QUESTION 131 (4 marks)
In controlled cultures without predators, a snail species survives over pH 5–9. In otherwise comparable cultures containing its predator, it occurs only at pH 6–7. The predator can survive throughout pH 5–9.
a) Identify the fundamental and realised pH ranges in these experiments. [2 marks]
b) Explain why the narrower occupied range does not, by itself, demonstrate a change in physiological pH tolerance. [2 marks]
Practice marking scheme
Answer
The tested fundamental pH range is 5–9; the realised range with the predator is 6–7. Predation can restrict realised distribution without changing the snail’s underlying pH tolerance.
Working
The predator-free experiment indicates that the snail can tolerate the broader pH range. With predation present, a biotic interaction reduces the occupied range. A restricted realised niche does not demonstrate that the species has lost its physiological ability to survive at pH 5, 8 or 9.
Marking criteria
- States fundamental tested range 5–9. [1 mark]
- States realised range 6–7 with predator. [1 mark]
- Identifies predation as a biotic restriction. [1 mark]
- Uses survival without the predator to distinguish occupation from physiological tolerance. [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.