Primary succession is the development of a living, organic community on a surface that does not have organic life, such as a rock or a glacier. The basic structure of these steps culminates in the creation of a soil that can then sustain a community of organic life from then on. Because the development of soil is necessary for all subsequent organic life to thrive, it is called "primary" or "first" succession, as it deals with the foundations of organic life.
Pioneer species refer to those species that can adhere to a lifeless, non-organic surface, and, from that, serve as the foundation of all subsequent life. Hence, the first step is the establishment of these species, most often lichens or mosses, on the surface.
Slowly, the cycles of birth, death and decay in the pioneer species begin to create a rudimentary soil. Animal wastes deposited on the surfaces can also contribute to that soil. Furthermore, the pioneer species can "catch" organic debris such as fallen leaves. This is the real "primary" step in primary succession, because the soil then serves as the basis for other forms of plant life beyond that the pioneer species.
Annuals and Perennials
This soil, having absorbed the nutrients of decaying pioneer species, can now support the development of annual plants. The existence of annuals, in turn, provides for a richer and deeper soil as dead stems, leaves and flowers decay over many years. Many perennials can grow where annuals do not. Perennials such as vetch often can grow in uninviting rocky areas where roots can extend into cracks in the rock.
Smaller plants are gradually joined by small shrubs such as alder. Spruce has a tendency to follow. Eventually, less hardy woods may compete with those rugged pioneer shrubs, depending on location and elevation. The steps that lead to softwood and hardwood forests normally occur in temperate climates, but not in the austere polar climates.
The final step is the growth of many perennial plants or, in many climates, forests. Hence, the decay of pioneer species and annual plants are necessary to provide just the right mix of nutrients (all other things being equal, such as moisture), necessary for the complex ecosystem of a forest to form. Succession is a very slow process, and can take centuries or longer to go from barren rock to forests.