Question 1
Paper 2Medium16 marksResearchers investigated factors affecting plant density in a meadow community. Plots received either ambient nutrient levels or additional nutrients. Herbivores either had access to the plots or were excluded. The table shows the mean density of established plants after 10 weeks.
| Nutrient level | Herbivores had access (plants m⁻²) | Herbivores excluded (plants m⁻²) |
|---|---|---|
| Ambient | 120 | 180 |
| Additional nutrients | 210 | 300 |
In a separate investigation, researchers tested juglone, released by the roots of black walnut trees (Juglans nigra), and penicillin, released by Penicillium mould.
| Source and substance tested | Response measured | Control (no substance) | Substance present |
|---|---|---|---|
| Black walnut; juglone | Germination of competing plant seeds after 5 days (%) | 80 | 40 |
| Penicillium; penicillin | Bacterial growth index (control set to 100) | 100 | 25 |
For the plant population, state which type of control is tested by (i) adding nutrients and (ii) excluding herbivores.
Calculate the mean increase in plant density associated with (i) nutrient addition and (ii) herbivore exclusion. Show your working.
Which type of control appears stronger in these plots? Justify your answer using your results from part (b).
Calculate the percentage decrease in the response for each substance, relative to its control.
Identify the process shown by each example in the second table. State one similarity and one difference between the processes.