Biology HL
Ecosystem Stability and Sustainable Harvest
- 1.
A sealed transparent mesocosm contains algae, decomposers, small grazers, water and mineral nutrients. Explain why it still needs an energy input, how matter can be recycled inside it, and why sealing the vessel does not guarantee long-term stability.
[4 marks] · no calculator - 2.
A hypothetical fish stock gains 900 tonnes through recruitment and growth in a year and loses 300 tonnes through natural mortality. Harvest is 750 tonnes. Ignore migration. Calculate the net biomass change and evaluate a proposal to call this harvest sustainable merely because the stock was large at the start.
[4 marks] - 3.
After fertiliser runoff enters a hypothetical lake, algal biomass rises and later dissolved oxygen falls as much of the algae dies. Explain the causal sequence and predict how reducing nutrient inputs could affect recurrence, without claiming immediate recovery is guaranteed.
[4 marks] · no calculator - 4.
A hypothetical fish stock begins a year at 2400 tonnes. Recruitment and growth add 500 tonnes, natural mortality removes 200 tonnes and harvest removes 450 tonnes. Ignore migration. Calculate the final biomass and the largest harvest that would leave biomass unchanged under these fixed one-year assumptions. Explain one reason this is not a guaranteed safe future quota.
[4 marks] - 5.
After nutrient runoff is reduced, a hypothetical lake continues to show low oxygen for several weeks. Suggest two mechanisms consistent with delayed recovery and explain why this observation alone does not show that reducing nutrient input was ineffective.
[3 marks] · no calculator