The Janzen - Connell (JC) hypothesis is one among hundreds of mechanisms proposed to explain the maintenance of high alpha diversity of tree species in tropical forests. It explicitly places natural enemies as agents of density/distance -dependent seedling mortality, thereby disadvantaging common tree species and preventing competitive exclusion. Constraints on the populations of common species could arise from either a greater diversity of natural enemies or greater damage from a similar number of natural enemies as rare species. We evaluated the first possibility by assessing damage patterns on leaves, as a proxy for natural enemies, on all seedlings within five permanent 1x200 m transects at La Selva Biological Station (Costa Rica) in January 2010. A total of 869 seedlings from 113 different tropical woody plant species were observed. Foliar damage patterns of each seedling were identified by the shape and position of the damage. To compare natural enemy species richness across host species, for each species with more than 3 seedlings we developed a rarefaction curve to which we fit a Michaelis-Menten function. The asymptote parameters, an estimate of the total number of natural enemy species, were compared among tree species that varied in seedling and mature tree abundance.
Results/Conclusions
In general, common species had more diverse damage patterns than rare species, indicating that they host a higher diversity of natural enemies. Conspecific seedling density based on all transects was positively (p = 0.01, r2 = 0.48) correlated with estimated total number of natural enemies hosted by the tree species. This finding is consistent with the idea that conspecific seedlings act as a sink of natural enemies. However, Penthacletra macroloba, the most common canopy tree species at La Selva, hosted a lower number of natural enemy species than expected. This exception is consistent with the idea that P. macroloba may have escaped some natural enemies and has thus become more abundant. More factors remain to be tested such as the effect of abiotic factors on the interaction between natural enemies and their hosts and the distance from adult tree. Nevertheless, our findings generally support that seedlings of more common tree species host a greater diversity of natural enemy species.
