When there is an extreme disturbance in the ecosystem, it means a general destruction of it, thus experiencing a mass extinction. Once this happens, there's usually just barren ground left. Since the barren ground doesn't provide suitable living conditions for most of the living organisms, the ones that need the lest resources to survive create the primary succession, usually the mosses and lichens. The organisms of the primary succession provide biomass, so the ground is mixing with it, thus new soil is forming. Once this happens, smaller plants and animals start to settle in, thus the secondary succession occurs. The animals and plants of the secondary succession enrich the soil, and set the basis for more complex ecosystem, so soon after larger plants and animals start to settle in.
