Interspecific neighbors change seed dispersal pattern of an avian-dispersed plant

Ecologists increasingly recognize that plant-plant facilitation can influence plant community structure. However, seed dispersal facilitation among plant neighbors that share seed dispersal agents has received little study. Seed dispersal facilitation among neighboring plants can be defined as an increase in the number, distances, and/or places reached by a plant's seeds that is due to the presence of co-fruiting-plant neighbors with which it shares frugivore seed dispersers. I experimentally tested the potential for seed dispersal facilitation and competition among co-fruiting-plant neighbors using Solanum americanum and Cestrum diurnum, two plant species that co-occur in open habitats of Puerto Rico and whose seeds are dispersed by the same bird species. I constructed S. americanum neighborhoods with and without C. diurnum (i.e., mixed and non-mixed) while controlling plant ripe-fruit crop and surrounding fruiting landscape in six replicate fields. I quantified seed dispersal using a grid of seed-collecting trays bearing bird perches placed at four distance classes from experimental plant neighborhoods. I found no difference in the quantity or rate of fruit removal between mixed and non-mixed neighborhoods. However, there were significantly more bird droppings with seeds collected from mixed neighborhoods than from monospecific S. americanum neighborhoods. Although the quantity of seed dispersal was no different among treatments, S. americanum seeds were distributed into more "packaging units" and reached more sampling trays when neighbored by C. diurnum. This was explained by the higher number of frugivores that visited mixed neighborhoods compared to Solarium-only neighborhoods, and by an associated increase in agonistic interactions in mixed neighborhoods. Therefore, the seed dispersal pattern of S. americanum was facilitated by the presence of fruiting C. diurnum neighbors. This study provides the first experimental evidence that seed dispersal kernels of endozoochorous plants are context dependent and not entirely a property of individual plant or disperser species.