Seed dispersal has long been an object of fascination to biologists and the general public alike. Examples abound of structures that have clearly evolved to promote dispersal by wind or on the outside or inside of animals, but it is only recently that attention has turned to the question of just how well these structures work and what happens to the seeds of all those species (the majority) with no obvious adaptations for dispersal. Few things in seed ecology have changed more in recent years than our understanding of seed dispersal.

Wind dispersal

Any structure that increases air resistance of the dispersule is likely to improve dispersal by wind. Some morphological adaptations impart lateral movement directly, but the great majority merely slow the rate of fall, relying on wind to provide the lateral motion (Augspurger, 1988). Wind dispersal has probably received more attention than all other dispersal modes, since it can be investigated (even if not totally satisfactorily) in the laboratory and is relatively amenable to mathematical models of varying complexity (Sharpe & Fields, 1982; Green, 1983; Matlack, 1987; Greene & Johnson, 1989, 1990, 1993, 1996; Hanson et al., 1990; Andersen, 1991).

These models are essentially of two sorts: (1) analytical models that describe seed densities directly (e.g. Greene & Johnson, 1989) and (2) individual-based models that simulate the movement of individual seeds (e.g. Andersen, 1991). Seed shadows are then produced by summing simulations for large numbers of seeds. See Jongejans & Schippers (1999) for a relatively simple individual-based model.