Alien plants versus alien herbivores: does it matter who is non-native in a novel trophic interaction?

Introductions of both plants and herbivorous insects have had tremendous impacts on the world's ecosystems. Novel herbivorous insect-plant interactions are important consequences of introductions of either plants or herbivorous insects. We contrast novel herbivorous insect-plant interactions that arise due to plant versus insect introductions with the aim of understanding whether the causes and consequences of the interaction depend on which party is non-native. The biotic context of the herbivore-plant interaction, in terms of mutualists, predators, and competitors can limit the prevalence of that interaction and varies between native and introduced ranges. Introduced plants can have a large, direct impact on their environment, whereas the impact of introduced herbivorous insects is often mediated through the plants that they consume.