Trophic island biogeography drives spatial divergence of community establishment

Food webs assemble via the interacting constraints of dispersal limitation and bottom-up trophic dependencies where consumers can only establish after their resources have arrived. These factors can affect assembly by influencing the expression of the regional species pool in local patches, but how this unfolds mechanistically remains unclear. Here, we use a large-scale grassland meta-community experiment to demonstrate how the independent influences of spatial factors and bottom-up constraints on insect trophic guilds interact to create divergent local insect communities. Bottom-up trophic dependencies tightly controlled the composition and abundance of specialist trophic guilds, resulting in different communities among islands because producer communities were mainly determined by patch size contingencies. Spatial isolation controlled the composition and abundance of generalist trophic guilds, resulting in different insect communities among islands based on distance from mainland. These results demonstrate that neither diet-based constraints nor the spatial characteristics of islands can predict the early structure of locally assembling food webs, with their establishment deriving instead from interactions between both processes.