next generation of plants.

Without decomposers the cycle would stall. Nitrogen, phosphorus and other essential elements would stay locked inside dead bodies and debris, and the soil would steadily lose the fertility that producers need. Bacteria also convert the nitrogen in animal waste into compounds that plants can absorb.

Adaptations are grouped by what they are: some are parts of the body, some are things the organism does, and some are processes running inside the body.

Notice the shape of each explanation: a named trait, matched to one specific challenge in the habitat, followed by the survival advantage it delivers. Functional adaptations are easy to overlook because they are invisible from outside — salt glands that excrete excess salt, heat-stable enzymes in hot springs, a lowered rate of ammonia production in alkaline water.

An adaptation is useful only against the conditions that produced it. It is a good answer to one particular question. Change the question — shift the temperature, the rainfall, the food supply or the mix of species around it — and the feature that once secured survival can become a handicap.

One clear case: many migratory birds time their journey by day length rather than temperature. When springs warm earlier than the birds' internal schedule expects, they arrive after the insects on which they feed their chicks have already peaked, and fewer chicks fledge. A trait once finely tuned to the old calendar now mistimes the whole breeding season.

Physical conditions can shift in the same way. Toads that rely on burying themselves in damp soil lose that refuge when prolonged drought, drainage works or groundwater extraction leave the ground dry and hard. A population with little genetic variation may contain no individual whose traits suit the new conditions, and it can decline sharply or disappear.

An ecosystem stays in equilibrium only while the conditions inside it stay roughly steady. Human populations change those conditions far faster than organisms can adapt, and the changes usually arrive together rather than one at a time. Four broad kinds of impact do most of the damage.

Pollution. Waste released into air, water or soil changes the physical factors that organisms depend on. Fertilisers washed off farmland add nutrients to a pond or river, so algae and surface plants multiply into a thick bloom. When those plants die, decomposers break them down using oxygen from the water, and the oxygen content drops so far that fish and other aquatic animals suffocate. Other pollutants act differently: heavy metals and pesticides accumulate in tissues and poison organisms, oil films coat feathers and gills, and plastic fragments are eaten because they look like food, filling digestive systems without providing energy.

Habitat loss and fragmentation. Clearing forest, draining marshland or building over grassland removes the shelter, nesting sites and food supply that a community needs. Even where some vegetation is left, the remaining patches may be too small or too far apart for organisms to move between them, breed successfully or escape seasonal shortages. Populations split into isolated groups, each less able to recover from disease or drought, and species that cannot survive in the changed conditions disappear locally.

Over-harvesting. When organisms are removed faster than they can reproduce, their numbers fall below the level needed to sustain the population. Over-fishing, logging of slow-growing trees, hunting for meat or trade, and collecting wild plants all take a harvest that the ecosystem cannot replace within a reasonable time. Once the breeding population is too small, recovery may take many years, and a species that plays a key role in a food web may take the rest of the community down with it.

Introduced species. Organisms moved into an area where they did not previously live are called introduced (alien) species. Some are brought in deliberately as crops, pets or pest control; others arrive accidentally in ballast water, packaging or soil. If the new habitat lacks a natural predator, competitor or disease to control them, their numbers grow quickly. They may eat native organisms, take the same food and space, spread parasites, or hybridise with close relatives, and the balance among the original populations is disturbed.

Sustainable living means meeting present needs while keeping ecosystems able to support future generations. It is not only about individual habits: it works at the level of the household, the community, the government and the economy at the same time.

• Cutting consumption at the source. Refuse what is not needed, reuse containers and clothing, repair rather than replace, and recycle materials such as paper, glass and metal. Less consumption means fewer raw materials extracted and less land cleared for them.
• Using energy efficiently. Switching to renewable sources such as solar power, improving insulation and taking public transport all reduce the greenhouse gases released per unit of activity, which slows warming and the climate shifts that follow.
• Making food choices carefully. Buying local and seasonal produce shortens transport, composting food waste returns nutrients to the soil instead of to landfill, and choosing sustainably caught fish keeps harvested populations above the level at which they can replace themselves.
• Protecting and restoring habitats. Nature reserves and marine parks set land aside by law, wildlife corridors let organisms move between patches of habitat, and replanting with native species rebuilds the communities that were lost.
• Regulating what people take and release. Fishing quotas, licences and closed seasons limit catches; selective logging rules allow only mature trees to be felled; import controls stop introduced species and products made from threatened wildlife from entering the country.
• Acting locally. Community clean-ups, school recycling drives, citizen surveys of local wildlife and pressure on businesses to change packaging all produce measurable change, and each one reduces the total demand placed on ecosystems.

The important idea is that conservation and human use are not opposites. An ecosystem managed so that its organisms can replace themselves keeps providing food, clean water and materials indefinitely, which is exactly what sustainability means.

Food-web questions look easy and are marked strictly, because almost every mark depends on the organism you name and the direction you claim energy moves in. Work through the diagram in a fixed order.

1. Find the producers first. The green plants sit at the base, always at the first trophic level. Nothing should point into them.
2. Follow the arrowheads, not the layout. An arrow shows the direction in which food and energy are transferred, so it runs from the organism being eaten to the organism that eats it. Read the arrows in the order they are actually drawn before you answer.
3. Count the chains when asked. Trace every separate route from a producer up to the top of the web, and write each one out.
4. Explain an effect one link at a time. "Explain how a fall in the number of water fleas affects the herons" needs a chain: fewer water fleas, so less food for the guppies, so guppy numbers fall, so the herons have less food and their population declines. Marks are usually given for each link, not for the conclusion alone.
5. Keep decomposers out of the chain. Fungi and bacteria recycle nutrients but do not appear as a trophic level in a food chain.

Questions that begin "describe the trend shown by the data" are a separate skill from explaining. The examiner has given you the figures and wants to see whether you can read them, so do not guess at causes and do not repeat the number you have already been given as your whole answer.

• State the direction first. "Both populations decreased over the period."
• Quote values from the table or graph. Give the starting and finishing figures with their units, and the years they belong to: "water fleas fell from 480 per litre in 2019 to 90 per litre in 2025".
• Describe the shape of the change. Say whether the fall was steady or sudden, and point to the period in which it was steepest or where a rise interrupted it.
• Compare the two sets if there are two. Say which fell further or faster, and whether they changed at the same time.
• Stop before the cause. Unless the question says "explain" or "suggest why", any statement about pollution, predators or fishing is outside the required answer and gains nothing.

Our pond study site supports this food web:

Table 1 records the population density of water fleas and guppies at the pond between 2019 and 2025.

Answer all parts. (Total: 10 marks)

1. Name one producer and one secondary consumer from the table above. (2 marks)
2. The producers in the pond store 8,000 J of energy. Assuming that 10% of the energy at each trophic level is transferred to the next, calculate the energy available to the secondary consumers. Show your working. (2 marks)
3. Describe the trend shown by both sets of data in Table 1. (2 marks)
4. A species of tilapia that does not normally live in the pond is released into it. Explain how this could cause the population of herons at the pond to decrease. (4 marks)