Factors influencing succession: Lessons from large, infrequent natural disturbances

Disturbance events vary in intensity size, and frequency, but few opportuni ties exist to study those that are extreme on more than one of these gradie nts. This article characterizes successional processes that occur following infrequent disturbance events that are exceptional in their great intensit y or large size. The spatial variability in disturbance intensity within la rge, infrequent disturbances (LIDs) often leads to a heterogeneous pattern of surviving organisms. These surviving organisms dictate much of the initi al successional pattern on large disturbances where the opportunities for s eeds to disperse into the middle of the disturbance are limited. The tradit ional distinction between primary and secondary succession is insufficient to capture the tremendous variability in succession following LIDs. Disturb ance size influences succession where long-distance colonization by propagu les is important. Observations from LIDs suggest the following interrelated hypotheses about trends in succession with increasing distance from seed s ources when disturbance intensity is high: (a) initial densities of organis ms will be lower; (b) nucleation processes, in which recovering patches ser ve as foci for additional colonization and expand spatially, will be more i mportant; (c) competitive sorting will be less important relative to chance arrival in determination of community composition, and (d) community compo sition will be initially less predictable; and (e) the rate of recovery of community composition will be slower. Prediction of succession following LI Ds without considering contingencies such as the abundance, types, and spat ial distribution of residuals, and distance to seed sources is likely to be unsuccessful for large portions of the landscape. Abundance and spatial ar rangement of survivors and arrival patterns of propagules may be the pivota l factors determining how succession differs between intense disturbances o f large and small extent.