Ecology — ecosystems, cycles & human impact.

HookGermany's insects quietly lost three-quarters of their weight

ModelThe ladder of organisation, and why nothing lives alone

Start with the vocabulary the whole topic is built from, because examiners hand out marks simply for using it precisely. A population is all the organisms of one species in a habitat; a community is all the populations of all the different species living there together; and an ecosystem is that community plus the non-living surroundings it interacts with — the soil, water, air and light. A habitat is just the place a particular organism lives. The ladder runs organism → population → community → ecosystem, and each rung contains the one below it.

The crucial idea is interdependence: within a community every species relies on others for food, shelter, pollination or seed dispersal, so removing one can ripple outwards to many. When the abiotic factors and the population sizes of all the species stay roughly constant over time, ecologists call it a stable community — the classic examples being an ancient oak woodland or a tropical rainforest, where numbers wobble year to year but never run away.

Feeding relationships are drawn as food chains, and every food chain on Earth begins with a producer — usually a green plant or alga that captures light energy by photosynthesis and stores it as the chemical energy in glucose. Producers are eaten by primary consumers (herbivores), which are eaten by secondary and then tertiary consumers (carnivores). A consumer that kills and eats another is a predator; the one eaten is its prey. In a stable community predator and prey numbers rise and fall in linked cycles: a glut of prey lets predators breed, the swelling predator population then eats the prey down, predators go hungry and decline, and the prey recovers — so the predator peak always lags a little behind the prey peak. Reading that lag off a graph is a favourite exam task, and the arrow in a food chain always points from the eaten to the eater, showing the direction energy and biomass flow.

MechanismCompetition, and the two kinds of factor that decide who wins

Organisms compete whenever a resource is scarce, and what they compete for differs by kingdom. Plants compete for light, space, water and mineral ions in the soil — which is why a fast-growing weed that shades its neighbours can crowd them out. Animals compete for food, mates and territory. To survive and reproduce, an organism must secure the resources it needs against everything else that wants them, and the outcome of that contest is what sets the size and distribution of a population.

The forces acting on a population divide cleanly into two lists, and keeping them straight is worth easy marks. Abiotic factors are the non-living conditions: light intensity, temperature, moisture (water availability), soil pH and mineral content, wind intensity and direction, the concentration of carbon dioxide for plants, and the concentration of dissolved oxygen for aquatic animals. Change one — drop the light under a new tree canopy, or drain a marsh — and the species that can no longer cope thin out while better-suited ones move in.

Biotic factors are the living pressures: the availability of food, the arrival of a new predator, the introduction of a new pathogen, and one species out-competing another for a shared resource. The two lists interact constantly. A cold summer (abiotic) may shrink an insect population, which starves the birds that eat it (biotic); a new fungal disease (biotic) may wipe out a tree whose shade other plants depended on. When an exam asks you to explain why a species is declining in one part of a habitat, the winning answer almost always names a specific factor from one of these two lists and then traces its effect through to the population — factor, mechanism, consequence.

ModelAdaptations — three flavours, and life where nothing should live

An adaptation is a feature that helps an organism survive and reproduce in the conditions where it normally lives, and the specification sorts them into three types. Structural adaptations are features of the body: a cactus stores water in a swollen stem, shrinks its leaves to spines to cut water loss and to deter grazers, and spreads shallow, wide roots to grab rain fast; a polar bear carries thick fat and dense fur for insulation and has a small surface-area-to-volume ratio that slows heat loss. Behavioural adaptations are things an organism does: many desert animals are nocturnal, sheltering through the fierce daytime heat and hunting in the cool of night; birds migrate to track food and warmth. Functional adaptations are processes inside the body: a camel can tolerate a large rise in body temperature and produces very concentrated urine to conserve water, and a desert kangaroo rat barely drinks at all, making metabolic water from its food.

At the outer edge of what life can tolerate live the extremophiles — organisms, usually bacteria, adapted to conditions that would kill almost anything else. They thrive in deep-sea hydrothermal vents at crushing pressure and scalding temperature, in volcanic hot springs, and in water so salty or acidic it strips other cells apart. Their enzymes are built to keep working where ordinary proteins would denature. Extremophiles matter beyond curiosity: the heat-stable enzymes copied from hot-spring bacteria are what make the DNA-copying technology behind modern genetics and forensics possible. The exam habit to build is to name the adaptation, classify it as structural, behavioural or functional, and — this is where the marks sit — say precisely how it improves survival in that particular environment.

