Impacts of forest fragmentation on interactions between plants and their insect herbivores and fungal pathogens

Natural enemies of plants, including fungal pathogens and insect herbivores, can maintain plant diversity if their harmful effects on seeds and seedlings are density-dependent (the Janzen-Connell hypothesis). As insect and fungal communities can be modified by anthropogenic habitat fragmentation, we conducted a field experiment to understand how fragmentation might affect the ability of natural enemies to maintain diversity. In 21 rainforest fragments in the Western Ghats, India, we suppressed insects and fungi with biocides and examined consequent changes in the survival of naturally dispersed tree seedlings. Seedling survival decreased with the density of conspecific seedlings in the same plot. This effect was reduced by fungicide applications, especially in large forest fragments. Insecticide increased seedling survival, but its effects were independent of fragment area and conspecific density. The effects of conspecific density and fungicide were predominantly driven by the most abundant species, Syzygium rubicundum. Overall, these results indicate that forest fragmentation can alter biotic processes that regulate plant diversity. However, the overall impact of fragmentation through this pathway may be limited to relatively few species.