Climate change and local host availability drive the northern range boundary in the rapid northward expansion of the eastern giant swallowtail butterfly

Aims Species distributions result from both biotic and abiotic interactions across large spatial scales. The interplay of these interactions as climate changes quickly has been understudied, particularly in herbivorous insects. Here, we investigate the relative impacts these influences on the putative northern range expansion of the giant swallowtail butterfly in North America. Location North America. Time period 1959-2018. Major taxa studied Eastern Giant swallowtail, Papilio cresphontes (Lepidoptera: Papilionidae); common hop tree, Ptelea trifoliata ; common prickly ash, Zanthoxylum americanum ; southern prickly ash, Zanthoxylum clava-herculis (Saphidales: Rutaceae). Methods We used data from museum collections and citizen science repositories to generate species distribution models. Distribution models were built for each species over two time periods (T1 = 1959-1999; T2 = 2000-2018). Results Models for P. cresphontes and associated host plants had high predictive accuracy on spatially-explicit test data (AUC 0.810-0.996). Occurrence data align with model outputs, providing strong evidence for a northward range expansion in the last 19 years (T2) by P. cresphontes . Host plants have shifted in more complex ways, and result in a change in suitable habitat for P. cresphontes in its historic range. P. cresphontes has a northern range which now closely aligns with its most northern host plant - continued expansion northward is unlikely, and historic northern range limits were likely determined by abiotic, not biotic, factors. Main conclusions Biotic and abiotic factors have driven the rapid northern range expansion in the giant swallowtail butterfly across North America in the last 20 years. A number of bioclimatic variables are correlated with this expansion, notably an increase in mean annual temperature and minimum winter temperature. We predict a slowing of northward range expansion in the next 20-50 years as butterflies are now limited by the range of host plants, rather than abiotic factors.