Many species attempt to colonize newly available space but only certain species are able to do so. These pioneer species modify the environment and make it suitable for the species in the next seral stage, which further modify the environment to make it suitable for the next stage, and so on. Each seral stage facilitates the next seral stage until the climax community which no longer modifies the environment. Tolerance model All species that can tolerate conditions on the site are present from the start. Succession is largely the process of tolerant species finding the site, but early arrivers modify the site so that some early arrivers can no longer tolerate the conditions.
Succession proceeds until only species that can tolerate the conditions are present.
Any species can colonize the early stages. However, early arrivers inhibit the colonization of later arrivals. Later arrivals can only get in if one of the earlier arrivals dies. Succession proceeds until all the plants present are long-lived species that resist physical and biological damage. The tolerance model seems to apply to few, if any, situations.