Date of Award:


Document Type:


Degree Name:

Master of Science (MS)


Watershed Sciences


Charles P. Hawkins


Species distribution models are frequently used in ecology to predict the spatial and temporal occurrence of organisms. Direct interpretation of these models assumes that the relationships between the organisms and their environment are manifestations of causal mechanisms. However, in general, the mechanisms producing these associations have not been experimentally validated, which questions our confidence in their interpretation and application. Temperature is one of the most important factors influencing the fitness and distribution of aquatic organisms, and studying the thermal physiology of aquatic invertebrates could provide a useful approach for validating predictions of the species distribution models.

Experimental thermal tolerance studies, which assess the physiological limits to temperature, should be useful in interpreting the causal basis for species distribution model predictions. Critical Thermal Maxima experiments are frequently used to measure the thermal tolerance of ectothermic organisms. They represent the temperature at which organisms exhibit disorganized locomotor activity to the point that they lose their ability to escape conditions that will promptly lead to death. Critical Thermal Maxima experiments could, therefore, provide a useful test of the inferred mechanisms of species distribution models.

The objective of my study was to determine if Critical Thermal Maxima experiments are associated with the thermal limits inferred from species distribution models. If the models accurately describe causal relationships between predicted distributions of organisms and environmental temperatures, and if the thermal maxima are associated with the limits to organism fitness, I expected to see a strong correspondence between model-derived and experimentally-derived thermal limits. A strong correspondence between model predictions and experimentally obtained thermal maxima would both validate a physiological interpretation of the species distribution models and justify the use of Critical Thermal Maxima experiments alone in predicting species distributions and responses to climate change.