Edexcel International GCSE Biology 4BI1: Ecology and the environment – Revision Notes

These notes condense Topic 4, Ecology and the environment (points 4.1–4.18), of the Pearson Edexcel International GCSE Biology (4BI1) specification, Issue 3, for the June and November series examined under Issue 3. The course is untiered; points 4.3B, 4.4B, 4.11B and 4.18B are bold B statements and are examined on Paper 2 only. For full explanations and worked examples, read the study guide first.

Then test yourself with the practice questions. The rest of the course is on the Edexcel IGCSE Biology hub, and the printable checklist lists every specification point. The free diagnostics show which topics need most work.

4(a) The organism in the environment

Definitions (4.1, 4.3B)

Term	Definition
Population	All the organisms of one species in an area at the same time
Community	All the populations of different species in an area
Habitat	The place where an organism lives
Ecosystem	A community and its abiotic environment, interacting
Biodiversity (Paper 2 only)	The variety of living organisms (number of species and how evenly they are represented) in an area

Method in steps: population estimate with quadrats (4.2)

1. Lay two tapes at right angles to form a grid over the area.
2. Use random numbers as coordinates; place the quadrat at each.
3. Count the species in each quadrat; use a fixed rule for edge plants.
4. Use at least ten quadrats; find the mean per quadrat.
5. Mean per m² = mean per quadrat ÷ quadrat area (m²).
6. Population = mean per m² × total area (m²).
7. To compare two areas, keep quadrat size, number and counting rule the same.

Worked reminder: a mean of 2.5 plants per 1 m² quadrat in a 400 m² field gives 2.5 × 400 = 1000 plants.

Distribution and biodiversity (4.4B) – Paper 2 only

• Distribution: place quadrats at regular intervals along a tape running across a change in conditions (for example, shade to open ground). Record species present or percentage cover at each point.
• Biodiversity: place quadrats at random in each area; record every species and how many of each. More species, more evenly spread = higher biodiversity.

Abiotic vs biotic (4.5)

Abiotic (non-living)	Biotic (living)
Temperature, light intensity, water, pH, soil minerals, oxygen in water, wind	Food supply, predation, competition, disease, parasites

Always link factor → process → effect on numbers. For example: less light → less photosynthesis → fewer plants survive.

Method in steps: explaining a change in population size

1. Identify the factor that changed (abiotic or biotic).
2. Name the process it affects (photosynthesis, feeding, reproduction, death rate).
3. State the effect on birth rate or death rate.
4. Conclude whether the population rises or falls, and suggest what limits it next.

For example: fewer predators → fewer prey eaten → death rate falls → prey population rises → food supply eventually becomes limiting and competition increases.

4(b) Feeding relationships

Trophic levels (4.6)

Producer → primary consumer → secondary consumer → tertiary consumer. Decomposers (bacteria, fungi) feed on dead material and waste at every level.

Food chain arrows point from the food to the feeder; they show the direction of energy flow.

Pyramids (4.7)

Pyramid	Measures	Always a true pyramid?
Numbers	Number of organisms per level	No (one tree, many insects)
Biomass	Dry mass per level	Almost always
Energy transfer	Energy passed to each level (for example, kJ per m² per year)	Yes

Energy transfer (4.8, 4.9)

Efficiency (%) = (energy passed to next level ÷ energy in the level) × 100

About 10% passes on. Energy is lost through:

• respiration, released as heat
• parts not eaten (roots, bones, fur)
• undigested food lost in faeces
• excretion (urea)
• movement and keeping warm (birds and mammals).

Worked reminder: primary consumers receive 5000 kJ and pass 450 kJ to secondary consumers. Efficiency = (450 ÷ 5000) × 100 = 9.0%. The other 4550 kJ was lost, mostly as heat from respiration.

Why food chains are short: after four or five transfers of about 10%, too little energy is left to support another population.

Substances (carbon, nitrogen compounds) are recycled. Energy is not; it leaves the chain as heat.

4(c) Cycles within ecosystems

Carbon cycle in steps (4.10)

1. Photosynthesis takes CO₂ from the air into plant carbohydrates.
2. Feeding passes carbon compounds along the food chain.
3. Respiration by plants, animals and microorganisms returns CO₂.
4. Decomposition: decomposers feed on dead matter and waste and release CO₂ by respiration.
5. Combustion of fossil fuels and wood releases CO₂.
6. Incomplete decomposition over millions of years forms fossil fuels.

Nitrogen cycle (4.11B) – Paper 2 only

Bacteria	Converts	Conditions
Nitrogen-fixing (soil, legume root nodules)	Nitrogen gas → nitrogen compounds	–
Decomposers	Proteins, urea, faeces → ammonium	–
Nitrifying	Ammonium → nitrite → nitrate	Need oxygen (aerated soil)
Denitrifying	Nitrate → nitrogen gas	Waterlogged, low oxygen

Plants absorb nitrate ions and use them to make proteins.

