The role of thermal tolerance in biological invasion

Abstract

Biological invasion and climate change are among the greatest threats to biodiversity currently. Several factors influence what makes a successful invasive species. Among these is the thermal tolerance of the invasive species. An organism’s thermal physiology underpins many aspects of its life, including the habitats in which it can survive, its reproduction and its development. Climate change is increasing global average temperatures at an unprecedented pace, which is increasing thermal stress on virtually all life around the world. These effects are particularly pronounced in ectotherms, which are reliant on environmental temperature to set their body temperature and drive their metabolism. Insects comprise a significant proportion of globally important invasive species. As well as being ectothermic, they have a complex life cycle with a number of developmental stages, all of which may be influenced by environmental temperatures. Invasive species can often tolerate or adapt to a broad range of environmental conditions. They may therefore be expected to have a broader thermal tolerance than their native counterparts, and than other invasives that have a narrower invaded range.

This research used a blowfly model to investigate how thermal tolerance differed across temperatures, using a static temperature assay to measure thermal knockdown time. Key foci included the effects of both developmental heat acclimation and adult heat tolerance on thermal performance. In Chapter 2, variation in adult heat tolerance was assessed across three temperatures (41℃, 42℃, 43℃) in five blowfly species invasive to New Zealand. I found that the two more globally invasive species in the study exhibited higher heat tolerance than the three less invasive Calliphora species, whose native ranges are generally more temperate. In Chapter 3, the effect of developmental temperature on adult knockdown time in a single species (Calliphora stygia) was assessed. I showed that C. stygia reared at the lowest temperature (18℃) had the highest knockdown time and the smallest body size, while those reared at 26℃ 1 exhibited the shortest knockdown time and an intermediate body size, with those reared at 22℃ being the largest.

Collectively, my results indicate that adult heat tolerance varies between invasive blowfly species, but that developmental acclimation temperature may not have a significant effect on adult heat tolerance. Given the increasing global temperatures seen with climate change, the effects of invasive species are likely to increase, with some species likely to do better than others in a warming world. Thermal performance will undoubtedly play a role in filtering out the ‘winners’ and ‘losers’, therefore further research into both heat and cold tolerance in both laboratory and field settings are crucial.