A student is investigating the relationship between soil moisture (an abiotic factor) and the abundance of a plant species in a grassland ecosystem. She uses random sampling with 10 frame quadrats. For each quadrat, she measures soil moisture (arbitrary units) and records the number of individual plants of the species. She then ranks the data for both variables. The ranks are shown in the table below.
| Site | Soil moisture rank | Plant abundance rank |
|---|---|---|
| 1 | 1 | 3 |
| 2 | 2 | 1 |
| 3 | 3 | 2 |
| 4 | 4 | 4 |
| 5 | 5 | 6 |
| 6 | 6 | 5 |
| 7 | 7 | 8 |
| 8 | 8 | 7 |
| 9 | 9 | 10 |
| 10 | 10 | 9 |
The formula for Spearman's rank correlation coefficient is:
where is the difference between the ranks for each pair, and is the number of pairs.
Outline how the student should proceed to determine whether there is a significant correlation between soil moisture and plant abundance. In your answer, include:
• the null hypothesis; • how to calculate the correlation coefficient; • how to use a critical value table; • the interpretation of the result.
Also outline:
• why random sampling is important in this investigation; • how the biodiversity of the grassland could be assessed at different levels.