Colonization, tolerance, competition and seed-size variation within functional groups

Ecologists interested in seed size have generally contrasted functional groups of plants but, recently, some have focussed on explaining the large range of seed size found within a functional group. A potentially important theoretical advance was the idea that seed number–seedling survival tradeoffs could explain the coexistence of species, in particular colonization–competition tradeoffs where smaller-seeded species are superior colonizers and larger-seeded species are superior competitors. However, recent models have placed limits on the potential of this approach, chiefly by showing that the asymmetry of competition must be strong. Also, although there is evidence from studies within functional groups that seed size does trade off against number of seeds and dispersal of those seeds, and that seed size is correlated with competitive ability among seedlings and tolerance of hazards during establishment, the available evidence suggests that SNSS tradeoffs do not make possible long-term coexistence without other forms of niche differentiation.