Ecosystems, communities & interdependence

An ecosystem is the interaction of a community of living (biotic) organisms with the non-living (abiotic) parts of their environment. AQA wants you to know the levels of organisation, from smallest to largest:

Each species depends on others for food, shelter, pollination and seed dispersal — this is interdependence. Remove one species and the whole community can be affected. A stable community is one where the species and environmental factors are in balance, so population sizes stay roughly constant.

Watch out: "community" means only the living things. Add the non-living factors (light, temperature, water, soil) and you have the full ecosystem.

Which level is it?

Competition in plants & animals

To survive and reproduce, organisms need resources from their surroundings. When resources are limited, organisms compete — and plants and animals compete for different things:

• Plants compete for: light, space, water and mineral ions from the soil.
• Animals compete for: food, mates and territory.

When you're asked "what are these organisms competing for?", pick the resources appropriate to whether they are plants or animals, then use any clues in the question (e.g. a shaded forest floor → competing for light).

Exam skill: AQA phrases this as "suggest the factors for which these organisms are competing." Give the resource and a reason tied to the habitat.

Competing for what?

Abiotic vs biotic factors

Abiotic factors are the non-living conditions that affect a community:

• light intensity · temperature · moisture levels
• soil pH and mineral content · wind intensity and direction
• carbon dioxide levels (for plants) · oxygen levels (for aquatic animals)

Biotic factors are the living influences on a community:

• availability of food · new predators arriving · new pathogens
• one species outcompeting another, so numbers fall too low to breed

Memory hook: abiotic = absence of life (physical/chemical). Biotic = "bio" = life. AQA asks you to explain how a change in one factor affects a given community, so always link the change to a population going up or down.

Abiotic or biotic?

Tap a factor, then tap the box it belongs in.

What are plants in a wood fighting over?

Adaptations: structural, behavioural, functional

Adaptations are features that let an organism survive where it normally lives. AQA groups them into three types:

• Structural — physical body features. e.g. a polar bear's thick fur and small ears reduce heat loss.
• Behavioural — the way an organism acts. e.g. birds migrating in winter; meerkats basking in the morning sun.
• Functional — internal processes (biochemistry / physiology). e.g. a camel producing very concentrated urine to conserve water; hibernation slowing metabolism.

Some organisms live in extreme environments — high temperature, high pressure or high salt concentration. These are extremophiles. AQA's named example: bacteria living in deep-sea vents.

Watch out: don't muddle the types. "Long legs" is structural; "hunts at night to avoid the heat" is behavioural; "makes concentrated urine" is functional.

Which type of adaptation?

Why do predator and prey numbers cycle?

Producers & food chains

Photosynthetic organisms — green plants and algae — are the producers of biomass for life on Earth. Every food chain begins with a producer that makes glucose by photosynthesis.

Producers are eaten by primary consumers (herbivores), which may be eaten by secondary consumers, then tertiary consumers. A predator kills and eats other animals; the animal eaten is its prey.

Misconception: the arrows in a food chain show the direction that energy and biomass flow — from the food to the feeder. They do not point "towards what gets eaten".

Read the food chain

Predator–prey cycles

In a stable community the numbers of predators and prey rise and fall in cycles. AQA expects you to interpret graphs of these cycles.

The logic: lots of prey → predators have plenty of food → predator numbers rise → they eat more prey → prey numbers fall → predators run short of food → predator numbers fall → prey recover… and the cycle repeats.

Misconception: the peaks are offset, not on top of each other. The prey peak comes first, and the predator peak lags behind it — because predators only increase after their food supply has already grown.

Read the cycle graph

Measuring distribution: quadrats & transects

Ecologists use quadrats and transects to find the distribution and abundance of species. This is Required Practical 9: measure the population size of a common species, and investigate how a factor affects its distribution.

• Quadrat — a square frame. Place it randomly (e.g. using coordinates from a random-number generator) to estimate abundance fairly. Count how many of a species fall inside, then find the mean per quadrat and scale up to the whole area.
• Transect — a line (tape) across the habitat. Place quadrats at regular intervals along it to see how a species changes across a gradient (e.g. from a shaded hedge into open field).

You need the maths terms mean, mode and median, and to be able to calculate arithmetic means and draw suitable graphs.

Why random? Random quadrat placement avoids bias so your sample fairly represents the whole area. A transect is the exception — there you place quadrats systematically along the line, on purpose, to study a gradient.

