Describe a method that could be used to determine the mean percentage cover of sea lavender (Limonium vulgare) in a salt marsh. Do not include information on the difficulties of working in a salt marsh.

Explain how succession results in a wide variety of animal species living in a salt marsh. Do not describe the process of succession in your answer.

Ecologists investigated the effect of two invertebrate herbivore species, the saltmarsh snail (Melampus bidentatus) and the meadow grasshopper (Chorthippus paralellus), on sea lavender growth in a salt marsh. They cleared experimental plots of vegetation in a salt marsh in East Anglia at an elevation of 1.2–1.6 metres above mean high water spring tides. They secured four fine mesh cages, W, X, Y and Z, over the soil surface in each plot and populated the cages as follows:

• W – No herbivores
• X – Four saltmarsh snails
• Y – Four meadow grasshoppers
• Z – Two saltmarsh snails and two meadow grasshoppers

After 18 weeks, the ecologists measured the mean percentage cover of sea lavender within each set of cages. The ecologists used a statistical test to find out whether the mean for each set of cages was significantly lower than the mean for set W.

Table 3 shows the probability ($P$) values that the ecologists obtained using this statistical test.

Table 3

Using all the information, evaluate the effect of the two herbivore species on sea lavender growth on salt marshes.