There is growing evidence that top–down biotic factors play an important role in plant community dynamics and are able to maintain the high plant diversity of primary tropical forests. However, the top–down impact by herbivores on the accumulation of plant biomass, richness and the community assembly process remains poorly known for the initial stages of secondary ecological succession. Here we test the top–down effects of natural enemies on the biomass, richness, diversity, community composition, trait dynamics and randomness during the assembly process on an early successional community in a lowland tropical forest in Papua New Guinea. We initiated secondary succession on 36 vegetation plots (5 × 5 m) organized in six blocks. After 1 year we evaluated successional community characteristics in each block containing a control plot and five treatment plots: experimentally excluding (a) insects by insecticides, (b) fungal pathogens by fungicides, (c) vertebrate predators and ants with exclosures and traps and experimentally adding (d) low and (e) high levels of a generalist herbivore species. Insects were responsible for increased plant diversity, shifts in plant community composition and a decrease in community weighted mean of leaf dry matter content. The lack of herbivores resulted in a diversifying effect but did not affect plant biomass and stem density. No other treatment had a significant effect on plant diversity during succession. The highest level of artificially increased herbivore density reduced plant biomass, while the fungicide treatment decreased stem density of woody plants. Using a null model approach, we showed that the removal of insects and their vertebrate predators increased randomness during the community assembly processes for woody plants, whilst the same effect was not found for the removal of fungi or elevated herbivore abundances. Synthesis. Early rainforest succession was not entirely plant driven. During the early succession of lowland tropical forest, insect herbivores enhanced plant diversity and caused shifts in community structure by promoting species with acquisitive leaf traits. As expected, the early successional communities were highly variable, but insects and vertebrate predators reduced randomness during the assembly process. Insect and vertebrate communities can thus impact the regeneration dynamics of tropical forests. In contrast, fungal pathogens were less important in maintaining high diversity of successional communities in our system.

The relative roles of plants competing for resources versus top‐down control of vegetation by herbivores, in turn impacted by predators, during early stages of tropical forest succession remain poorly understood. Here we examine the impact of insectivorous birds, bats, and ants exclusion on arthropods communities on replicated 5 × 5 m of pioneering early successional vegetation plots in lowland tropical forest gaps in Papua New Guinea. In plots from which focal taxa of predators were excluded we observed increased biomass of herbivorous and predatory arthropods, and increased density, and decreased diversity of herbivorous insects. However, changes in the biomass of plants, herbivores, and arthropod predators were positively correlated or uncorrelated between these three trophic levels and also between individual arthropod orders. Arthropod abundance and biomass correlated strongly with the plant biomass irrespective of the arthropods' trophic position, a signal of bottom‐up control. Patterns in herbivore specialization confirm lack of a strong top‐down control and were largely unaffected by the exclusion of insectivorous birds, bats, and ants. No changes of plant–herbivore interaction networks were detected except for decrease in modularity of the exclosure plots. Our results suggest weak top‐down control of herbivores, limited compensation between arthropod and vertebrate predators, and limited intra‐guild predation by birds, bats, and ants. Possible explanations are strong bottom‐up control, a low activity of the higher order predators, especially birds, possibly also bats, in gaps, and continuous influx of herbivores from surrounding mature forest matrix.