How does species diversity change as succession occurs?
In short, changes to soil content increases nutrients available which in turn spawns the growth of new foliage which increases diversity.
"Initially, there is a very low number of species-just the pioneer species that are able to start inhabiting the area (like lichens and mosses). Thus, there is very little species diversity. As the soil becomes richer in nutrients, more plant life is able to inhabit the area. This obviously directly increases diversity, but also increases diversity further in that a higher number of plants in the area draws more animals. As new and larger species, like trees, inhabit the area, new ecological niches are created that further promote an increase in species diversity." -source