Energy available for life history allocations or investments into growth, reproduction, activity, and somatic maintenance depends on how much and what type of resources organisms acquire from the environment, combined with how acquired resources are processed through metabolic pathways. Consequently, both environmental (e.g. resource availability) and physiological (e.g. metabolic) constraints on energy availability directly contribute to the maintenance of widescale variation in life histories across the tree of life. Variation in life history spans a single continuum from a fast (fast growth, early reproduction, and short lifespan) to slow (slow growth, late reproduction, and long lifespan) “pace-of-life” that shapes population and ecosystem dynamics. The timing of life history allocations evolves to maximize fitness based on environmental conditions, but environments are becoming more variable, due to global climate change. Organisms may be able to adapt to novel environments by changing their life history but, predicting if and how life history evolution will buffer species from consequences of global change requires a mechanistic understanding of life history, motivating this dissertation work. Decades of work has investigated the roles of energetic constraints in shaping life history allocations, but environmental constraints on resource acquisition and physiological constraints on metabolism are often considered independent of one another. Therefore, complex interactions between resource acquisition, metabolism, and life history allocations, along with their role in governing life history evolution, remains uncharacterized in any single system.

This dissertation sheds new light on the mechanistic underpinnings of life history by testing the central hypothesis that to meet changing energetic demands of life history allocations through the life cycle, coordinated shifts in resource acquisition and metabolism are required, and therefore underpin the expression of alternative life histories. While large-scale diversity in life histories is found across the tree of life, variation in life history strategies is also present among individuals in species with life history polymorphisms. I leveraged one model system with a life history polymorphism, wing-polymorphic field crickets (Gryllus spp.). Within populations of wing-polymorphic field cricket species, individuals are either flight-capable (long-winged, LW) or flightless (short-winged, SW) in early adulthood. The polymorphism is maintained by an allocation-based trade-off between flight and oogenesis: long-winged crickets preferentially allocate resources to support flight muscle maintenance and somatic fat storage, enhancing dispersal capability, while short-winged crickets preferentially allocate resources to support oogenesis at a younger age, enhancing early life fecundity. Both long- and short-winged forms (morphs) co-occur within populations and the polymorphism is present in multiple independently evolving lineages of field crickets. Thus, this system provides a powerful model in which we can investigate mechanisms of life history evolution acting on genetically similar organisms, both within and across species. I studied two closely related wing-polymorphic cricket species, the variable field cricket (Gryllus lineaticeps) (Chapter 1 and 3) and the sand field cricket (Gryllus firmus) (Chapter 2 and 3). The variable field crickets (G. lineaticeps) that I worked with were derived from wild-collected individuals and maintained in a genetically polymorphic laboratory population, providing morphs with a mixed genetic background and naturally segregating combinations of alleles that evolved under natural selection. In complement, the sand field crickets (G. firmus) that I worked with were derived from family lines following long-term artificial selection on wing-length, which maximized phenotypic and genetic differences between flight-capable and flightless crickets. In these species, I advanced our understanding of the behavioral and physiological basis of life history, by characterizing the associations between resource acquisition, metabolism, and life history allocations in crickets investing in either flight or reproduction. To do so, I applied an integrative approach that spanned levels of the biological hierarchy (whole organism, tissues, cells, and organelles) on variable timescales (within and across life stages). Combined, my findings suggest that both resource acquisition and metabolism are fine-tuned based on energetic demands of life history, and thus represent fundamental processes that have evolved to enable organisms to alter the timing of major life history events.

