| Summary: | The Janzen-Connell (JC) hypothesis is a leading explanation for plant-species diversity in tropical forests. It predicts that host specialist natural enemies drive the disproportionate mortality of con specific juveniles close to adults, particularly when density is high. The survival of heterospecifics is thought to be unaffected because apparent competition is generally weak or absent. Consequently, if the JC effect is sufficiently intense, adults will not be replaced by conspecific individuals and species turnover will be promoted at forest sites, which has the potential to greatly enhance diversity. This thesis first demonstrates experimentally that natural enemies are capable of driving overcompensating density dependence in the seedlings of a tropical tree and that these natural enemies do not generate apparent competition. The potential for these short-term processes to develop into long-term recruitment inhibition is assessed by showing that adults are rarely replaced by con specific individuals, using Barro Colorado Island 50 ha plot datatset. However, self-replacement rates are shown to vary with landscape-scale abundance, adult-tree size and growth form. Finally, the diversity enhancing potential of self-replacement inhibition is measured using a simulation model in which the strength of density dependence is manipulated and the JC mechanism is compared to several alternative mechanisms for enhancing coexistence.
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