I think that it's Photosynthesis?
Answer:
There are two types of ecological succession:
Primary succession:
Primary succession occurs in areas which were previously devoid of life. There were no organisms living in this area before. For example, lands after new glaciers or volcano eruptions. Firstly, microorganisms begin to habitat this land, followed by plants like lichens, shrubs etc. Finally, complex life evolved from these.
Secondary succession:
Secondary succession arises in areas where life existed before but was destroyed due to natural circumstances like flood, fire etc. Small grasses inhabit this land first which are taken over by trees over period of time.