In Chapter 1, I disentangled the reciprocal interaction between resource acquisition and life history allocations, by using a geometric framework of nutrition experimental approach. I hypothesized that high demands of biosynthesis for life history allocations drive elevated resource acquisition requirements and increase performance costs of resource limitations. To test this hypothesis, I characterized resource acquisition (dietary preferences and intake) and life history allocations, across the juvenile to adulthood transition in both male and female variable field crickets (Gryllus lineaticeps), when feeding on artificial diets varying in nutrient composition (carbohydrate- or protein-biased). I used Gryllus lineaticeps crickets recently derived from natural populations for this study, to avoid potential effects of long-term lab adaptation on nutritional preferences and feeding behavior. Consistent with my hypothesis, long-winged crickets increase feeding as juveniles to meet elevated caloric requirements associated with flight muscle synthesis. However, nutritionally imbalanced diets did not affect allocations to flight because long-winged juveniles behaviorally adjusted feeding to meet caloric requirements irrespective of diet composition and palatability. As adults, males ate less than females and life history allocations of males were insensitive to diet, due to lower energetic costs of reproduction for males compared to females in early adulthood. In females, the onset of adulthood was accompanied by a shift in dietary preferences to more protein-biased diets in short-winged crickets investing in large scale and rapid oogenesis, but not in long-winged crickets investing in flight. Unlike juveniles, adult females did not fully behaviorally compensate for variation in diet composition and incurred large caloric and protein deficits on a carbohydrate-biased diet. This in turn resulted in a constraint on reproductive investment by short-winged females; on carbohydrate-biased diets, short-winged females could no longer meet biosynthetic demands of oogenesis, resulting in small ovaries. This finding broadly suggests that when energetic and nutrient demands of life history allocations are large, behavioral adjustments in feeding may not be sufficient to buffer organisms from energetic constraints caused by resource limitations. Additionally, on a carbohydrate-biased diet, ovary size of short-winged and long-winged crickets was similar suggesting that without sufficient protein acquisition, the adaptive benefit of higher early life fecundity associated with the typical short-winged life history strategy is lost. Together these findings reveal that strong associations between resource acquisition and life history allocations dictate fitness consequences of reductions in environmental resource availabilities in a life stage, sex, and life history specific manner.

In Chapter 2, I focused on the association between metabolic and life history evolution. In the sand field cricket, Gryllus firmus, I determined how high energetic demands of life history allocations to dispersal-capability drives the evolution of larger metabolic capacities through biochemical and molecular changes in tissues and organelles. I hypothesized that tissue-specific increases in either mitochondrial content or function result in larger metabolic capacities and improved locomotor performance in flight-capable long-winged compared to flightless short-winged crickets, due to the high aerobic demands of flight. I measured a suite of metabolic traits across levels of the biological hierarchy, from organismal metabolic rates and running performance in vivo, to metabolic function of tissues and organelles in vitro. When running on a treadmill, flight-capable long-winged crickets reach higher maximal metabolic rates and had an enhanced endurance compared to flightless short-winged crickets. Consistent with my hypothesis, long-winged crickets also exhibited increases in mitochondrial content of dorsoventral flight muscle and modifications to the electron transport system composition within mitochondria of the fat body, a tissue responsible for fuel storage and mobilization, which resulted in enhanced mitochondrial bioenergetic capacities. Therefore, metabolic pathways are remodeled in a tissue-specific manner to increase energy production capacities required for high levels of activity. Since this study was conducted in sand field crickets (Gryllus firmus) artificially selected for flight-capability or reproduction, I further concluded that differences in metabolic capacities arose in response to selection on a single life history trait, dispersal-capability. Overall, this work provides fresh insight into the links between life history and metabolism, suggesting that dispersal capability should be explicitly considered as a potential factor driving the evolution of metabolic capacities.

In Chapter 3, I further investigated the interaction between metabolism and life history by determining if mitochondrial function and life history allocations change concurrently across early adulthood. I hypothesized that energetic requirements increase with biosynthetic demands of life history and elicit concurrent elevations in mitochondrial respiratory function. To test this hypothesis, I assessed life history allocations and mitochondrial function across the first week of adulthood, in both long- and short-winged variable field (Gryllus lineaticeps) and sand field (Gryllus firmus) crickets. In both species, mitochondrial function of morphs diverged through time, along with their allocations to either flight or reproduction. In long-winged crickets, mitochondrial bioenergetic capacity for energy production increases through early adulthood, peaks at the time of dispersal, but rapidly declines when a switch to reproduction is made post-dispersal. In short-winged crickets, mitochondrial bioenergetic capacity declines gradually as ovarian synthesis is completed. Despite different evolutionary histories (Gryullus firmus- artificially selected vs. Gryllus lineaticeps- genetically admixed backgrounds), patterns of mitochondrial plasticity were consistent between species, suggesting that mitochondrial plasticity is likely an important general adaptation shaped by life history evolution. Overall, these findings support the conclusion that adaptive plasticity in mitochondrial function enables shifts in life history allocations, advancing our understanding of the metabolic basis of life history.

