Lowland primary rainforests in the tropics are remarkable for their extraordinary species diversity, starting from plant communities and cascading to higher trophic levels. The species diversity is accompanied by extreme complexity of food webs, making the understanding of forest ecosystems extremely difficult. Here we review ecological trends in forest food web structure along key ecological gradients – latitudinal, elevational, and successional, with the aim of explaining the ecology of high species diversity and food web complexity. We argue that community-level censuses of food webs need to be followed by manipulative experiments as a feasible, perhaps the only feasible, approach to understanding their assembly mechanisms. We can claim to understand tropical ecosystems only when we are able to predict their composition and dynamics. We can achieve this only by combining ecological and phylogenetic analyses as the contemporary structure of forest ecosystems is a result of evolutionary history shaped by current ecological conditions. We illustrate these concepts by examples from a global network of study sites, with a particular focus on our work in Papua New Guinea.