Available at: https://digitalcommons.calpoly.edu/theses/2062
Date of Award
MS in Biological Sciences
With temperatures rising globally, assessing the possible impacts of the changing climate becomes more and more urgent. Ectotherms are excellent indicators of potential climatic ramifications on biodiversity because of their heavy reliance on the environment for their thermoregulation. Studies have historically looked at thermal tolerance values to establish predictive models for population and species extinctions.
In chapter 1, we looked at recent studies that suggest that thermal tolerance may be a plastic trait and test the effects empirically. Most studies are based on captive lizards acclimated to laboratory conditions that do not necessarily reflect natural environments, and if thermal tolerance is plastic and affected by the recent thermal history of the animal, then the data may not be accurate. We tested the critical thermal maximum of the western fence lizard, Sceloporus occidentalis, in all four seasons, both under field (same-day) and short (two-day) lab-acclimated conditions. We found that thermal tolerance varied seasonally, with the lowest values in the summer in both same-day and two-day acclimated lizards. Additionally, we found that the thermal tolerance of lizards tested on the same day was higher in spring than in fall, but two days of acclimation to lab conditions eliminated this difference. We also tested the thermal tolerance of lizards housed at several constant acclimation temperatures for one or three weeks and compared these values to those of lizards housed in a terrarium allowing thermoregulation, and to same-day lizards. While the thermal tolerance of all lab-acclimated lizards was higher than that of same-day lizards, there was no significant difference in thermal tolerance among any of the acclimation treatments. Overall, our results show that thermal tolerance may be plastic in some situations in S. occidentalis, but that this species overall shows little plasticity in response to acclimation.
In chapter 2, we evaluated the thermal environment of S.occidentalis using operative temperature models. Using operative temperature models combined with field lizard body temperatures and a lab-determined selected body temperature range, we evaluated the thermal environment of Sceloporus occidentalis to identify habitat quality, thermoregulatory effectiveness, and thermal exploitation index. Additionally, we used two predictive climate change models at a 1°C and 2°C increase to project the potential changes in habitat quality in the future. The thermal quality was highest for shady microhabitats, lowest for sunny microhabitats, and intermediate for mixed sunny/shady microhabitats. S. occidentalis were able to maintain their body temperatures in their Tset range for 6 hours, indicating the ability to exploit multiple microhabitats. Neither climate change scenario (1°C or 2°C increase) placed S. occidentalis at risk of extinction, likely because the coastal field site has a relatively mild climate. However, both scenarios greatly decreased the thermal quality of the environment, causing S. occidentalis to lose up to 2.5 hours of activity time per day. This highlights that even animals that inhabit mild climates are likely to experience sub-lethal effects of climate change.