Insects and non‐woody plants slow down tropical forest succession: A community‐wide experiment in Papua New Guinea

Abstract Ecological succession is a complex community‐wide process whose theoretical principles require further development. Here, we test the hypothesis that the succession of woody plants in tropical rainforests is determined by bottom‐up plant competition, rendering top‐down control by insect herbivores insignificant. Over 18 months of rainforest secondary succession, we removed insects and non‐woody plants from replicate 5 × 5 m plots in a factorial experiment at 700 and 1700 m elevation in Papua New Guinea. At 700 m elevation, insect removal increased biomass, reduced diversity and altered the species composition of woody plants, while removal of non‐woody plants increased both biomass and diversity of woody plants and altered their species composition. At 1700 m, the effect of insects on woody plant biomass and species composition disappeared while the effect of non‐woody plants on woody plant biomass, diversity and species composition became stronger. The removal of insects did not increase the proportion of alien species in the woody vegetation as predicted by the enemy‐release hypothesis. The increased disturbance caused by removing non‐woody plants also did not promote alien plants. Synthesis . The importance of top‐down insect herbivory decreased, while the importance of bottom‐up plant competition with non‐woody plant species increased with elevation, representing a gradient of increasing environmental stress in the succession of woody plants.