A drought-tolerant grass is present in some parts of the eastern area of a valley.
Researchers divided the eastern area into 20 equal sections. They recorded whether the grass was present in each section at the end of four consecutive years. The same sections were surveyed each year.
| Survey year | Mean annual rainfall / mm | Sections where grass was present |
|---|---|---|
| 1 | 760 | 4 |
| 2 | 700 | 7 |
| 3 | 630 | 11 |
| 4 | 570 | 15 |
The researchers also selected 36 equal plots where the grass was absent. Each plot was next to a section containing the grass. The plots were randomly allocated to three treatments, with 12 plots in each treatment. All conditions were kept the same except the proportion of natural rainfall reaching the soil.
| Natural rainfall reaching soil / % | Plots where grass became established after two years |
|---|---|
| 100 | 3 out of 12 |
| 70 | 7 out of 12 |
| 40 | 10 out of 12 |
A scientist claimed that decreasing rainfall caused the grass to become more widespread within the eastern area.
Evaluate the scientist's claim. Use calculations and evidence from both tables.
In year 5, the mean annual rainfall is 500 mm and no other environmental factor changes.
Suggest the number of the 20 sections in which the grass may be present. Justify your answer.
83 exam-style questions on AQA GCSE Biology 7.2 Organisation of an ecosystem, covering 7.2.1 Levels of organisation, 7.2.2 How materials are cycled, 7.2.3 Decomposition, and 7.2.4 Impact of environmental change (HT only). Each one has a worked solution and a mark scheme showing where the marks go.