Clements (1916, 1928) proposed that six basic processes drive succession: nudation (denudation), migration (dispersal), ecesis (establishment), competition, reaction (site modification by organisms) and stabilization (development of a stable endpoint). With some modifications (see Chapter 3), these concepts still provide a useful framework to study succession (Glenn-Lewin et al., 1992). Denudation is the process of disturbance that creates a barren substrate and is the only process largely independent of the biota. If the site is not completely new (e.g. a recent lava flow, a newly emerged island or a recently formed delta), denudation removes existing plants and animals before succession begins. The degree of removal of the biota determines whether primary or secondary succession ensues on these latter sites (see Chapter 1). In this chapter we examine the often dramatic disturbances that create the initial conditions for primary succession.
Physical environment and disturbance
Our understanding and acceptance of change in the natural world has changed slowly. Early humans often attributed natural disturbances to supernatural forces, and they recorded them as catastrophic, inexplicable or unusual events (Scarth, 1999). Nineteenth century geologists established that landscape changes (e.g. uplift and erosion of landforms) were usually gradual (Lyell, 1850; Davis, 1899), leading to a dynamic view of both landforms and the origin and extinction of organisms as formulated in 1859 in Darwin's Origin of Species (Leakey, 1979). Studies in the Swiss Alps, initiated in 1842 and continuing to the present, quantified the movements of glaciers (Aellen, 1981).