Tropical forests are home to thousands of tree species, with forests consisting of many rare species and a handful of common species. Many ecologists are interested in understanding how so many species can coexist and persist for a long time. One of the hypotheses explaining species coexistence is that species-specific enemies (e.g. pathogens, seed predators, herbivores) are keeping common species in control. When trees fruit, many fruits, hence seeds, drop near fruiting trees so many seedlings of the same species occur near these fruiting trees. Species-specific enemies also occur where seed and seedling densities are high, so most seedlings near their mothers may all die, but will open up space for other species to occupy. When enemies are ‘picky’ as to which species to attack, and attack plants of same species (conspecifics) that are near mother plants (distance dependence) in high densities (density dependence), common species are at disadvantage because they are more likely to encounter these enemies that are usually limited in dispersal ability.
In one of the projects I conducted in Costa Rica, I showed that the spatial pattern of offspring shifts outward as plant size increases by recording all plant sizes of five animal-dispersed tree species (Sugiyama 2015). Simulation shows that such shifts happen when survival increases when going farther from the fruiting tree and I indeed found an increase in seedling survival when going farther from fruiting trees. This suggest that when you lose animals from forests due to disturbances such as deforestation and fragmentation, and seeds are not dispersed by animals, trees may keep producing a lot of seeds but most of them may simply die.
Many tropical tree species depend on animals for seed dispersal because trees themselves cannot move. Being dispersed away from fruiting trees may increase the chance of seed survival by escaping the high mortality zone around their mothers. However, fruit-eating New World leaf-nosed bats bring the fruits back to their feeding roosts, so even though the seeds get dispersed, they end up in clumps under these feeding roosts, instead of under fruiting trees. This seems to defeat the whole purpose of seed dispersal from the plants’ perspective. Why do bat-dispersed trees invest in producing nutritious fruits if dispersers they want to attract may not be doing a good job? Although density effects and distance effects are often confounded in nature, do they have separate effects? Is being dispersed away more important than being in low densities? Are negative density-dependent effects reduced away from fruiting trees? Using a Neotropical bat-dispersed species gave us a unique opportunity to resolve this paradox of clumped seed dispersal. We mapped the entire population of a bat-dispersed species in a 50 ha plot on Barro Colorado Island (BCI) in Panama to answer these questions. In our study species, there were no negative density effects and being dispersed away from fruiting trees indeed improved seedling survival (Sugiyama et al. 2018). What was more, we found positive effects of being dispersed to bat feeding roosts on seedling survival, despite the fact that the seeds end up in high densities. These findings resolve the apparent paradox of clumped seed dispersal, and may help explain the existence of many species with clumped seed dispersal.