Primary succession begins with the input of seeds, spores, animals and organic matter or by vegetative expansion from adjacent habitats. Distance alone plays a major role in determining what reaches a new site, so the landscape context of a new surface is crucial. Most species fail to disperse beyond some short distance and the total propagule density is low, so chance plays a large role in governing which species reach isolated surfaces. Thus the initial vegetation of isolated sites can be highly variable. Early species composition is governed by a complex suite of interacting forces, often leading to a chaotic mosaic of early species association (see sections 5.3.4 on predictability and 5.4.1 on stability).
In this chapter, we explore colonization and establishment phenomena and their consequences for primary succession. We focus first on pre-dispersal effects such as pollination and seed set, then on dispersal mechanisms. Finally, we explore the factors that affect establishment and the repeatability of the first species assemblages. Most primary seres are colonized by propagules dispersed by wind, water or animals. We ask, ‘How do different degrees and types of isolation and substrate types affect colonization?’ We then examine the factors that permit establishment and the consequences of differential longevity.
Primary succession lets us evaluate community assembly without the confounding influences of residual vegetation. We can search for patterns among sites of similar origin and between sites created by different processes. In this way, critical factors can be highlighted.