Competition and facilitation in bird -dispersed plants
Seed dispersal is essential to maintain plant community diversity, with frugivores providing seed dispersal to many species (up to 90% of tropical forest species). Thus, to understand the ecology of many plant communities one must understand the relationship between frugivore-plant interactions and plant distribution.
I used a simulation model to investigate how abundance of frugivores and the aggregation of plants affected frugivory, dispersal, and plant-neighborhood interactions. I found that dispersal was reduced by increasing aggregation of plants and that aggregation also limited the distribution of seed-dispersal services among plants. Plant neighborhoods were important in determining fruit removal and seed-dispersal, but whether plants competed or facilitated dispersal services was contingent on the degree of plant aggregation and abundance of frugivores.
I experimentally tested the potential for competition or facilitation to occur between two plant species that share frugivores. I found that Solanum americanum plants neighbored by Cestrum diurnum obtained more dispersal than Solanum neighbored by conspecifics. Furthermore, I found evidence that seed-dispersal facilitation can affect the distribution pattern of natural populations of mistletoes growing on the dioecious Cecropia schreberiana. Female Cecropia trees were twice as infected as males, with females having twice as many mistletoes as males. This was explained by a female-biased visitation pattern of birds that fed on mistletoe and Cecropia fruits, where Cecropia fruits facilitated the establishment and dispersal of mistletoes.
Last, tracking the fate of dispersed seeds and seedlings is central to many plant-frugivore ecology questions but remains difficult to measure in the field. I developed a technique to trace seeds to mother plants using 15N-labelled Urea leaf-feeding. To those studying seed dispersal and seedling establishment, this technique provides a non-fading internal marker able to trace seedlings to mother plants.
In conclusion, interactions involving facilitation and directional dispersal can leave clear prints on plant species distribution and heterogeneity. I suggest that understanding community patterns and dynamics of animal-dispersed plant populations must take into account the intricacies of frugivore-mediated interactions.