Responses of dispersal agents to tree and fruit traits in Virola calophylla (Myristicaceae): implications for selection

Variation in traits affecting seed dispersal in plants has been attributed to selection exerted by dispersal agents. The potential for such selection was investigated in Virola calophylla (Myristicaceae) in Manú National Park, Peru, through identification of seed dispersal agents and of tree and fruit traits significantly affecting the quantity of seeds dispersed. Seventeen bird and one primate species (the spider monkey, Ateles paniscus) dispersed its seeds. Spider monkeys dispersed the majority of seeds (a minimum of 83% of all dispersed seeds). Visitation by dispersal agents depended only on the quantity of ripe fruit available during a tree observation. In contrast, seed removal increased with both greater quantity of ripe fruit and aril: seed ratio. When analyzed separately, seed removal by birds increased only with greater aril: seed ratio, whereas seed removal by spider monkeys was affected by the quantity of ripe fruit and phenological stage. The finding that dispersal agents responded differently to some tree and fruit traits indicates not only that dispersal agents can exert selection on traits affecting seed dispersal, but also that the resulting selection pressures are likely to be inconsistent. This conclusion is supported by the result that the proportion of the seed crop that was dispersed from individual trees, which accounted for cumulative dispersal by all agents, was not influenced by any tree or fruit trait evaluated. Comparing these results with those from studies of V. sebifera and V. nobilis in Panama revealed that the disperser assemblages of these three Virola species were congruent in their similar taxonomic representation. In Panama the proportion of V. nobilis seed crop dispersed was related positively to aril: seed ratio and negatively to seed mass, a result not found for V. calophylla in Peru. The greater importance of dispersal by primates versus birds in V. calophylla, relative to V. nobilis, may explain this difference. Thus, variation in disperser assemblages at regional scales can be another factor contributing to inconsistency in disperser-mediated selection on plant traits.