Ecology

IGCSE Edexcel Biology
4.1–4.17 Ecosystems, food chains, energy flow, and the carbon cycle
Key Concepts: Ecosystems consist of communities of organisms interacting with their environment. Energy is transferred through food chains and webs, but some is lost at each stage. The carbon cycle describes how carbon moves between organisms and the environment.

Section A — Ecosystems and Populations

1. Define the following terms. [6]

a) Population

b) Community

c) Ecosystem

2. State two biotic and two abiotic factors that affect the size of a population. [4]

Section B — Food Chains and Food Webs

3. Define the terms producer, primary consumer, and secondary consumer. Give an example of each from a named food chain. [6]
4. Consider the food chain: Grass → Rabbit → Fox → Flea [4]

a) Identify the producer and the apex predator.

b) If the rabbit population decreases, predict what might happen to the fox population and the grass. Explain your answer.

5. State two advantages of using a food web compared to a single food chain to describe feeding relationships in an ecosystem. [2]

Section C — Energy and Ecological Pyramids

6. Explain why energy is lost between each trophic level in a food chain. State at least three reasons. [3]
7. Approximately what percentage of energy is transferred from one trophic level to the next? [1]
8. Explain why pyramids of biomass are usually pyramid-shaped and rarely inverted. [2]
9. Compare a pyramid of numbers with a pyramid of biomass. State one situation where a pyramid of numbers might be inverted. [3]

Section D — The Carbon Cycle

10. Describe how carbon enters living organisms from the atmosphere. [2]
11. State three processes that return carbon dioxide to the atmosphere. [3]
12. Explain the role of decomposers in the carbon cycle. [2]
13. Explain how the burning of fossil fuels affects the carbon cycle and global temperatures. [3]

Total marks: 41

Mark Scheme

1. a) All individuals of the same species living in the same area at the same time [2]; b) All the populations of different species living in the same area [2]; c) A community of organisms plus their non-living (abiotic) environment [2] [6]
2. Biotic (any two): predation, disease, competition for food/mates, parasitism; Abiotic (any two): temperature, light, water availability, soil pH, salinity [4]
3. Producer: photosynthetic organism that makes its own food; e.g. grass; Primary consumer: organism that eats a producer; e.g. rabbit; Secondary consumer: organism that eats a primary consumer; e.g. fox [6 — 2 per term]
4. a) Producer: grass; apex predator: flea [1]; b) Fox population decreases (less food) [1]; grass population increases (less being eaten) [1]; [allow other valid chains of reasoning] [4]
5. Any two: shows that many animals eat more than one type of food; more accurately represents feeding relationships; shows what happens if one species is removed [2]
6. Any three: energy lost in respiration (heat); energy lost in movement; energy not consumed (e.g. bones/fur); energy lost as waste (faeces/urine); energy used for reproduction [3]
7. About 10% [1]
8. Because energy is lost at each trophic level, the total biomass at each level decreases [1]; so there is less and less biomass at higher levels, producing a pyramid shape [1] [2]
9. Pyramid of numbers uses the count of organisms; pyramid of biomass uses total dry mass [1]; a pyramid of numbers can be inverted when one large organism (e.g. one oak tree) supports many smaller ones (e.g. caterpillars) [1]; pyramids of biomass are rarely inverted [1] [3]
10. Plants absorb CO₂ from the atmosphere during photosynthesis [1]; CO₂ is converted into organic molecules (glucose/carbohydrates) [1] [2]
11. Any three: respiration (by animals, plants, microorganisms); combustion/burning of fossil fuels or wood; decomposition/decay by microorganisms [3]
12. Decomposers (bacteria and fungi) break down dead organic matter [1]; they respire, releasing CO₂ back to the atmosphere, and return minerals to the soil [1] [2]
13. Burning fossil fuels releases CO₂ that was stored millions of years ago [1]; this increases the concentration of CO₂ in the atmosphere [1]; CO₂ is a greenhouse gas — it traps heat (infrared radiation) from the Earth, causing global temperatures to rise (enhanced greenhouse effect) [1] [3]