IGCSE Biology: ecosystems, energy flow and human impact

Key ideas board by board

Cambridge 0610 Core: the Sun as the principal energy source, food chains and webs, producers, consumers, herbivores, carnivores, decomposers, trophic levels, pyramids of numbers and biomass, the carbon cycle, population, community and ecosystem, factors affecting population growth and the phases of the sigmoid curve. Topic 20 adds food production, habitat destruction, deforestation, sewage and fertiliser pollution, plastics, methane and carbon dioxide with the enhanced greenhouse effect, and conservation of forests, fish and endangered species. Supplement adds pyramids of energy, why energy transfer is inefficient, why eating crops is more efficient than eating livestock, the nitrogen cycle, explaining the sigmoid curve, the full eutrophication sequence and conservation methods in more detail.

Edexcel 4BI1: population, community, habitat and ecosystem; the quadrat practical (4.2); abiotic and biotic factors; trophic levels including decomposers; pyramids of number, biomass and energy transfer; the 10% figure; the carbon cycle; sulfur dioxide and carbon monoxide pollution; greenhouse gases (water vapour, carbon dioxide, nitrous oxide, methane, CFCs) and the enhanced greenhouse effect; sewage and eutrophication. Biodiversity and its practical (4.3B, 4.4B), the nitrogen cycle (4.11B) and deforestation (4.18B) are Paper 2 only.

Worked example 1: energy transfer efficiency

Grass contains 20 000 kJ per m^2 per year. Rabbits feeding on it gain 2 000 kJ, and foxes gain 180 kJ from the rabbits. Efficiency from grass to rabbits = 2 000 / 20 000 x 100 = 10%. From rabbits to foxes = 180 / 2 000 x 100 = 9%.

Explain where the rest went: energy is transferred to the environment as heat from respiration, used in movement, lost in faeces and urine, and in parts of organisms not eaten (bones, roots). This is why there is less energy at each level, why pyramids of energy are always pyramid shaped, and why food chains usually have fewer than five trophic levels.

Applied to food (Cambridge Supplement 19.2.19): eating crop plants is more energy efficient than eating livestock fed on crops, because one fewer trophic level means less energy lost, so the same land feeds more people.

Worked example 2: estimating a population with quadrats

A student places ten 0.25 m^2 quadrats at random in a 50 m by 40 m field and counts 32 daisies in total. Mean per quadrat = 32 ÷ 10 = 3.2. Density = 3.2 ÷ 0.25 = 12.8 daisies per m^2. Field area = 50 x 40 = 2 000 m^2. Estimated population = 12.8 x 2 000 = 25 600 daisies.

Method points examiners reward: place quadrats at random using coordinates from a random number generator, so results are not biased; use enough quadrats to get a representative sample; decide a rule for plants partly inside the quadrat. To compare two areas (Edexcel 4.2), use the same number and size of quadrats in each.

Eutrophication in order (Cambridge Supplement, Edexcel 4.17)

The oxygen is used by the bacteria, not by the algae. This is the step most answers miss.

Common mistakes

• Saying energy is "lost" with no detail. Say how: respiration releasing heat, movement, egestion, parts not eaten.
• Drawing food chain arrows the wrong way. Arrows show the direction of energy flow: grass -> rabbit -> fox.
• Saying energy is recycled. Nutrients (carbon, nitrogen) are recycled; energy flows through and leaves as heat.
• Saying fish die in eutrophication because algae use the oxygen or because the fertiliser is poisonous.
• Confusing the greenhouse effect with the ozone layer.
• Calling the decomposers in the nitrogen cycle "nitrogen-fixing bacteria". Decomposers produce ammonium; nitrifying bacteria make nitrate; nitrogen-fixing bacteria turn nitrogen gas into nitrogen compounds; denitrifying bacteria return nitrogen gas to the air.

Exam technique

Ecology questions mix calculation, data and extended writing. For food web questions such as "what happens to the number of X if Y is removed", give a direction and a reason, and consider more than one effect if the web allows it: "more grass would be eaten because there are more rabbits, but foxes may also eat more voles instead". For pyramids, draw to scale when asked and label the trophic levels.

"Discuss" questions on monocultures, intensive farming or conservation need advantages and disadvantages. "Explain" questions on the greenhouse effect need the chain: more carbon dioxide and methane, more infrared (heat) radiation absorbed and re-emitted back to Earth, rising average temperatures, climate change, with a biological consequence such as habitat loss or changes in species distribution.

How one-to-one lessons help

Ecology looks easy, which is why students under-prepare it, then lose marks on wording and calculations. A tutor works through efficiency and quadrat calculations until they are routine, rehearses the eutrophication and nitrogen cycle sequences, and uses past data questions to practise describing trends with numbers. For Gulf students, the tutor can also use local examples such as mangroves or overfishing to practise applying ideas to unfamiliar contexts.

Self-check

1. Define population, community, habitat and ecosystem.
2. Calculate energy transfer efficiency and explain where the rest of the energy goes.
3. Draw pyramids of numbers, biomass and energy, and explain why they differ.
4. Describe the carbon cycle with all its processes.
5. Estimate a population from quadrat data.
6. Explain eutrophication in six steps.
7. Explain the enhanced greenhouse effect and one consequence.
8. (Cambridge Supplement, Edexcel Paper 2) Describe the nitrogen cycle and the roles of bacteria.