Acacia trees release droplets of sugar-rich liquid from their stems. Ants take this liquid and are often observed chasing leaf-eating insects away from the stems.
Researchers selected 50 similar acacia branches and divided them equally between five treatments. Each branch was enclosed in a transparent, ventilated sleeve. The researchers placed the same number of leaf-eating insects in every sleeve but varied the number of ants.
After six days, the researchers measured the mean percentage of leaf area damaged in each treatment.
| Number of ants in each sleeve | Mean leaf area damaged in % |
|---|---|
| 0 | 48 |
| 4 | 31 |
| 8 | 19 |
| 12 | 14 |
| 16 | 13 |
Describe the relationship shown by the results.
Use the results to calculate the percentage decrease in mean leaf area damaged between the treatment with no ants and the treatment with 12 ants.
Explain why the interaction observed on the acacia stems can be classified as mutualism.
During a drought, the acacia trees released less sugar-rich liquid. Explain how this could alter the percentage of leaf area damaged by insects.
Suggest why the final two treatments produced almost the same result.
82 exam-style questions on Edexcel GCSE Biology 9.1 Ecosystems and interdependence, covering 9.1.1 Levels of organisation in ecosystems, 9.1.2 Abiotic and biotic factors, 9.1.3 Importance of interdependence, 9.1.4 Parasitism and mutualism, and 9.1.5 Determining number of organisms in an area. Each one has a worked solution and a mark scheme showing where the marks go.