Your turn — mean per quadrat

Why do predator and prey numbers cycle?

Where does the carbon in a rotting leaf go?

The carbon cycle

All materials in the living world are recycled. The carbon cycle returns carbon from organisms to the atmosphere as CO₂, ready for plants to use in photosynthesis again.

Key processes: photosynthesis (plants take in CO₂), respiration (plants, animals and microbes release CO₂), combustion (burning fuels/wood releases CO₂), and decomposition (microbes break down dead matter and release CO₂).

Role of microbes: decomposers return carbon to the atmosphere as CO₂ and mineral ions to the soil. Note AQA says you are not expected to study the nitrogen cycle.

Which process?

Where does the carbon in a rotting leaf go?

The water cycle

The water cycle provides fresh water for plants and animals on land before it drains back into the seas. Water is continuously evaporated and precipitated.

Energy from the Sun evaporates water; it cools and condenses into clouds; it falls as precipitation (rain, snow); it flows over and through the land, supplying fresh water, and drains back to the sea. This links to GCSE Chemistry (the Earth's early atmosphere).

Watch out: the water cycle is driven by the Sun's energy — it is the physical evaporate/precipitate loop that keeps supplying fresh water.

Water cycle process

Decomposition & rate of decay

Decomposers (microorganisms) break down dead material. The rate of decay depends on three conditions — AQA wants you to explain each:

• Temperature — warmer speeds up decay (enzymes work faster), but too hot denatures the enzymes and decay slows/stops.
• Water — moisture is needed for the reactions and for microbes to grow; dry conditions slow decay.
• Oxygen — most decomposers respire aerobically, so more oxygen speeds decay. Without oxygen (anaerobic), decay produces methane — used in biogas generators as a fuel.

Gardeners and farmers give optimum conditions for rapid decay in a compost heap; the compost is a natural fertiliser.

Misconception: decomposers recycle nutrients (carbon → CO₂, minerals → soil). They do not recycle energy — energy is not cycled, it is lost as heat and ultimately passes out of the ecosystem.

Your turn — rate of decay

Impact of environmental change

Environmental changes affect the distribution of species in an ecosystem. AQA lists three kinds of change you must be able to evaluate:

• Temperature — e.g. warming shifts where a species can live.
• Availability of water — drought can push species out of an area.
• Composition of atmospheric gases — e.g. changing CO₂ or O₂ levels.

These changes may be seasonal (birds migrating), geographic (moving to a new region), or caused by human interaction (pollution, warming). You should evaluate the impact when given data.

Higher tier & Biology-only: this whole sub-topic (4.7.2.4) is separate Biology only and Higher tier only — you won't be assessed on it in Combined Science or on Foundation-tier Biology papers.

Environmental change

Which wood survives the disease better?

What is biodiversity?

Biodiversity is the variety of all the different species of organisms on Earth, or within an ecosystem.

High biodiversity makes ecosystems stable: it reduces the dependence of one species on another for food, shelter and maintaining the physical environment. The future of the human species relies on us maintaining a good level of biodiversity — yet many human activities are reducing it.

Link it up: waste/pollution, deforestation and global warming (coming next) all reduce biodiversity — you should be able to explain how.

Why does biodiversity matter?

Waste & pollution: air, water, land

A growing human population and rising living standards mean more resources used and more waste produced. Unless waste is handled properly, pollution results — and pollution kills plants and animals, reducing biodiversity. AQA groups pollution by where it happens:

• Water — from sewage, fertiliser and toxic chemicals.
• Air — from smoke and acidic gases.
• Land — from landfill and toxic chemicals.

Exam link: the acidic gases here connect to GCSE Chemistry (atmospheric pollutants from fuels). Always finish with the consequence: pollution → fewer species → lower biodiversity.

Which kind of pollution?

Which wood survives the disease better?

Land use, peat bogs & deforestation

Humans reduce the land available for other species by building, quarrying, farming and dumping waste.

Peat bogs: destroying peat to make garden compost reduces this habitat and the variety of species living there (lower biodiversity). Worse, decay or burning of the peat releases CO₂. There is a conflict between cheap compost for food production and conserving peatlands.

Deforestation in tropical areas happens to:

• provide land for cattle and rice fields;
• grow crops for biofuels.

Consequences to state: deforestation reduces biodiversity, reduces the CO₂ removed by photosynthesis, and releases CO₂ when trees are burned — all feeding into global warming.

