Abstract / Summary
Understanding the complexity of interactions in quantitative terrestrial food webs is a key challenge in ecology, particularly in tropical habitats where reports are scarce. Documenting tri-trophic interactions in these ecosystems is essential to advance understanding of how community dynamics and specialisation shape network structure. Here we present the largest and most comprehensive plant-gall-parasitoid network to date. We sampled galls and reared parasitoids from three sites in lowland rainforest near Madang, Papua New Guinea, and combined them into a single quantitative tri-trophic network. Network statistics were calculated, and gall traits (taxon, apparency, openness, plant module, and pubescence) were tested as predictors of parasitoid richness. We also analysed the potential for apparent competition (PAC) among galls. The network comprised 32 host plant species, 78 galling insect species, and 102 parasitoid species represented by 820 individuals. Monophagous species dominated both gallers and parasitoids, producing high specialisation (H2' = 0.99 for plant-gall and 0.90 for gall-parasitoid subwebs). Gall apparency and gall-forming taxon significantly predicted parasitoid assemblage size: conspicuous galls formed by Cecidomyiidae hosted more parasitoid species. One third of gall species shared no parasitoids, and the remainder showed very low PAC, suggesting limited interspecific enemy-mediated interactions but potential for intraspecific density dependence. The existence of highly specialised tri trophic food webs in tropical rainforests has been questioned, given the challenges posed by extreme specificity. Such networks require parasitoids specialised to herbivores that are themselves restricted to single plant species, raising doubts about their feasibility in diverse ecosystems. Our study demonstrates that these networks do occur, with gall forming insects and their parasitoids providing clear evidence of highly specialised tri trophic interactions in tropical forests. We also discuss the potential existence of a latitudinal gradient in gall-parasitoid specialisation.