Practise O/L exam questions on Ecosystems and Biodiversity with worked solutions
2020 · Paper II · 6 marks
The table below shows the population of organisms in a freshwater pond over three years.
Year 1: Algae 10 000 / Small fish 500 / Large fish 50
Year 3: Algae 5 000 / Small fish 400 / Large fish 30
(i) Suggest ONE reason for the decline in algae population.
(ii) Explain the relationship between small fish and large fish populations shown in the data.
(iii) Define biodiversity and state ONE reason why it is important.
Show worked solution
(i) Reduced light penetration (eutrophication / pollution reducing photosynthesis); or increased grazing by small fish (accept any valid environmental cause).
(ii) As large fish decrease (fewer predators), small fish numbers could initially increase — but the data shows both decline together, suggesting a common stressor (e.g. habitat loss / pollution) or time lag in predator-prey dynamics.
(iii) Biodiversity = the variety of living organisms in an ecosystem. It is important because it maintains ecosystem stability and provides food, medicines, and ecosystem services.