AQA GCSE Biology 8461: Ecology – Revision Notes

These revision notes cover 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 examined on Paper 2 at both Foundation and Higher tier, every May/June. Only section 4.7.2.4 is Higher tier only, and it is marked below.

For full explanations and worked examples, read the Ecology study guide first. Test yourself with the Ecology practice questions. The course hub is AQA GCSE Biology and the printable checklist lets you tick off each statement. For links across the whole course, see Key ideas revision notes.

Key definitions (4.7.1)

Term	Meaning
Ecosystem	Interaction of a community of living organisms (biotic) with the non-living (abiotic) parts of their environment
Population	All the organisms of one species in a habitat
Community	All the populations of different species in a habitat
Interdependence	Each species depends on others for food, shelter, pollination, seed dispersal etc.; removing one can affect the whole community
Stable community	Species and environmental factors in balance, so population sizes stay fairly constant
Extremophile	Organism living in extreme conditions (high temperature, pressure or salt), e.g. bacteria in deep-sea vents
Biodiversity	Variety of all the different species on Earth or within an ecosystem
Food security	Having enough food to feed a population

What organisms compete for

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

Abiotic factors (7): 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 (4): availability of food; new predators arriving; new pathogens; one species outcompeting another so numbers are too low to breed.

Adaptations are structural (body features), behavioural (what the organism does) or functional (processes inside the body).

Method in steps – explaining the effect of a changed factor

1. Name the factor and say how it changed (use the data).
2. State the direct effect on one organism (survival, growth or breeding).
3. Follow the knock-on effect through the food web or competition.
4. Say what happens to population sizes, and quote figures if given.

Sampling – Required practical 9 (4.7.2.1)

Method in steps – estimating population size

1. Lay out a grid; use random numbers for coordinates.
2. Place a quadrat at each coordinate; count the species.
3. Repeat for many quadrats; calculate the mean per quadrat.
4. Divide by quadrat area to get the number per m².
5. Multiply by the total area.

Method in steps – investigating distribution

1. Run a transect (tape) across the change, e.g. shade to open ground.
2. Place a quadrat at regular intervals along it.
3. Record the species (count or % cover) and measure the abiotic factor at each point.
4. Repeat transects and plot the species against distance or the factor.

Averages. Mean = total ÷ number of values. Median = middle value when in order. Mode = most common value.

Worked reminder. Mean of 3.0 daisies in 0.25 m² quadrats → 3.0 ÷ 0.25 = 12 per m².

Predator–prey cycles

• Prey rise → more food → predators rise (with a lag).
• Predators rise → more prey eaten → prey fall.
• Prey fall → less food → predators fall. Repeat.

Cycling and decay (4.7.2.2–4.7.2.3)

Process	Effect on atmospheric CO₂
Photosynthesis (plants, algae)	Removes CO₂
Respiration (plants, animals, microorganisms)	Returns CO₂
Decay by microorganisms	Returns CO₂ to air; returns mineral ions to soil
Burning (fuels, peat, wood)	Returns CO₂

Water cycle: evaporation and precipitation; provides fresh water on land before draining to the sea. The nitrogen cycle is not required.

Rate of decay increases with: warmth (up to the optimum for enzymes), water, and oxygen. Compost = natural fertiliser. Anaerobic decay → methane → biogas fuel.

Required practical 10: effect of temperature on the rate of decay of fresh milk, measured by pH change.

Rate formula: rate = change ÷ time (e.g. pH units per minute, g per week).

Worked reminder. Milk falls from pH 6.7 to 5.5 in 40 minutes: 1.2 ÷ 40 = 0.03 pH units per minute. The pH falls because the microorganisms decaying the milk produce acid.

Environmental change (4.7.2.4) – Higher tier only

Factors: temperature, availability of water, composition of atmospheric gases. Causes: seasonal, geographic, human interaction. Evaluate: describe the shift, link it to the change, state limitations of the data.

Human impact (4.7.3)

Pressure	Key points
Water pollution	Sewage, fertiliser, toxic chemicals
Air pollution	Smoke, acidic gases
Land pollution	Landfill, toxic chemicals
Land use	Building, quarrying, farming, dumping waste
Peat bogs	Destroyed for garden compost; habitat and biodiversity lost; decay or burning releases CO₂
Deforestation	For cattle, rice fields and biofuel crops
Global warming	Rising CO₂ and methane contribute

Maintaining biodiversity: breeding programmes; protecting and regenerating rare habitats; field margins and hedgerows; less deforestation and CO₂ emission; recycling instead of landfill.

