Tolerance Model - Models of Succession
In this model, the presence of early successional species is not essential, that is, any species can start succession. Some species are competitively superior and they eventually predominate in the climax community. Species that are more tolerant of limited resources, replace the other species.
Figure: Tolerance Model
Succession proceeds either by the invasion of later species or by a thinning out of the initial colonists, depending on the starting conditions. The crux of the whole thing is that there is a competitive hierarchy in which later species can outcompete earlier species and can also invade in their absence.