The Role of Cold Tolerance in the Geographic Distribution of the Giant Swallowtail Butterfly (Papilio cresphontes)

Abstract

Climate change is a well-known factor driving species range shifts. These shifts are often attributed to the relaxation of harsher climates at the northern range limit. Specifically, lack of cold tolerance as a constraint on range expansion at higher latitudes is one hypothesis. However, few, if any, studies have tested this hypothesis during a critical season: fall, when organisms are subjected to sporadic low temperature exposure but may not have become cold tolerant yet. In this study, I investigated the impact of low temperature on the larvae of the Giant swallowtail butterfly, Papilio cresphontes, at its northern range edge by integrating physiological experiments and species distribution modelling. First, the cold hardiness of the larvae was tested in a laboratory by determining their supercooling point, chill coma temperature and survival at three low temperatures. I found that the supercooling point was -6.6°C, that larvae use a freeze avoidant strategy, and that larvae enter a chill coma at 2.4°C. I also found that exposure to -2°C did not lead to a high rate of mortality, but that larval survival was impeded by temperatures below the SCP with temperatures below SCP (-8°C) produced high mortality (10-12% survival). Second, to determine the importance of low temperatures at a broad scale, I compared species distribution models of P. cresphontes based only on environmental data derived from other sources to models that also included the physiologically-derived parameters I generated experimentally. The modelling revealed that growing degree days and precipitation best predicted the distribution of P. cresphontes. The cold tolerance variables did not explain much variation in habitat suitability of P. cresphontes distribution. As such, the modelling results were consistent with the experimental results: low temperatures in the fall are unlikely to limit the distribution of P. cresphontes. Further investigation into the ecological relevance of the physiological thresholds determined here will improve our understanding of range expansion of P. cresphontes due to climate change. This study demonstrates that a combination of approaches provides a strong test of hypotheses related to the factors that limit species distributions.