Trophic levels and biomass (4.7.4)

Level	Name	Example role
1	Producer	Plants and algae
2	Primary consumer	Herbivore
3	Secondary consumer	Carnivore eating herbivores
4	Tertiary consumer	Carnivore eating carnivores

• Apex predator: carnivore with no predators.
• Decomposers secrete enzymes outside; small soluble molecules diffuse in.
• About 1 % of incident light energy is transferred by producers; about 10 % of biomass passes to the next level.
• Losses: egestion (faeces); waste (CO₂ and water from respiration; water and urea in urine); glucose used in respiration.

Method in steps – efficiency of biomass transfer

1. Efficiency = biomass in higher level ÷ biomass in lower level × 100.
2. Or give it as a fraction of mass.
3. Round sensibly and include %.

Worked reminder. 300 g → 27 g: 27 ÷ 300 × 100 = 9.0 %.

Food production (4.7.5)

• Threats to food security: rising birth rate; changing diets moving scarce food around the world; new pests and pathogens; environmental change (rains failing); cost of agricultural inputs; conflict affecting water or food.
• Intensive farming: limit movement and control temperature so less energy is transferred to the surroundings; high-protein feed. Some people object on ethical grounds.
• Fisheries: net size control and quotas keep stocks at breeding level.
• Biotechnology: Fusarium → mycoprotein (grown on glucose syrup, aerobic, harvested and purified); GM bacterium → human insulin; GM crops such as golden rice.

Must-know distinctions

• Abundance (how many; random quadrats) vs distribution (where; transect).
• Abiotic (non-living) vs biotic (living) factors.
• Aerobic decay (CO₂) vs anaerobic decay (methane).
• Egested (never absorbed; faeces) vs excreted waste (urea in urine; CO₂ and water from respiration).
• Biomass pyramid vs food chain: the pyramid shows mass at each level, drawn to scale.
• Quota (limits how much is caught) vs net size (limits which fish are caught).

Quick self-test

1. Four 1 m² quadrats contain 3, 5, 0 and 4 plants. Find the mean. Estimate the number in a 200 m² field.
2. A level holds 2400 g of biomass; the next holds 180 g. Calculate the efficiency.
3. About how much biomass passes from 3000 g of producers to primary consumers?
4. Leaf litter falls from 50 g to 32 g in 6 weeks. Calculate the rate of decay.
5. Name the trophic level of a herbivore.
6. Give two things plants compete for.
7. Give two abiotic factors for aquatic animals and plants.
8. Name the gas made by anaerobic decay.
9. What does Fusarium grow on to make mycoprotein?
10. Why do larger net mesh sizes help conserve fish stocks?
11. (Higher tier only) Name the three environmental changes in 4.7.2.4.
12. Give two reasons peat bog destruction is harmful.

Answers

1. Mean = 12 ÷ 4 = 3.0 per m²; 3.0 × 200 = 600 plants.
2. 180 ÷ 2400 × 100 = 7.5 %.
3. About 10 % → about 300 g.
4. (50 − 32) ÷ 6 = 3 g per week.
5. Level 2 (primary consumer).
6. Any two of: light, space, water, mineral ions.
7. Oxygen level (aquatic animals); carbon dioxide level (plants). Temperature and light intensity also accepted.
8. Methane.
9. Glucose syrup, in aerobic conditions.
10. Young, small fish escape, survive and breed, so the stock is maintained.
11. Temperature; availability of water; composition of atmospheric gases.
12. Habitat and biodiversity are lost; decay or burning of peat releases carbon dioxide.

Where marks are usually lost

• Stopping at the mean per quadrat and not scaling up to the whole area.
• Using quadrats in chosen “good” places – say random coordinates for abundance.
• Drawing a pyramid of biomass without scaling the bars, or with level 1 at the top.
• Saying biomass is “used up” – name the losses: faeces, urea, respiration.
• Explaining decay with “bacteria like it warm” – say enzymes work faster, and denature if too hot.
• Mixing up interdependence (species rely on each other) with competition (species want the same resource).
• Writing about the nitrogen cycle, which is not required.
• In “evaluate” questions, giving only benefits – add a cost or conflict and a conclusion.
• Calling mycoprotein a bacterial product – it comes from the fungus Fusarium.

Official syllabus

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