Competition for dispersal agents among tropical trees: influences of neighbors

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Abstract
We distinguish factors influencing seed dispersal that are potentially under the control of parent plants from those that are not in a Panamanian population of the neotropical nutmeg, Virola surinamensis (Myristicaceae). In the first category, we find that individual variation in crop size and nutritional components of the aril failed to explain any variation in the proportion of fruits taken from fruiting trees. The ratio of edible aril to indigestible seed explained a significant but small (13%) portion of variation in the fraction of fruits taken by tropical birds. These results suggest that the potential for ongoing natural selection on fruit and crop characteristics by dispersal agents exists, but is periodic or weak. On the other hand, the number of fruits available to birds within 50 m of any given Virola tree profoundly influenced the extent to which it secured dispersal. More than three times as much variation (42%) in relative dispersal could be explained by the number of “competing” Virola fruits during the peak of an individual tree's fruiting season. When isolated plants are eliminated from the analysis, the depressive effect of competition with neighbors explains 72% of the variation in individual dispersal. These results suggest that birds seek out clumps of fruiting trees, but that intraspecific competition for a limited disperser assemblage occurs within the clumps. Our results are a first step towards separating attributes of the plant that are potentially under the influence of natural selection by dispersal agents from emergent population effects that can override individual advantage in crop or fruit characteristics.