QUESTION 132 (5 marks)
A mound-building invertebrate creates moist refuges used by several other species. It contributes only 0.2% of community biomass. In a replicated removal experiment, the table gives mean species richness after one year. Initial richness was similar. A leaf-feeding species contributes 18% of biomass.
| Treatment | Mean species richness |
|---|---|
| No removal | 20 |
| Mound builder removed | 14 |
| Leaf feeder removed | 19 |
a) Identify the stronger keystone-species candidate and justify with two features of the data. [3 marks]
b) Explain a mechanism by which its removal could reduce species richness. [2 marks]
Practice marking scheme
Answer
The mound builder is the stronger keystone candidate: its removal is associated with a decline from 20 to 14 species despite low biomass. Losing moist refuges could restrict other species.
Working
The control mean is 20. Removing the mound builder reduces mean richness by six species; removing the much more abundant leaf feeder reduces it by one. The larger ecological effect relative to biomass supports a keystone role. Removal of refuges can change microhabitat conditions and exclude organisms requiring moisture; a keystone need not be a top predator or the most abundant species.
Marking criteria
- Selects the mound-building invertebrate. [1 mark]
- Uses the larger richness decline, 20 to 14 versus 19. [1 mark]
- Links the effect to its small 0.2% biomass share. [1 mark]
- Identifies loss of moist refuges or altered microhabitat conditions. [1 mark]
- Links those changes to reduced survival/occupation by dependent species. [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.