What causes an ecosystem to move from a primary successional stage towards a relatively-stable mature ecosystem?

The correct answer is "secondary succession, an ecological succession step where an area previously occupied by living beings is disrupted".

Explanation:

In ecological succession, an ecosystem changes and develops through a series of steps. The first step, is the primary successional stage where rocks or other sedimentary structures are colonized by microorganisms. The second step, where the ecosystems changes towards a relatively-stable mature stage, occurs when an area previously occupied by living beings is disrupted. This step is named secondary succession, and the most clear example of this phenomena occurs when wildfire clears oak and hickory forests.