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 (Papilio cresphontes) in North America. We leveraged data from multiple citizen science repositories (iNaturalist, eButterfly, GBIF) to generate species distribution models. Distribution models were built for each species (P. cresphontes and three host plants) over two time periods (T1 = 1959-1999; T2 = 2000-2018).
Results/Conclusions
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.
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. These findings demonstrate the value of citizen science data in exploring distributional shifts of important linked species across large temporal and spatial scales, and the role that these data can play in shaping our predictions about future distributions under changing environmental conditions.