When a glacier retreats it leaves behind bare rock and gravel with no soil. Over many years this becomes colonised by living organisms. The table shows measurements taken at increasing distances from the edge of a retreating glacier. The distance from the glacier represents the time since the ground was exposed.
| Distance from glacier / m | Nitrogen in soil / g m⁻² | Number of plant species |
|---|---|---|
| 100 | 1 | 4 |
| 400 | 9 | 14 |
| 700 | 18 | 24 |
| 1000 | 25 | 34 |
Using the table, describe how the number of plant species changes with distance from the glacier.
[2]
The first plants to grow on the bare ground are pioneer species that can fix nitrogen. Explain how these pioneer species allow a succession to occur.
[3]
Calculate the mean rate of increase in the number of plant species between 100 m and 1000 m from the glacier. The ground at 100 m was exposed 30 years ago and the ground at 1000 m was exposed 300 years ago. Give your answer in species per year.
[2]