AQA GCSE Biology 8461: Ecology – Study Guide

This study guide teaches Topic 7, Ecology (sections 4.7.1 to 4.7.5), of the AQA GCSE Biology (8461) specification, for first teaching 2016 with exams from 2018 (version 1.0). Ecology is assessed on Paper 2, alongside Homeostasis and response and Inheritance, variation and evolution. Both tiers sit this topic; the one Higher tier only subsection, 4.7.2.4, is labelled below. The exams run every May/June for the life of the specification.

Use it with the Ecology revision notes and the Ecology practice questions. The course hub is AQA GCSE Biology and the printable checklist lists every statement. The Key ideas guide shows how this topic links to the rest of the course.

What this topic covers

Spec	What you must be able to do	Tier
4.7.1.1–4.7.1.4	Describe levels of organisation, interdependence and competition; explain effects of abiotic and biotic factors; explain adaptations; extremophiles	Both
4.7.2.1	Food chains, sampling with quadrats and transects (Required practical 9), mean/median/mode, predator–prey cycles	Both
4.7.2.2	Carbon and water cycles; role of microorganisms	Both
4.7.2.3	Factors affecting decay; compost; biogas (Required practical 10)	Both
4.7.2.4	Evaluate how environmental change affects species distribution	Higher tier only
4.7.3.1–4.7.3.6	Biodiversity, pollution, land use, peat, deforestation, global warming, maintaining biodiversity	Both
4.7.4.1–4.7.4.3	Trophic levels, pyramids of biomass, efficiency of biomass transfer	Both
4.7.5.1–4.7.5.4	Food security, farming techniques, sustainable fisheries, biotechnology	Both

The nitrogen cycle is not required.

Communities, factors and adaptations (4.7.1)

An ecosystem is the interaction of a community of living organisms (biotic) with the non-living (abiotic) parts of their environment. The levels of organisation run from an individual organism, to a population (one species), to a community (all the populations), to the ecosystem.

Competition. Plants compete for light, space, water and mineral ions from the soil. Animals compete for food, mates and territory. In an exam you may be given a habitat and asked to suggest what is being competed for: pick resources that are actually limited in that setting.

Interdependence. Each species depends on others for food, shelter, pollination, seed dispersal and so on. Removing one species can affect the whole community. A stable community is one where all the species and environmental factors are in balance, so population sizes stay fairly constant.

Abiotic factors (listed in the spec): 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: availability of food; new predators arriving; new pathogens; one species outcompeting another so its numbers are no longer sufficient to breed.

When asked to explain the effect of a change, give a chain: change → what it does to one organism → knock-on effect on others. For example, a new pathogen kills many rabbits → less food for foxes → fox numbers fall; grass is grazed less → grass cover rises.

Adaptations let organisms survive in the conditions where they normally live. They can be:

• structural (body features, such as thick fur or small leaves)
• behavioural (what the organism does, such as migrating)
• functional (processes inside the body, such as producing very concentrated urine).

Extremophiles live in extreme environments: high temperature, high pressure or high salt concentration. Bacteria living in deep-sea vents are extremophiles.

Food chains, sampling and predator–prey cycles (4.7.2.1)

Photosynthetic organisms are the producers of biomass for life on Earth. Every food chain starts with a producer that synthesises molecules, usually a green plant or alga making glucose by photosynthesis. Producers are eaten by primary consumers, then secondary and tertiary consumers. Consumers that kill and eat other animals are predators; those eaten are prey.

In a stable community, predator and prey numbers rise and fall in cycles. The predator peak comes after the prey peak: more prey means more food, so more predators survive and breed; more predators then eat more prey, so prey numbers fall; with less food, predator numbers fall, and the cycle repeats.

Required practical 9: sampling

You measure the population size of a common species and use sampling to investigate the effect of a factor on its distribution.

• Quadrats placed at random (using coordinates from a random number generator on a grid) estimate abundance. Count the organisms in each quadrat, find the mean, scale up to the whole area.
• A transect is a line (a tape) across a habitat where a factor changes, such as light from a hedge into a field. Place quadrats at regular intervals along it and record the species and an abiotic factor at each point. This shows distribution.

You must understand mean, median and mode, calculate arithmetic means, and plot suitable graphs with sensible scales.

Worked example 1. A student places ten 0.25 m² quadrats at random on a lawn 40 m × 25 m and counts daisies: 4, 0, 2, 7, 2, 5, 1, 2, 6, 1.

Total = 30, so mean = 30 ÷ 10 = 3.0 daisies per quadrat
In order: 0, 1, 1, 2, 2, 2, 4, 5, 6, 7
Median = middle of 5th and 6th values = (2 + 2) ÷ 2 = 2
Mode = 2 (appears three times)
Daisies per m² = 3.0 ÷ 0.25 = 12
Lawn area = 40 × 25 = 1000 m²
Population estimate = 12 × 1000 = 12 000 daisies

Note the mean (3.0) is higher than the median (2) because a few quadrats had high counts.

Cycles and decomposition (4.7.2.2–4.7.2.3)

Many materials cycle through the abiotic and biotic parts of an ecosystem. All materials are recycled to provide the building blocks for future organisms.

Carbon cycle.

• Plants and algae take in carbon dioxide from the air for photosynthesis.
• Carbon passes along food chains as carbon compounds when animals eat.
• Respiration by plants, animals and decomposing microorganisms returns carbon dioxide to the atmosphere.
• Carbon cycle diagrams usually also show combustion (burning fuels, wood or peat) releasing carbon dioxide.

