Explanation:

The lichens are the pioneer species and invade the bare rocks. They secrete acids which dissolve the rocks, help in their weathering and formation of soil. 
Mosses grow in with the lichens to form a mat over the substratum. Mosses decompose rocks making the substrate suitable for the growth of higher plants. Since mosses form dense mats on the soil, they reduce the impact of falling rain and prevent soil erosion.