As sessile organisms, trees heavily depend on various animal dispersers at the cost of resource investment. Studies have found high offspring mortality near maternal trees where seed/seedling densities are high, which has been one of the major explanations for the significance of seed dispersal. In the tropics, bats are often the most common mammal but, unlike many other animal dispersers which scatter seeds at low densities, bats transport fruits from maternal trees to their feeding roosts, where they defecate and deposit seeds in high densities. While this behavior may diminish the advantage of seed dispersal, bat-dispersal may still be beneficial if a relatively small proportion of seeds dropped under non-maternal or non-roost trees show exceptionally high survival. However, assessing the survival of seedlings from those sites requires continuous spatial data because sub-sampling may not fully capture those rare events. To test whether bat-dispersal is truly beneficial, we tagged and mapped the entire post-germination population of Calophyllum longifolium(Calophyllaceae), a bat-dispersed canopy tree species, throughout the entire 50 ha plot of Barro Colorado Island (BCI). We tested whether potential negative density-dependent mortality outweighed the positive distance-dependent survival in a system where density and distance from maternal trees are naturally decoupled.
Results/Conclusions
With a complete census of all life stages across the entire 50-ha plot, our study revealed that differences between the spatial distribution of seedlings and larger size classes already existed before individuals reached 1 cm diameter at breast height (dbh). Recruits still occurred under maternal trees and potential roosts where seedlings experienced high mortality within the past year. These high mortality zones for early-stage individuals differed from where larger saplings died, which might have resulted from demographic changes in tolerance against distance/density dependence or temporal changes in roost locations. Conspecific density of same-size-class (within 10 m) and larger-size-class (within 20 m) neighbors had significant negative impacts on 1-yr survival of early-stage individuals. In contrast, distance from the nearest conspecific adult had positive effects on 1-yr survival of seedlings, and many seedlings dispersed away from conspecific individuals in low density survived. While negative density-dependent mortality had greater effect in predicting survival of early-stage individuals than distance from nearest conspecific adults, the benefit of bats seem to outweigh the negative effects, which may explain the more than doubling of the population of Calophyllum in the past three decades.