Year of Award

2023

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Organismal Biology, Ecology, and Evolution

Other Degree Name/Area of Focus

Ecology & Evolution

Department or School/College

Division of Biological Sciences

Committee Chair

Zachary Cheviron

Commitee Members

Art Woods, Bret Tobalske, Eric Riddell, Nathan Senner

Keywords

Range shifts, thermoregulatory costs, mammals, snow, climate change

Subject Categories

Biophysics

Abstract

Species geographic ranges are shifting in the face of contemporary climate warming, and documenting range shifts is crucial to our understanding of the underlying drivers mediating movement in geographic range limits. Studies on elevational range shifts with climate change are beginning to accrue within the literature, though observed shifts are idiosyncratic and difficult to predict. Some species may respond to warming temperatures by shifting their range limits upslope, where temperatures are cooler owing to the adiabatic lapse rate. However, species may also respond to warming temperatures in an elevation-dependent manner: if changes in snow depth expose overwintering organisms to colder air temperatures, they could shift upwards, where snowpack is deeper, or downard, where air temperatures are warmer. Physiological tolerance is often cited as a major driver of species distributions, but studies rarely connect organismal physiology to changes in climate and concomitant shifts in species’ elevational ranges. Even fewer studies focus on the factors governing elevational range shifts in mammals at the seasonal scale. Advances in the field of biophysical ecology allow for the direct translation of changes in environmental conditions over time to currencies directly relevant to organismal fitness. Here, we apply mechanistic models of heat flux to test whether changes in thermoregulatory costs underlie observed range shifts in 41 species of small mammals in the Colorado Rocky Mountains. We gathered biophysical and behavioral data on focal species from museum specimens and the literature, and quantified microhabitat conditions with high-resolution forcing data and newly-developed microclimate models. We calculated the change in both cooling and heating costs at historical range limits across the past century to test whether changes in thermoregulatory requirements drove movement at both lower and upper elevational range limits. Overall, changes in thermoregulatory costs did not explain elevational range shifts in our system. Mammals largely decreased heating costs, and increases in cooling costs were negligible. Changes in thermoregulatory costs varied across seasons and elevation. Our results suggest that movement at elevational limits in small mammals is largely driven by other abiotic and biotic factors rather than thermoregulatory costs alone. We submit that future research on the drivers of elevational range shifts in mammals would benefit from a more holistic and nuanced approach, rather than focusing solely on changes in the physiological costs of thermoregulation.

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© Copyright 2023 Ryan T. Mahar