Climate change poses a substantial threat to organisms and ecosystems that interact with other stressors including changes in land use. Changes including shifts in temperature and precipitation, more extreme events, and alterations of the timing of biological events are predicted to have substantial impacts on organisms and ecosystems around the world.
Governments around the world are working together to limit global average temperature increases to a magnitude that will reduce impacts on societies and ecosystems to an acceptable level. The Intergovernmental Panel on Climate Change (IPCC) and other scientists have been asking: what impacts will a 1.5°C or 2°C climate warming have on organisms and ecosystems? These temperature targets have been chosen because models and observations suggest that higher targets would have dramatic impacts.
Translating shifts in air temperature into impacts on organisms is an important challenge because multiple environmental (air and ground temperature, solar radiation, and wind speed) and biological factors (size, shape, color, and composition of organisms) combine to determine the body temperatures organisms experience. Organisms can also spatially and temporally alter how they interact with environmental factors by adjusting their locations or timing of activity.
Much of the research examining climate change impacts has focused on ectothermic organisms because they produce minimal internal heat and thus rely on environmental sources of heat to reach body temperatures suitable for their activity. Radiation from the sun is an important source of heat that can dramatically raise ecotherm body temperatures above air temperatures. Indeed, many ectotherms require shading to avoid thermal stress (Sunday et al., 2014). When shady sites are available, many ectotherms can behaviorally thermoregulate to maintain temperatures that allow optimal performance by moving between sunny and shady sites and by altering their activity times. A valuable approach for understanding how environmental conditions influence organisms is to quantify how well ectotherms can perform functions over a range of body temperatures. This quantification is termed a thermal performance curve (Sinclair et al., 2016).
This activity focuses on lizards, which have often been used to study climate change impacts because their performance is highly sensitive to temperature and many lizards use thermoregulation to adjust external heat sources. We examine how environmental conditions determine the body temperatures of lizards and whether they experience thermal stress.
Introducing Operative Temperature
We can predict the body temperature of an organism in a particular environment by summing heat exchanges associated with solar and thermal radiation and with the animal’s contact with the air and surfaces. This activity uses a model to estimate heat gains and losses in order to predict body temperatures. These models are referred to as energy budget or biophysical models. When we use such a model (or a biomimic) to predict equilibrium (steady-state) body temperatures for a particular organism in a particular environment we call the prediction an operative temperature.
Thermal Performance Curve
The graph on the right is a thermal performance curve (TPC) of a lizard species Pseummodromus hispanicus. TPCs are constructed with body temperature on the x-axis and the rate of a performance that influences survival or reproductive success on the y-axis. The lower and upper limits of the body temperature range over which performance is above zero are called critical thermal minima and maxima, CTmin and CTmax, respectively. The optimal temperature (Topt) is the body temperature at which the organism attains the highest performance. These three metrics (CTmin, Topt, and CTmax) are often reported as a simple way to describe a TPC.
Most TPCs exhibit a gradual rise with increasing body temperature before dropping sharply if Topt is exceeded. The rapid drop beyond Topt results in species quickly experiencing thermal stress if their body temperatures deviate from their preferred range. Organisms with Topt that are adapted to their environment can thus be threatened by even small increases in environmental temperatures (Tewksbury et al., 2008).
What is Thermal Safety Margin (TSM)?
Thermal safety margin (TSM) is a metric that indicates how much more the temperature can increase before the environment turns uninhabitable. TSM is variably defined but here we estimate TSM as the temperature difference between a lizard’s operative temperature and its critical thermal maximum (Huey et al., 2012). A positive TSM means the environmental temperature is within the thermal tolerance of the organism and a negatinvert raster to date TSM implies that the operative temperature surpasses the critical thermal maximum. In this platform, we use TSM to visualize and address the danger some species might face from increased temperature.
Climate and zenith angle data are long-term (1961-1990) monthly averages for 15km grids from Kearney et al. (2014, microclim: Global estimates of hourly microclimate based on long-term monthly climate averages). We use air temperature 1 cm above the ground assuming a soil substrate, ground temperature at depths of 0 cm under ground assuming a soil substrate, and wind speed 1cm above ground. Shade level of 0% and 100% correspond to "exposed" and "covered".
Elevation data are from NOAA ETOPO1 Global Relief Model, grid-registered ETOPO1 Ice Surface.
We estimated lizard optimal temperatures using a biophysical model from Campbell and Norman (2000, Environmental Biophysics)
The biophysical model was parameterized using lizard mass and size (snout-vent length, SVL) data from the following compilation: Meiri. 2010. "Length–weight allometries in lizards."
Lizard optimal temperatures and critical thermal maximum were obtained from the following compilation: Bennett et al. 2018. GlobTherm, a global database on thermal tolerances for aquatic and terrestrial organisms.
We estimate shifts in TSMs following 1.5°C or 2°C warming by adding that magnitude of warming to the monthly air and ground temperature averages. We thus omit potential changes to other environmental variables.
Huey, R. B., Kearney, M. R., Krockenberger, A., Holtum, J. A., Jess, M., & Williams, S. E. (2012). Predicting organismal vulnerability to climate warming: roles of behaviour, physiology and adaptation. Philosophical Transactions of the Royal Society B: Biological Sciences, 367(1596), 1665-1679.
Sinclair, B. J., Marshall, K. E., Sewell, M. A., Levesque, D. L., Willett, C. S., Slotsbo, S., ... & Huey, R. B. (2016). Can we predict ectotherm responses to climate change using thermal performance curves and body temperatures? Ecology Letters, 19(11), 1372-1385.
Sunday, J. M., Bates, A. E., Kearney, M. R., Colwell, R. K., Dulvy, N. K., Longino, J. T., & Huey, R. B. (2014). Thermal-safety margins and the necessity of thermoregulatory behavior across latitude and elevation. Proceedings of the National Academy of Sciences, 111(15), 5610-5615.
Tewksbury, J. J., Huey, R. B., & Deutsch, C. A. (2008). Putting the heat on tropical animals. Science, 320(5881), 1296-1297.
Select a species and explore their distribution, current status and the risk they may face from increasing temperature.