Island biogeography theory and the assembly patterns of native versus non-native forest insects

Forest tree diversity helps shape the assembly of herbivorous insect communities in forests worldwide, and Island biogeography theory explains the role of tree range area as a driver of variation in native insect richness per tree species. However, it is unclear if these biogeographical principles apply to non-native insect herbivores. We created an extensive list of tree-insect associations for tree species native to Palearctic Europe bioregion, and investigated how both native and non-native insect herbivore species richness per tree may be influenced by various factors including the range area of the tree and the tree type (conifer or angiosperm). Models were further subsetted to compare drivers for the richness of different feeding guilds (folivores, sap feeders or wood borers) and host breadth (specialist or generalist). Tree range area had a positive effect on native, but not non-native, insect species richness per tree, while non-native richness was affected by host type and had a positive correlation with native herbivore richness on the same tree species. Host breadth grouping did not influence the difference in the effect of tree range on native versus non-native insect richness, but native folivores had a stronger relationship with range distribution area compared to the other groups. We conclude that the assembly of non-native herbivorous insect species on trees occurs through fundamentally different mechanisms than the co-evolutionary processes proposed by the framework of Island biogeography theory.