Tardigrades are microscopic, aquatic animals that are an emerging model system for studying physiological responses to a wide range of environmental stressors. Past research has explored tardigrades’ extreme tolerance to desiccation and radiation, but little is known about the phylum’s tolerance to subzero temperatures, especially under more ecologically relevant conditions. Here, I use Hypsibius exemplaris—a lab-friendly cosmopolitan species, with a publicly available genome—as a model species to characterize the physiological response of active, hydrated tardigrades to low temperatures ranging from 1°C to -20°C. I develop methods to characterize numerous cold tolerance phenotypes in tardigrades, through multiple lenses: high-accuracy measurement of survival, plasticity of chill coma, and comparative proteomics and ice-affinity purification of cold-treated tardigrades (aimed at identifying underlying molecular mechanisms of cold tolerance and putative tardigrade-specific ice-binding proteins).

In Chapter 1, I develop new methods to score mortality of cold-exposed tardigrades using the live/dead cell-marker SYTOX Green, to improve the accuracy of survival-based assays. I determined that visualization of SYTOX Green uptake was a more accurate and more specific method of determining mortality in tardigrades, compared to the traditional methods of scoring survival based on locomotion. This new method for determining survival allows for higher throughput of scoring tardigrade phenotypes and provides differentiation of live, dead, and injured tardigrades (showing localized regions of damage in the tardigrade body-plan, as they first experience cellular death). Using this new assay, I monitored survival of hydrated mixed-staged adult H. exemplaris over a range of acute exposure times (2-120 hours) and subzero temperatures (-10°C, -15°C, and -20°C), in order to dissect the impacts of time, temperature, and their interaction on cold tolerance. Tardigrades exposed to -10°C showed little SYTOX Green uptake and high survival across all timepoints, and samples supercooled rather than froze. Survival of tardigrades exposed to -15°C was highly variable, and depended on the water-state of the cold exposed samples (liquid or frozen). When exposed to -20°C, tardigrade survival declined exponentially with increasing exposure time and nearly all samples froze. Although I predicted that freezing may drive tardigrade mortality based on these results, I surprisingly found that tardigrades incubated with the ice-nucleating bacteria Pseudomonas syringae had significantly improved survival after cold exposure, illustrating the importance of ice-formation dynamics and environmental microbes. Lastly, a 3-week thermal acclimation of tardigrades to mild cold (1°C & 4°C) did not significantly improve survival to low temperature, while acclimation to 15°C (vs the standard culture condition of 20°C) did. This work suggests that H. exemplaris is sensitive to ecologically relevant cold and assays developed here will be useful for exploring the thermal physiology of additional tardigrade species. More so, evidence collected here suggests that this tardigrade species is capable of surviving environmental freezing, under certain ecologically relevant conditions.

The goal of Chapter 2 was to explore cold tolerance phenotypes of tardigrades at low but non-freezing temperatures. Although no such phenotype was documented in past tardigrade literature, I predicted that tardigrades would experience a state of reversible paralysis known as chill coma, with exposure to sufficiently low temperatures and exposure times. Chill coma is often a non-lethal phenotype commonly seen in insects, but it nonetheless can impact the ecology and overall fitness of an organism living in winter conditions, as it prevents individuals from feeding, reproducing, and evading predators. Chill coma has been demonstrated to be a plastic phenotype in response to a variety of environmental conditions, as seen in insects. I predicted that the amount of time required for tardigrades to recover from chill coma (known as chill coma recovery time or CCRT) would vary with low temperature and exposure time. By analyzing video recordings of adult H. exemplaris recovering from cold-exposure, I observed that tardigrades experience a cold-induced state of reversible paralysis indicative of chill coma after 12h at 1°C. The impact of longer mild cold exposures depended on temperature: above -1°C, recovery times decreased with increasing duration of exposure, indicating that beneficial acclimation may have occurred. Below -1°C to -4°C, recovery times increased with increasing exposure duration, suggesting that sublethal damage may have accumulated. I also observed that chill coma recovery time was plastic depending on photoperiod acclimation. Tardigrades that were acclimated to shorter-day photoperiods (winter-like) for 2 weeks had significantly faster CCRTs than tardigrades acclimated to longer-day photoperiods (summer-like). In summary, this chapter characterizes a new cold tolerance phenotype in tardigrades, as well as its plasticity. Determining the underlying mechanisms that control the chill coma plasticity in tardigrades will further illustrate similarities or differences between the insect and tardigrade nervous system, in future work.