4(d) Human influences on the environment

Air pollutants (4.12)

Pollutant	Source	Biological consequence
Sulfur dioxide	Burning sulfur-containing fossil fuels	Acid rain: damages leaves and kills trees; acidifies lakes so fish die; irritates lungs
Carbon monoxide	Incomplete combustion (car exhausts)	Binds to haemoglobin, so less oxygen carried; can be fatal

Greenhouse gases (4.13, 4.14)

Water vapour, carbon dioxide, nitrous oxide, methane, CFCs.

• CO₂: burning fossil fuels, deforestation
• Methane: cattle, rice paddies, landfill
• Nitrous oxide: breakdown of nitrogen fertilisers by soil bacteria
• CFCs: old aerosols and refrigerators

Enhanced greenhouse effect in steps (4.15)

1. Short-wave radiation from the Sun warms the Earth’s surface.
2. The surface emits longer-wave infrared radiation.
3. Greenhouse gases absorb and re-radiate some of it back towards the Earth.
4. More gas → more re-radiated → enhanced greenhouse effect.
5. This may lead to global warming: melting ice and rising sea levels, flooding, changed rainfall, extreme weather, habitat loss, species moving or dying out, lower yields in some areas.

Sewage vs eutrophication (4.16, 4.17)

	Sewage	Eutrophication
Cause	Organic waste enters water	Nitrates and phosphates leached from fertiliser
First effect	Bacteria feed on the sewage and multiply	Algal bloom; plants below die (no light)
Oxygen fall	Bacteria respire aerobically	Decomposing bacteria respire aerobically
Result	Fish and aerobic invertebrates die	Fish and aerobic invertebrates die

In both cases the bacteria, not the algae, cause the oxygen to fall.

Deforestation (4.18B) – Paper 2 only

• Leaching: mineral ions washed out of unprotected soil.
• Soil erosion: no roots to hold soil; rivers silt up, flooding.
• Evapotranspiration reduced: less water vapour, less rain, drier climate locally.
• Carbon cycle: less CO₂ absorbed; burning or decay releases more.
• Atmospheric gases: more CO₂, less O₂ released.

Must-know distinctions

• Population (one species) vs community (all species).
• Habitat (a place) vs ecosystem (living + non-living, interacting).
• Nitrifying (makes nitrate, needs oxygen) vs denitrifying (removes nitrate, low oxygen).
• Pyramid of numbers (can be any shape) vs pyramid of energy (always a pyramid).
• Greenhouse effect (natural, keeps Earth warm) vs enhanced greenhouse effect (extra warming from human gases).

Quick self-test

1. Define ecosystem.
2. A producer level holds 15 000 kJ. Using 10% transfer, how much energy reaches the secondary consumers?
3. Give two reasons energy is lost between trophic levels.
4. Why are quadrats placed at random when estimating population size?
5. Mean = 2.5 per 1 m² quadrat; field area 400 m². Estimate the population.
6. Name the process that returns carbon to the air from fossil fuels.
7. Which bacteria convert ammonium to nitrate? (Paper 2 only)
8. Why do denitrifying bacteria thrive in waterlogged soil? (Paper 2 only)
9. How does carbon monoxide reduce oxygen transport?
10. Name the five greenhouse gases in the specification.
11. In eutrophication, what directly uses up the dissolved oxygen?
12. Give one effect of deforestation on soil. (Paper 2 only)

Answers

1. A community of organisms and its abiotic environment, interacting.
2. 15 000 → 1500 kJ (primary) → 150 kJ (secondary).
3. Any two: respiration (heat); parts not eaten; faeces; excretion; movement.
4. To avoid bias, so the sample represents the whole area.
5. 2.5 × 400 = 1000.
6. Combustion.
7. Nitrifying bacteria.
8. Little oxygen is present; they convert nitrate to nitrogen gas in these conditions.
9. It binds to haemoglobin, so less oxygen can be carried by red blood cells.
10. Water vapour, carbon dioxide, nitrous oxide, methane, CFCs.
11. Aerobic respiration by decomposing bacteria.
12. Leaching of mineral ions, or soil erosion.

Where marks are usually lost

• Defining a community without saying “different species”.
• Drawing food-chain arrows from the eater to the food.
• Stating “10% is transferred” without explaining where the other 90% goes.
• Writing “energy is recycled” or “energy is used up”.
• Scaling a quadrat mean by total area without first converting to per m².
• Omitting random placement or the number of quadrats in a method.
• Saying algae use up the oxygen in eutrophication.
• Mixing up nitrifying and denitrifying bacteria, or naming bacteria species (not required).
• Linking carbon monoxide to acid rain, or sulfur dioxide to haemoglobin.
• Listing greenhouse gases without saying how human activity adds each one.

Official syllabus

Pearson Edexcel International GCSE in Biology (4BI1) specification, Issue 3, first examination June 2019, Pearson Education Limited – Topic 4, Ecology and the environment.