What is missing from the ecosystem during primary succession

competition

Explanation:

After an ecosystem has been destroyed because of certain catastrophe and all of the space that it occupied is totally cleared and open for overtaking, the primary succession comes in action. When the primary succession occurs, there's no competition, as pretty much everything has been destroyed from the previously existing ecosystem. The pioneers of the primary succession are organisms that do not require a lot to prosper, such as the mosses and lichens. As they start to cover the barren land, little by little they manage to change the composition of the rocks, thus resulting in the formation of the first soils. As that happens, the secondary succession comes on the horizon as now it has conditions for it, so the species from it overtake and replace the species form the primary succession.