The two major benefits of seed dispersal are: (1) departure from the parent plant, which usually avoids sibling competition and reduces seed/seedling mortality by predators or pathogens; and (2) colonization of new sites. Seed density usually decreases with distance from the parent plant, following a unimodal leptokurtic distribution (with a peak at or close to the source, followed by rapid decline and a long, more or less fat, tail; Figure 1). Deviations from this conventional seed shadow shape can result from patchiness of habitat structure or from other ecological factors such as the behavior of frugivores, which can promote nucleation process due to preference for certain sites (for instance, by depositing seeds under particular trees used for resting). The tail of the distribution can be, in fact, as important as the modal portion of the curve, as
seeds in such tail have the potential to spread the parental genes to long distances as well as to maintain genetic connectivity among distant populations (see below).
Frequently, the benefits of leaving the immediate vicinity of the mother plant depend upon the advantages obtained by (1) increasing the distance to it and (2) avoiding a highly intense sibling competition. Therefore, the effects of both factors (seed density and distance from the parent plant) are not easy discernible without field experimentation.
Regarding the advantage of colonizing new sites, seeds that leave the mother plant have the capacity of occupying vacant habitats and suitable microhabitats for germination and growth. This allows, for instance, the latitudinal or altitudinal migration of many plants in front of climate changes, the recolonization of a land after a volcanic eruption, the colonization by many herbs and shrubs of an abandoned field, enhancing thus the ecological succession. There are also species that have 'directed seed dispersal', benefiting from it as seeds are deposited in sites or microsites that are especially suitable for germination and seedling establishment. Seed of mistletoes, for instance, are usually defecated by birds on host twigs, which are required for the successful germination and seedling recruitment.
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