QUESTION 123 (6 marks)
The curves show cumulative plant-species counts from randomly located equal-sized quadrats in habitats X and Y. Within each habitat, no previously recorded species is removed from the cumulative total.
a) Compare the observed richness rankings at 6 and 18 quadrats, referring to the graph. [2 marks]
b) Explain why the six-quadrat comparison could give a misleading ranking of the habitats. [2 marks]
c) Explain what the flattening curves suggest about additional sampling, and state one limit of that inference. [2 marks]
Practice marking scheme
Answer
At 6 quadrats Y appears richer (20 versus 12). By 18 quadrats X is richer (30 versus 22). Late flattening supports more complete coverage, while six quadrats missed many X species.
Working
Early species accumulation can be slower where species are rare or clumped. At six quadrats the observed richness is not yet close to the later X plateau. Around 18–24 quadrats both curves flatten, with X near 30 and Y near 22. A plateau suggests diminishing discoveries under this protocol, without proving every species was found.
Marking criteria
- States Y > X at 6 quadrats, with values 20 and 12. [1 mark]
- States X > Y at 18 quadrats, with values 30 and 22. [1 mark]
- Identifies incomplete sampling in X at six quadrats. [1 mark]
- Links different discovery rates to rare or clumped species, or differing completeness. [1 mark]
- States that additional quadrats are yielding few new species. [1 mark]
- Notes that unobserved rare species may remain or that other methods/times may reveal more. [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.