Water cycle. Water evaporates (and plants lose water by transpiration), condenses, and falls as precipitation. This provides fresh water for plants and animals on land before it drains into the seas.

Microorganisms (decomposers) break down dead material and waste. They return carbon to the air as carbon dioxide and return mineral ions to the soil.

Decay is faster when it is warm (enzymes work faster, up to an optimum), moist (microorganisms need water) and there is plenty of oxygen (for aerobic respiration). Too hot, and enzymes denature. Gardeners and farmers provide these conditions so waste decays quickly into compost, a natural fertiliser. Anaerobic decay produces methane; biogas generators use this to make a fuel.

Required practical 10: decay of milk

You investigate the effect of temperature on the rate of decay of fresh milk by measuring pH change. As bacteria break down the milk, acid is produced, so the pH falls.

Worked example 2. At 30 °C, milk falls from pH 6.7 to pH 5.5 in 40 minutes.

Change in pH = 6.7 − 5.5 = 1.2
Rate = 1.2 ÷ 40 = 0.03 pH units per minute

The same idea works with mass: compost that falls from 800 g to 560 g in 12 weeks decays at (800 − 560) ÷ 12 = 20 g per week.

Environmental change (4.7.2.4) – Higher tier only

Changes in temperature, availability of water and composition of atmospheric gases affect where species are found. Changes may be seasonal, geographic or caused by human interaction. When you evaluate data, say what moved, link it to the named change, and note limits of the evidence (one site, one year, other factors changing).

Biodiversity and human impact (4.7.3)

Biodiversity is the variety of all the different species of organisms on Earth, or within an ecosystem. High biodiversity makes ecosystems stable by reducing the dependence of one species on another for food, shelter and maintenance of the physical environment.

A growing human population and higher standard of living use more resources and make more waste.

• Pollution in water (sewage, fertiliser, toxic chemicals), in air (smoke, acidic gases) and on land (landfill, toxic chemicals) kills plants and animals, reducing biodiversity.
• Land use: building, quarrying, farming and dumping waste take land from other species.
• Peat bogs are destroyed to make garden compost. This removes a habitat and its biodiversity, and the decay or burning of peat releases carbon dioxide. There is a conflict between cheap compost for food production and conserving peatland.
• Deforestation in tropical areas provides land for cattle and rice fields and for crops grown for biofuels.
• Global warming: rising carbon dioxide and methane levels contribute to it. Biological consequences you can describe include species moving to new ranges, loss of habitat and changes in distribution.

Maintaining biodiversity – programmes include:

• breeding programmes for endangered species
• protecting and regenerating rare habitats
• reintroducing field margins and hedgerows where farmers grow one crop
• some governments reducing deforestation and carbon dioxide emissions
• recycling resources rather than dumping waste in landfill.

Evaluation questions expect both sides: the benefit to biodiversity and the cost or conflict (money, land for food, jobs).

Trophic levels and biomass (4.7.4)

Level	Organisms	Name
1	Plants and algae	Producers
2	Herbivores	Primary consumers
3	Carnivores that eat herbivores	Secondary consumers
4	Carnivores that eat carnivores	Tertiary consumers

Apex predators are carnivores with no predators. Decomposers secrete enzymes into the environment; small soluble food molecules then diffuse into the microorganism.

A pyramid of biomass has trophic level 1 at the bottom, with bar widths drawn to scale. Producers transfer about 1 % of incident light energy for photosynthesis. Only about 10 % of the biomass at each level passes to the level above. Biomass is lost because:

• not all ingested material is absorbed – some is egested as faeces
• some absorbed material is lost as waste: carbon dioxide and water in respiration, water and urea in urine
• large amounts of glucose are used in respiration.

Worked example 3. Biomass: producers 5000 g/m², primary consumers 450 g/m², secondary consumers 40 g/m².

Efficiency (level 1 → 2) = 450 ÷ 5000 × 100 = 9.0 %
Efficiency (level 2 → 3) = 40 ÷ 450 × 100 = 8.9 % (2 s.f.)

Because so little biomass passes up, less biomass is available at each higher level, so fewer organisms can be supported there.

Food production (4.7.5)

Food security means having enough food to feed a population. Threats: rising birth rate in some countries; changing diets in developed countries moving scarce food around the world; new pests and pathogens; environmental changes such as rains failing; the cost of agricultural inputs; conflicts affecting water or food.

Farming techniques. Limiting animals’ movement and controlling the temperature of their surroundings reduce energy transfer to the environment, so more biomass goes into growth. Some animals are fed high-protein food. Some people have ethical objections to these intensive methods.

Sustainable fisheries. Fish stocks are declining. Control of net size (larger mesh lets young fish escape to breed) and fishing quotas help keep stocks at a level where breeding continues.

Biotechnology. The fungus Fusarium produces mycoprotein, a protein-rich food suitable for vegetarians; it is grown on glucose syrup in aerobic conditions and the biomass is harvested and purified. A genetically modified bacterium produces human insulin to treat diabetes. GM crops could give more food or food with better nutritional value, such as golden rice.

Common errors

• Writing “energy” when the spec says biomass for pyramids – read the question.
• Drawing the predator peak before the prey peak.
• Forgetting to scale up from quadrat area to m², then to the whole area.
• Saying decay is faster at any higher temperature – enzymes denature if too hot.
• Listing respiration as the only biomass loss; faeces and urine count too.

Official syllabus

AQA GCSE Biology (8461) specification, for first teaching 2016, exams from 2018, version 1.0, published by AQA – section 4.7 Ecology.