Non-native plants reduce abundance, richness, and host specialization in lepidopteran communities

The impact of non-native plant invasions on ecosystems has been controversial because obvious local effects have not yet led to the global extinction of any native plant species on continents and large islands. We suggest that extinction is not the appropriate measure of impact on ecosystem function and present evidence that non-native plant invasions or the replacement of native plants with non-native ornamentals results in significant bottom-up reductions of energy available for local food webs. Using replicated common gardens we compared Lepidoptera species richness and abundance on native plants, non-native congeners of those natives, and non-native species with no close relatives in the study area. Non-native plants supported significantly fewer caterpillars of significantly fewer specialist and generalist species even when the non-natives were close relatives of native host plants. However, the effect size was smaller in the latter category indicating phylogenetic similarity to local natives may positively impact herbivory. Cluster analysis revealed that a non-native plant congener often supports a lepidopteran community that is a subset of the similar, but more diverse community found on its native congener. The proportion of the Lepidoptera community consisting of specialist species was about five times larger across native species within sites compared to non-native plant species. In addition, species accumulation trajectories suggested that in a fully sampled community the differences between the Lepidoptera supported by native and non-native plants may be even greater than presented here.