Arthropods on native versus alien woody plants: understanding variation across host plant taxonomy and geography

Alien plants have generally been shown to support reduced arthropod abundance, biomass, and diversity, but inferences have typically come from studies limited in taxonomic and geographic scope. Here, we make use of data from a unique citizen science project, Caterpillars Count!, that consists of nearly 70,000 standardized surveys of woody plant branches across eastern North America on over 100 plant species. From these data we find that caterpillar (i.e. larval Lepidoptera) occurrence was nearly 4 times higher, beetle (order Coleoptera) occurrence was 50% higher, and spider (order Araneae) occurrence was 17% higher on native versus alien plants. The occurrence of hoppers and allies (suborder Auchenorrhyncha) was slightly higher on alien plants, while there was little difference in occurrence on native versus alien plants for ants (family Formicidae) or true bugs (suborder Heteroptera). Species richness of caterpillars, spiders, and beetles was also higher on average on native versus alien plants. The increased occurrence of caterpillars on native plants was consistent across plant families, whereas differences between native and alien plants for the other arthropod groups were highly variable across plant families. Caterpillar occurrence varied widely both within and between plant families, suggesting the importance of other plant species characteristics beyond native versus alien origin. Finally, we show that caterpillar occurrence appears to increase with latitude on alien plants at a faster rate than it does on native plants, but that this difference can be attributed to shifts in the identity and composition of plant species across the latitudinal gradient within our dataset. These findings have implications for how the increased spread of alien plants may impact ecosystems and food webs, and for land managers seeking to mitigate those changes.