Primary succession is when there is literally nothing of the ecosystem left. Everything is wiped clean. There's barely any life sustaining soil. The area has to essentially start from scratch. Typically though this occurs when a brand new piece of land is created. The only example I can think this would occur is on a newly formed volcanic island. There's no soil, plant, or animals on a new volcanic island and has to develop from the very beginning
Secondary succession is where a previous ecosystem gets destroyed. The basic plants and nutrition filled soil is there but larger plants and animals still aren't. I guess this would occur most likely in a fire.
Hope this helped!
