Mechanisms of Dispersal

Seeds are dispersed in a great variety of ways. The morphological devices that enhance dispersal are usually quite evident and interpretable. Thus, for instance, we find wind-borne diaspores bearing wings, hairs, or plumes that increase air resistance and slow the rate of fall (a dispersal syndrome named anemochory), seeds that float in the water by means of a buoy (hydrochory), seeds with hooks or barbs that adhere to the exteriors of animal vectors (exozoochory), seeds with elaiosomes for ant dispersal (myrmechochory), or diaspores with flesh appendages or coverings that are consumed by animals which later eject the seeds (endozoochory, within which we distinguish ornithochory, saurochory, etc., depending upon the taxonomical order of the frugivore in question). Some plants disperse their offspring ballistically, by the explosive opening of the fruits or the springing of a trip lever. It is also not uncommon that plants combine two or three modes of dispersal; we thus find plants that are dispersed ballistically and by ants (e.g., Viola), others dispersed by ants, birds, and mammals (e.g., Myrtus), etc. Other species lack any evident dispersal device, which makes us wonder whether dispersal is less advantageous in these species or how they achieve effective dispersal.

The dispersal mode ofseeds has commonly been associated with seed size; thus, for instance, species with mammal-dispersal syndrome have significantly larger seeds than those with bird-dispersal syndrome. This occurs even within a genus, such as in Pinus, in which seeds weighing less than c. 100 mg are wind dispersed whereas heavier seeds tend to have adaptations for bird dispersal. However, the usefulness of dispersal syndromes has often been questioned, especially for vertebrate seed-dispersal syndromes, which have been found to be only minimally attributable to plant adaptations to dispersers and, on the contrary, appear to reflect more the influence of plant phylogeny on fruit traits. In fact, after accounting for phylogeny, fruit size has been found to be the only trait out of a large number of fruit traits considered in a review, significantly associated to dispersal.

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