Year of Award

2023

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Organismal Biology, Ecology, and Evolution

Department or School/College

Division of Biological Sciences

Committee Chair

H. Arthur Woods

Commitee Members

Winsor H. Lowe, Bret W. Tobalske, Dean Jacobsen, Wilco. C. E. P. Verberk

Keywords

Ecophysiology, thermal tolerance, metabolism, climate change, flow, acclimation

Publisher

University of Montana

Subject Categories

Entomology | Other Physiology | Terrestrial and Aquatic Ecology

Abstract

How do organisms respond to environmental challenges and to environmental change? These questions occupy a central place in ecology and answering them will help us to understand why species live where they do, how organisms are affected by human activities, and, ultimately, how to choose among alternative conservation strategies. These questions are difficult, however, for two reasons. First, environmental challenges often involve multiple, interacting stressors. Second, individual responses can be modified by behavioral, morphological, and physiological plasticity. My dissertation investigates how interactions between temperature and oxygen influence the performance and survival of aquatic insects and how plasticity allows individuals to mitigate temperature-oxygen challenges.

Understanding temperature-oxygen interactions is important for aquatic insects because, in water, oxygen availability is very low (compared to in air). This oxygen problem can be exacerbated by warming because rising temperatures cause metabolic demand for oxygen to increase exponentially. Risings temperature can thus cause oxygen demand to surpass supply, depressing performance and survival. Yet, how well nymphs can mitigate the effects of temperature-oxygen challenges via plasticity remains poorly understood.

In chapter one, I demonstrate the importance of ‘the oxygen problem’ for aquatic insects by showing that tissue oxygen levels are far lower in aquatic than terrestrial insects. Results suggest that levels of internal oxygen are actively regulated by aquatic insects to establish stronger oxygen gradients and higher rates of oxygen flux, necessary for living in water. In chapter two, I present a literature review that examines how climate change threatens insects in high-elevation streams. Overall, the outlook is bleak due to both discrete and interacting challenges from warming temperatures, shifting flow regimes, and increasing levels of ultraviolet radiation and salinity. However, populations may also cope with changing conditions via plasticity and local adaptation. In chapters three and four, I use laboratory experiments to investigate the capacity of aquatic insects to mitigate temperature-oxygen challenges via plasticity. In chapter three, I presented aquatic insects with gradients in temperature, oxygen, and flow and measured how nymphs relocated among them. Nymphs readily moved to microclimates with higher flows when ambient conditions depressed oxygen availability or increased oxygen demand. In chapter four, I exposed nymphs to long-term normoxic or hypoxic temperature ramps, with ecologically relevant ramping rates and diel thermal variation and measured nymph performance, survival, and morphological and physiological plasticity. Overall, both temperature and exposure duration, but not oxygen, affected the long-term performance and survival of nymphs. Individuals acclimated strongly throughout the long-term ramp, however, allowing them to mitigate the effects of exposure duration and to cope better with chronically hot and hypoxic conditions. Collectively, my research shows that while temperature-oxygen interactions can reduce the survival of aquatic insects under some scenarios, plasticity allows nymphs to strongly mitigate their effects. Indeed, because of plasticity, individuals may be more resilient to immediate consequences of oxygen-limitation than previously recognized.

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© Copyright 2023 Jackson H. Birrell