The goal of Chapter 3 was to characterize how the tardigrade proteome responds to cold exposure, as well as to discover and functionally verify candidate proteins that help tardigrades perturb freeze damage—especially novel candidate ice-binding proteins. Here, I used comparative proteomics to broadly determine how the protein repertoire shifts in H. exemplaris due to ecologically relevant cold conditions. Cohorts of 1000+ adult tardigrades were exposed to a cold treatment down to -10°C (or a control treatment of 20°C), and proteins were extracted from whole-animals via optimized sample preparation methods for data-independent acquisition mass spectrometry (DIA-MS). In a parallel set of samples, I performed a specialized ice-affinity purification method (IAP) and DIA-MS to enrich for and identify proteins that bind to ice. I predicted that tardigrades utilize a wide range of biochemical mechanisms related to cold tolerance, as seen in insects, as well as IBPs or other cold tolerance proteins that are unique to the phylum, with novel structures or functions. Gene ontology (GO) enrichment analysis of proteins are significantly more abundant after cold illustrated changes in neuron and neuromuscular function, calcium ion balance, cell division processes (such as apoptosis), and glucose metabolism. GO enrichment of proteins less abundant after cold treatment suggested changes in lipid metabolism, cuticle production, development, and redox. Ice-affinity purification and a bioinformatics pipeline identified a ranked list of novel candidate ice-binding proteins that tardigrades may utilize to survive freezing. This study is the first comparative proteomics study exploring the impacts of cold stress, in any eukaryotic animal, and results provide a framework for what molecular mechanisms tardigrades may use to survive cold.

This dissertation fills a gap in knowledge about the fundamental physiology and molecular mechanisms that an especially cold-hardy microscopic organism (tardigrades, specifically Hypsibius exemplaris) uses to survive subzero temperatures. Development of new phenotypic assays allowed for the rigorous characterization of the plasticity of tardigrade survival and chill coma, in response to time, temperature, and their interaction. Despite tardigrades’ reputation for being virtually indestructible, I determined numerous time and temperature-dependent conditions that result in increased mortality and reduced performance (CCRT), that may impact tardigrade ecology and winter-time behavior. Interestingly, I also determined that tardigrade’s survival to environmental freezing improves drastically when animals are exposed to ice-nucleating microbes, which may be found in their natural environments. Finally, I determined a number of molecular mechanisms that may underlie cold tolerance in tardigrades, as illustrated by the first-ever comparative proteomics study of any animal in response to cold, along with generating a ranked list of novel, tardigrade-specific candidate ice-binding proteins. Overall, I hope that the tools and findings of this work will help establish tardigrades as an increasingly important and tractable model system, in the field of comparative physiology and beyond.

Global climate change is altering not only average temperatures, but also the distribution and abundance of precipitation. During winter in montane habitats, changing precipitation patterns mean changing snow fall, which will have profound ecological impacts. Snow is an effective insulator and decouples below-snow temperatures from air temperatures, providing a stable environment and determining microclimate conditions in the soil. A wide range of organisms take advantage of stable below-snow winter environments by overwinter in soil. Winter soil microclimate modulates the degree of stress, performance, and ultimately fitness for organisms that overwinter in the soil. During the coldest portions of winter, the insulating effect of snow protects organisms from potentially damaging or even lethal cold stress. However, protection from cold extremes comes at a cost: in ectothermic animals, energy consumption is dependent on body temperature, and so relatively warm below-snow conditions drain energy stores more quickly. In winter resources are limited and energy stores cannot be replenished, and complete depletion of energy stores can cause mortality. Even if organisms survive, post-winter fitness may also be impacted by energy depletion by reducing reserves available for growth and reproduction. We don’t yet understand the impacts of changing winter snow will have on overwintering organisms in montane environments. I hypothesize that winter snow cover, through regulation of winter microclimate, modulates the cause of stress and mortality (cold damage or energy depletion) for montane ectotherms overwintering in soil. By better understanding the impacts of changing winter snow cover on montane ectotherms, we may be better able to identify processes and populations that are vulnerable to climate change.

My dissertation aims to expand our understanding of the impact of snow cover on overwinter stress on insects that overwinter in the soil, by examining the impact of snow cover along elevation gradients, then extending the energy use model developed in Chapter 1 to test for impacts of plasticity. Finally I examine how overwinter conditions alter physiological processes upon emergence from overwintering. This work shows that winter snow cover has far reaching impacts on overwintering ectotherms, which are not equal across elevation, and have effects that can carry over into the growing season, expanding our understanding of changing winters and population impacts of climate change.

