QUESTION 121 (5 marks)
A 5000 m² reserve contains dry habitat covering 70% of its area and wet habitat covering 30%. Equal numbers of random quadrats are placed in the two strata. Their mean plant densities are 2 plants m⁻² in dry habitat and 12 plants m⁻² in wet habitat. Treat these as representative density estimates.
a) Estimate the total plant abundance. Show your working. [3 marks]
b) Explain why multiplying the unweighted mean of the two densities by the total reserve area would overestimate abundance. [2 marks]
Practice marking scheme
Answer
Estimated abundance is 25 000 plants. Using the unweighted mean density of 7 plants m⁻² would give 35 000 and overrepresent the smaller wet stratum.
Working
Dry area is 3500 m² and wet area is 1500 m². Estimated counts are 3500×2 = 7000 and 1500×12 = 18 000; total 25 000. The reserve-wide density is 5 plants m⁻². Equal sample allocation does not make the habitat areas equal.
Marking criteria
- Calculates areas 3500 and 1500 m². [1 mark]
- Calculates stratum estimates 7000 and 18 000. [1 mark]
- Adds to obtain 25 000 plants. [1 mark]
- States that the high-density wet habitat is only 30% of the reserve. [1 mark]
- Explains why equal weighting overrepresents wet habitat; accepts comparison with 35 000. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.