MethodRequired practical 9 — counting what you cannot count

You cannot count every daisy on a football field or every limpet on a shore, so ecologists sample: they measure a small, representative part and scale up. The core tool is the quadrat, a square frame of known area (commonly 0.25 m², a half-metre square) that you place on the ground and inside which you count individuals or estimate the percentage cover of a plant.

To estimate the size of a whole population, the quadrats must go down at random positions — you generate random coordinates (like a grid reference from a random-number app) and place the quadrat there. Random placement is not laziness; it is the whole point, because it removes the unconscious bias of a student who drifts towards the interesting-looking patches. You take the mean number per quadrat, convert it to a number per square metre, and multiply by the total area. This gives an estimate, and calling it an estimate — not a count — is itself a mark, because you have only measured a fraction of the field.

When the question is not 'how many?' but 'how does the population change across the habitat?' — say, how plant cover changes from the open field into the shade of a wood — you switch to a transect. You lay a tape in a straight line along the gradient you care about and place quadrats at regular, fixed intervals (a belt transect). That regular spacing is deliberately not random, because here you want to track how the community shifts against a changing abiotic factor such as light. The same data set also drills the maths skills the spec lists: from a set of quadrat counts you should be able to pull the mean (add and divide), the median (the middle value) and the mode (the most common value).

MechanismHow atoms get reused: the carbon and water cycles, and decay

The Earth has a fixed stock of carbon atoms and water molecules; life keeps working only because they are used, released and used again. In the carbon cycle, photosynthesis removes carbon dioxide from the air and locks its carbon into the glucose — and then the proteins and fats — of plants. That carbon passes along food chains as animals eat. It returns to the atmosphere as carbon dioxide through three routes: respiration by plants, animals and microorganisms; combustion when wood or fossil fuels burn; and decomposition, in which microorganisms break down the dead. The water cycle runs on the Sun's energy: evaporation (and transpiration from plants) lifts water vapour, which condenses into clouds and falls as precipitation, providing the fresh water land organisms need before it drains back to the sea. In both cycles the unsung workers are decomposers — bacteria and fungi that feed on dead material and, in doing so, hand the trapped mineral ions back to the soil for plants to reuse.

Decomposition (a separate-science, Biology-only topic) speeds up or slows down with three conditions, and gardeners and composters exploit every one. Decay is fastest when it is warm (up to the point where high temperatures denature microbial enzymes), moist (microbes need water and their digestive enzymes work in solution), and rich in oxygen (most decomposers respire aerobically). Starve the microbes of oxygen and decay goes anaerobic, producing methane — the basis of biogas generators, which capture that methane from rotting waste or manure and burn it for fuel. A well-managed compost heap is simply this biology run on purpose: warm, damp, turned to let air in, so microbes recycle garden waste into fertiliser within weeks.

DataTrophic levels, pyramids of biomass and the 10% that gets through

Each feeding stage in a food chain is a trophic level, numbered from the bottom (all of these leaves are separate-science, Biology only). Level 1 is the producers; level 2 the herbivores or primary consumers; level 3 the carnivores that eat them (secondary consumers); level 4 the top carnivores or apex predators, which no other animal hunts. Decomposers sit outside the numbered ladder, breaking down dead organisms and wastes at every level and returning mineral ions to the soil.

Draw the amount of living material — the biomass — at each level as a bar, stack the bars with producers at the bottom and scale each bar's width to the biomass it represents, and you get a pyramid of biomass. It tapers as you climb, and it does so for a hard quantitative reason. Producers capture only about 1% of the light energy that reaches them, and thereafter only roughly 10% of the biomass at one trophic level is passed on to the next. The other ~90% is lost along the way: not all of an organism is eaten (roots, bones, shells are left); not all of what is eaten can be absorbed, so some leaves the body as faeces (egestion); a large share is respired away to power movement and — in mammals and birds — to keep the body warm; and nitrogenous waste leaves as urea in urine. Because so little gets through, food chains rarely stretch beyond four or five links: there is simply not enough biomass left to support a sixth.

CaseThe human thumbprint: biodiversity, pollution, land and climate

Biodiversity is the variety of all the different species of organisms on Earth, or within one ecosystem, and a high biodiversity is what makes an ecosystem stable — species buffer one another against change, so the whole community depends less on any single one. Human activity is pushing biodiversity down on several fronts at once, and the driver behind all of them is the same: a human population that has passed eight billion, combined with rising standards of living that mean each person uses more raw materials and produces more waste.

