Abstract/Summary

The successful production and development of seeds can be fraught with challenges, as plants must simultaneously attract pollinators and escape enemies which feed on the developing seeds. Interactions with insect pollinators and predators can be dependent on the local abundance of resources (flowers or seeds) available to them, sometimes causing variation in seed production and survival across gradients of host plant densities. Where insects are host-specific, density-dependent patterns of seed predation can promote the coexistence of tree species by suppressing the recruitment of species which are locally abundant. Although this mechanism has received substantial attention, density-dependent seed mortality caused by insects in the canopy has been largely overlooked. In this thesis, I explore the impacts of plant-insect interactions on seed production and pre-dispersal seed development to better understand the potential roles of these interactions in plant community dynamics. I first investigate the phenomenon of premature fruit drop in a community of tropical forest trees and conclude that seed losses at the pre-dispersal stage are significant and could be triggered by pre-dispersal seed predators. In the same forest, I then explore how premature fruit drop varies with the local density of conspecific trees, demonstrating that seed mortality associated with premature fruit drop is highest where conspecific trees are locally abundant. Next, I investigate the parallel processes of fruit set and fruit drop through field surveys focusing on a tropical (Jacaranda copaia) and a temperate (Crataegus monogyna) tree species. At the population-level I find conspecific density-dependent patterns of premature fruit drop but no evidence to suggest that fruit set varies with conspecific density in either of the studied tree populations. Taken together, the results presented in this thesis provide evidence that density-dependence in plant-insect interactions at the pre-dispersal stage can be widespread, and highlight the potential role of these interactions in promoting the coexistence of plant species in natural forest systems.