Peat & carbon

Global warming & maintaining biodiversity

Levels of carbon dioxide and methane in the atmosphere are increasing and contribute to global warming. Biological consequences you should be able to describe include: loss of habitat (e.g. rising seas / melting ice), changes in species distribution and migration patterns, and reduced biodiversity.

To reduce our negative effects, scientists and citizens run programmes to maintain biodiversity:

• breeding programmes for endangered species;
• protection and regeneration of rare habitats;
• reintroducing field margins and hedgerows on farmland that grows a single crop;
• governments reducing deforestation and CO₂ emissions;
• recycling resources rather than dumping waste in landfill.

Nature of science: the scientific consensus on global warming rests on systematic reviews of thousands of peer-reviewed publications — that's why it is treated as reliable even though any single study can be uncertain.

Boosting biodiversity

Why do food chains stop after a few links?

Trophic levels

Trophic levels number an organism's position in a food chain, starting at Level 1:

• Level 1 — Producers: plants and algae, which make their own food.
• Level 2 — Primary consumers: herbivores that eat plants/algae.
• Level 3 — Secondary consumers: carnivores that eat herbivores.
• Level 4 — Tertiary consumers: carnivores that eat other carnivores. An apex predator is a carnivore with no predators.

Decomposers break down dead plant and animal matter by secreting enzymes into the environment; the small soluble molecules produced then diffuse back into the microorganism.

Note: this whole 4.7.4 section (trophic levels, pyramids, biomass transfer) is separate Biology only (triple) — not tested in Combined Science.

Match the organism to its trophic level

Tap an organism on the left, then tap its correct trophic level on the right.

Pyramids of biomass

A pyramid of biomass shows the relative amount of biomass (mass of living material) at each trophic level. Level 1 (the producer) is always at the bottom, and each bar is drawn to scale.

You should be able to construct an accurate pyramid from data — draw each bar's width in proportion to the biomass, centred, with the producer at the base.

Misconception: a biomass pyramid is (almost) always the right way up — biomass decreases at each higher level because so little is passed on. It is not the same as a pyramid of numbers.

Read the pyramid

Transfer of biomass (the 10% rule)

Producers transfer only about 1% of the light energy that reaches them into biomass through photosynthesis. After that, only about 10% of the biomass at each trophic level is transferred to the level above.

Biomass is lost between levels because:

• not all material is eaten, and not all eaten material is absorbed — some is egested as faeces;
• some absorbed material is lost as waste — CO₂ and water from respiration, and water and urea in urine;
• large amounts of glucose are used in respiration.

Why so few top predators? Because only ~10% passes up each level, there is far less biomass at the top — so an ecosystem can only support a small number of organisms at high trophic levels.

Your turn — biomass transfer

Your turn — efficiency of transfer

Why do food chains stop after a few links?

How do we keep a growing world fed?

Food security, farming & the sea

Food security is having enough food to feed a population. Biological factors that threaten it include: a rising birth rate; changing diets moving scarce food around the world; new pests and pathogens; environmental changes (e.g. failed rains → famine); the cost of agricultural inputs; and conflicts affecting water or food.

Efficient food production (factory farming): transfer of energy from farm animals to the environment can be restricted by limiting their movement and controlling the temperature of their surroundings; some animals are fed high-protein food to increase growth. This raises ethical objections for some people — you should be able to evaluate the pros and cons.

Farming the sea (sustainable fisheries): fish stocks are declining, so they must be kept at a level where breeding continues. Fishing quotas and controls on net size (letting young fish escape) help stocks recover.

Why restrict movement/keep warm? Animals then waste less biomass on movement and on keeping warm by respiration, so more biomass goes into growth — making food production more efficient.

Efficient food production

Biotechnology solutions

Modern biotechnology cultures large quantities of microorganisms for food, and modifies organisms to meet the needs of a growing population:

• Mycoprotein — the fungus Fusarium is grown on glucose syrup in aerobic conditions; the biomass is harvested and purified into a protein-rich food suitable for vegetarians.
• Insulin from bacteria — a genetically modified bacterium produces human insulin, which is harvested and purified to treat people with diabetes.
• GM crops — genetically modified crops can give more food or better nutrition, e.g. golden rice.

Exam link: the GM techniques here connect to genetic engineering in Topic 4.6. Remember Fusarium needs aerobic conditions and a glucose syrup food source.

Biotechnology

How do we keep a growing world fed?

The whole of 4.7 Ecology

You've covered the full AQA 4.7 specification. Press Finish to see your score.