QUESTION 124 (5 marks)
Seedlings of one species are grown in a controlled experiment with two maintained soil-moisture levels and two light treatments. Each treatment starts with 30 seedlings and is repeated six times. Other conditions are held similar. The table gives mean survivors after four weeks.
| Soil moisture | Full light | Shade |
|---|---|---|
| Low | 3 | 15 |
| High | 24 | 25 |
a) Describe how the effect of shade differs between the two moisture levels. Use numerical evidence. [2 marks]
b) Give a biological explanation consistent with the greater benefit of shade under low moisture. [2 marks]
c) Explain why the repeated treatments are useful. [1 mark]
Practice marking scheme
Answer
Low-moisture survival increases from 3 to 15 with shade; high-moisture survival changes from 24 to 25. Shade has a much larger survival association under low moisture.
Working
At low moisture, shade adds 12 survivors on average; at high moisture it adds only one. The species is strongly limited by water in full light. Shade can reduce evaporative water stress even when maintained soil-moisture levels differ. The treatment effect should be compared within each moisture level rather than assumed constant across them.
Marking criteria
- Identifies increases of 12 at low moisture and 1 at high moisture. [1 mark]
- States that shade has the larger effect under low moisture. [1 mark]
- Identifies water stress or greater water loss in full light. [1 mark]
- Links shade to reduced water stress and increased survival. [1 mark]
- Explains that replication reduces reliance on a single unusual trial or reveals variation. [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.