Dispersal has two functions in species diversification: one is in determining the range of a species and the other is in bringing about gene flow within and among populations (Levin and Kerster 1974). The seed-dispersal ability of a plant species significantly influences the regional distribution of plants (Howe and Smallwood 1982
Nathan and Muller-Landau 2000). Because there are geographical, ecological and/or physical limitations in seed dispersal and establishment, most plant species are not distributed globally. However, some species have the capability for long-distance seed dispersal that depends on agents such as wind, sea currents or bird migration and consequently such species are distributed across extremely wide ranges (Cain et al. 2000
Takayama et al. 2006).