A rocky shore is an ecosystem with diverse communities of seaweeds (macroalgae) and marine invertebrates. Some herbivorous organisms feed on the seaweeds. Succession on a rocky shore eventually results in a wide variety of animal species inhabiting the shore.
Describe a method that could be used to determine the mean percentage cover of seaweed on a rocky shore. Do not include information on the difficulties of working on a rocky shore.
Explain how succession results in a wide variety of animal species living on a rocky shore. Do not describe the process of succession in your answer.
Marine biologists investigated the effect of two grazer species, the common limpet (Patella vulgata) and the common periwinkle (Littorina littorea), on seaweed growth on a rocky shore. They cleared experimental plots of rock surface on a rocky shore in South West England at a tidal height of 1.5–2.0 metres above chart datum. They attached four large mesh cages, A, B, C and D, to the rock surface in each plot and populated the cages as follows:
After 26 weeks, the biologists measured the mean percentage cover of all seaweed within each set of cages. The biologists used a statistical test to find out whether the mean for each set of cages was significantly lower than the mean for set A.
Table 3 shows the probability (PPP) values that the biologists obtained using this statistical test.
| Set of cages | PPP value |
|---|---|
| B | =0.002= 0.002=0.002 |
| C | =0.684= 0.684=0.684 |
| D | =0.352= 0.352=0.352 |
Using all the information, evaluate the effect of the two grazer species on seaweed growth on rocky shores.