More waste, unless handled, means more pollution, which the spec splits three ways. Water is polluted by sewage, by fertiliser washing off fields, and by toxic chemicals. Land is polluted by landfill and by toxic chemicals such as pesticides and herbicides. Air is polluted by smoke and by acidic gases like sulfur dioxide, which returns as acid rain. Pollution kills organisms directly and reduces biodiversity wherever it lands. Humans also simply take the land other species need — for building, quarrying, farming and dumping waste. A pointed example is peat: peat bogs are waterlogged habitats that build up over thousands of years and lock away enormous amounts of carbon, yet they are dug up for cheap compost and fuel, and draining or burning that peat both destroys a rare habitat and releases its stored carbon as carbon dioxide.

The same appetite drives deforestation in the tropics — forests cleared for timber, for cattle grazing and rice fields (both of which release methane), and to grow crops for biofuels. Cutting the forest removes trees that were absorbing carbon dioxide, and burning them releases more, while countless species lose their home. All of this feeds global warming: rising levels of carbon dioxide and methane trap more of the Sun's heat, and the biological consequences are the ones examiners want named — loss of habitat as ice melts and low-lying land floods, shifts in the distribution of species as they track the conditions they can tolerate, changes in migration patterns, and an overall fall in biodiversity. (For Higher-tier candidates, leaf B7.10 asks you to link a specific environmental change — in temperature, in water availability, or in the composition of atmospheric gases — to exactly these changes in where organisms can live.)

CasePutting biodiversity back: conservation and its trade-offs

Because biodiversity underpins stable ecosystems — and, ultimately, the clean water, pollination and crops humans rely on — governments and scientists run programmes to protect it, and the specification lists the main tools. Breeding programmes for endangered species, often in zoos, keep a population alive and boost its numbers for release. Rare habitats such as heathland, mangroves and coral reefs are legally protected and actively regenerated. On farmland, reintroducing field margins and hedgerows — strips left wild around the edges of crop fields — gives insects, birds and small mammals somewhere to live in an otherwise bare landscape. Governments set targets to reduce deforestation and the carbon emissions driving warming, and encouraging households to recycle rather than send waste to landfill cuts both pollution and the land lost to dumping.

None of this is free, and the honest exam answer weighs the conflict. Conservation costs money and can clash directly with human needs: protecting a forest may deny local people the farmland or timber income they depend on; a field margin left wild is a strip that grows no saleable crop; a breeding programme is expensive and no substitute for the wild habitat a species really needs. The strongest evaluation names a real trade-off — economic development and food production on one side, long-term biodiversity and ecosystem stability on the other — and then reaches a supported judgement rather than simply asserting that protecting nature is good.

CaseFeeding ten billion: food security, farming and biotech

Food security means having enough safe food to feed a population, and a whole cluster of separate-science leaves examines why it is under threat and what biology offers in response. The pressures the spec lists: a rising birth rate and growing population; changing diets in wealthier countries, which pulls scarce food resources across the globe; new pests and pathogens attacking crops and livestock; environmental change such as drought and flooding making land unfarmable; the rising cost of farming inputs; and conflicts that disrupt supplies of food and water.

One response is to farm animals more efficiently — and here the biomass logic from earlier pays off directly. Because animals lose most of their biomass to respiration and movement, limiting those losses leaves more energy for growth: restricting an animal's movement and keeping it in a warm, controlled indoor environment means less energy is 'wasted' on moving and on generating body heat, so more of its feed becomes meat, and high-protein feed pushes growth further. It works, but it is squarely contested on ethical grounds — animal welfare, crowding, disease spread and heavy antibiotic use — and a good answer states both the efficiency case and the objection. In the oceans, sustainable fisheries tackle overfishing that has crashed stocks like North Atlantic cod, using fishing quotas that cap the catch and controls on net mesh size so that young fish slip through and survive to breed, keeping the breeding population large enough to renew the stock.

Finally, biotechnology engineers food and medicine on an industrial scale. The fungus Fusarium is grown aerobically in giant fermenters on glucose syrup, then harvested and purified into mycoprotein — the high-protein, low-fat basis of Quorn, suitable for a vegetarian diet. Genetically modified crops are engineered for higher yields or new traits: some carry a bacterial gene making them resistant to insect pests, and 'golden rice' is engineered to make beta-carotene, a source of vitamin A intended to prevent the childhood blindness caused by vitamin A deficiency. The same genetic-engineering toolkit lets bacteria be reprogrammed to churn out human insulin for people with diabetes. GM is powerful and controversial — the exam rewards you for holding both the promise and the public concern in view.

VocabularyKey terms the mark scheme pays for

TrapsMisconceptions that cost marks

ExamWhat examiners want