The process of change in the species structure of an ecological community over time, after a period of disturbance, is called ecological succession. It has two types: the primary and the secondary succession.
In primary succession, the environmental condition is considered to be harsh, with little or no soil present. It usually takes place after some form of disturbance, like a wildfire or a lava flow that destroy a wide vast of forest and its ecosystems.
Grass species are usually the first organisms to start the process of primary succession because they can thrive in harsh condition, like the lack of soil and water.