Recruitment trade-offs and the evolution of dispersal mechanisms in plants

In this study we place seed size vs. seed number trade-offs in the context of plant dispersal ability. The objective was to suggest explanations for t he evolution of different seed dispersal mechanisms, in particular fleshy f ruits, wind dispersal and the maintenance of unassisted dispersal. We sugge st that selection for improved dispersal may act either by increasing the i ntercept of a dispersal curve (log seed number vs. distance) or by flatteni ng the slope of the curve. 'Improved dispersal' is defined as a marginal in crease in the number of recruits sited at some (arbitrary) distance away fr om the parent plant. Increasing the intercept of the dispersal curve, i.e. producing more seeds, is associated with a reduction in seed size, which in turn affects the recruitment ability, provided that this ability is relate d to seed size. If recruitment is related to seed size there will be a recr uitment cost of evolving increased seed production. On the other hand, a fl attening of the slope by evolving dispersal attributes is likely to be asso ciated with a fecundity cost. An exception is wind dispersal where smaller (and hence more numerous) seeds may lead to more efficient dispersal. We de rive two main predictions: If recruitment is strongly related to seed size, selection for improved dispersal acts on the slope of the dispersal curve, i.e. by favouring evolution of dispersal attributes on seeds or fruits. If , on the other hand, recruitment is only weakly related to seed size (or no t related, or negatively related), selection for improved dispersal favours increased seed production. Despite its simplicity, the model suggests expl anations for (i) why so many plant species lack special seed dispersal attr ibutes, (ii) differences in dispersal spectra among plant communities, and (iii) adaptive radiation in seed size and dispersal attributes during angio sperm evolution.