Examining the relative contributions of latitudinal variation and phenotypic plasticity to thermal tolerance in arthropods

Climate change is one of the greatest challenges facing biodiversity today. To understand the responses of ectothermic organisms to increasingly variable climates, it is essential that we understand the patterns of variation in thermal tolerance that occur across spatial and temporal scales. Thermal tolerance measures such as critical thermal limits (CTLs) are at the heart of physiological studies and provide information on the range of temperature at which ectotherms are able to function, and their variation gives key insights into climate change responses. However, within the thermal physiology literature there are several key areas in which controversy has arisen. Notably, in the effects of the rates of temperature change used to assess CTLs, and the effects of acclimation treatments on the same. Furthermore, there is conflicting evidence as to whether low-latitude ectotherm species will be more at risk than high-latitude species from climate change effects, based on the expectations of established macrophysiological rules. The overall aim of this thesis is therefore to investigate these three key areas of controversy in little-studied arthropod groups.
The experimental conditions under which thermal tolerance traits, such as critical thermal limits (CTLs), are assessed can significantly affect the outcomes of such trials. Not only can this lead to erroneous conclusions regarding the thermal tolerance of organisms, but also misinform estimates of vulnerability of which metrics are calculated from thermal tolerance measures. I examined the effects of rate of temperature change on critical thermal limits (CTLs) in two beetle (Tenebrio molitor and Cyrtobagous salviniae), and six Collembola species, using a range of environmentally relevant rates of change; and then examined how the resulting variation in CTLs might feed over into estimates of climate sensitivity based on these measures. Rate of change effects were found to vary between species, latitudinal sites and acclimation treatments. For the Collembola, these effects resulted in significantly different estimates of climate sensitivity between latitudinal groups (subtropical, temperate and polar). The time course of the attainment and reversal of an acclimation response for CTLs was also investigated and found to be significantly different in T. molitor and C. salviniae. These results also suggested that the beetles would have limited capacity to respond to temperature changes over short time scales (hours), such as extreme thermal events; the frequency of which is predicted to increase with future climate scenarios. At a local scale, the thermal tolerance and climate sensitivity of C. salviniae was examined across South Africa to predict whether this species' effectiveness as a biocontrol agent would be affected by predicted increases in temperature in the future. Given the possible benefits of warmer winter temperatures and the possible constraints of higher summer temperatures, the future success of C. salviniae will depend on careful monitoring and strategic reintroductions where necessary. At a broad spatial scale, I examined whether support could be found for several macrophysiological rules that predict thermal tolerance at large spatial scales. Using fifteen Collembola species from latitudinally distinct sites, I found support for Janzen's hypothesis (increasing thermal tolerance breadth with increasing latitude), Gause's rule (negative relationship between basal thermal tolerance and plasticity) and Brett's rule (greater variation in upper compared to lower thermal limits); corroborating a few analyses at global scales using other taxonomic groups. These broad spatial patterns form an important part of understanding ectotherm responses to climate changes at a global scale.
In summary, this study found that the thermal history and experimental conditions under which thermal tolerance estimates are obtained, affect both assumptions of organismal thermal tolerance and estimates of sensitivity to climate warming. These are important considerations when attempting to describe both small and large-scale responses of ectotherms to climate change.