Chapter 1 establishes the energetic impacts and thermal challenges of changing snow cover along elevation gradients. Along elevational gradients snow cover increases but air temperature decreases, and it is unknown how these opposing gradients impact performance and fitness of organisms overwintering in the soil. I developed experimentally validated ecophysiological models of cold and energy stress over the past decade for the montane leaf beetle Chrysomela aeneicollis, along five replicated elevational transects in the Sierra Nevada mountains in California. Cold stress peaks at mid-elevations, while high elevations are buffered by persistent snow cover, even in dry years. While protective against cold, snow increases energy stress for overwintering beetles, particularly at low elevations, potentially leading to mortality or energetic trade-offs. Declining snowpack resulting in drier winters, will predominantly impact mid-elevation populations by increasing cold exposure, while high elevation habitats may provide refugia.

Chapter 2 focuses on the impacts of plasticity in metabolic-rate-temperature relationships in dormant organisms and their impacts on energy use estimates, expanding on the model developed in Chapter 1. Ecophysiological energy use models predict long-term energy use from metabolic rates, but we do not know the degree to which plasticity in metabolic intensity or thermal sensitivity impact energy use estimates. I quantified metabolic rate-temperature relationships of dormant willow beetles (Chrysomela aeneicollis) monthly from February to May under constant and variable acclimation treatments. Metabolic intensity increased through time, and acclimation altered both metabolic intensity and the thermal sensitivity. However, incorporating these two types of metabolic plasticity into energy use models did not improve energy use estimates, validated by empirical lipid measurements. Together, this indicates that while metabolic rate temperature relationships are plastic during winter, incorporating this plasticity does not improve prediction of energy use made by ecophysiological models, partly due to large individual variability in energy reserves.

Chapter 3 examines the impact that overwintering environment plays on prioritization of physiological processes upon emergence from dormancy, based on changes in gene expression. During winter, dormant organisms conserve resources through metabolic suppression and minimizing cellular processes. The transition from dormancy to active season requires a quick reversal of these processes and large-scale physiological transitions, enabling organisms to begin exploiting favorable environmental conditions. The impact of overwinter microclimate on the physiological processes involved in transitioning from dormancy to active season in insects are unknown. I conducted a field snow-cover manipulation and then profiled gene expression of willow leaf beetles Chrysomela aeneicollis during the transition out of dormancy using RNA-seq. Upon emergence from dormancy, beetles first prioritize up-regulation of transcripts associated with digestion and nutrient acquisition. The prioritization of nutrient acquisition is followed by investment into reproduction, but the timing is sex-specific with females investing sooner. Winter snow cover impacted the timing of these processes, with beetles that overwintered below snow being several days ahead of beetles that overwintered without snow. This highlights the importance of winter microclimate in regulating critical life history transitions.

Seasonality shapes major life-history strategies and adaptations across the tree of life. The alternating nature of seasons for growth and reproduction and seasons of adverse conditions can put conflicting selective pressures on energy use. Growing seasons often drive evolution of fast-paced processes and activity, while adverse seasons like winter drive evolution of dormancy and energy conservation. Thus, energy use strategies in seasonal environments have profound impacts on fitness. Integrating fitness across seasons and organismal responses to environmental change is key to understanding how populations will respond to ongoing global climate change. Energetics can play two essential roles in mediating organismal fitness in seasonal environments: energy reserves can determine winter survival, and energy at the end of winter can determine reproductive success. Survival and reproduction are both critical components of fitness, and thus energy allocation strategies may favor one over the other, revealing potential trade-offs when energy is limited. However, we do not know how environmental variation and predictability affect energy allocation strategies. Understanding the impacts of environmental change in seasonal environments will help us make better predictions of population responses and vulnerability to climate change.My dissertation aims to expand our understanding of overwintering energy use strategies, then explore how variable environments affect energy allocation strategies under a trade-off between maintenance and future reproduction. Finally, I test my predictions of energy allocation strategies under realistic winter conditions. Winter performance can determine summer performance, linking life cycles in seasonal environments. This work reveals the extent to which stochastic and predictable variation in seasonal environments can affect selective pressures and thus fitness in overwintering organisms. The carry-over effects of winter fitness into the growing season and vice versa expand our understanding of population responses to variable and changing environments. Chapter 1 explores the role of group behavior on energy conservation in overwintering aggregations, a potential energy use strategy that remains widely underexplored. The convergent ladybeetle (Hippodamia convergens) overwinters in massive aggregations, making it an ideal system for testing the effect of aggregation size on metabolic rates in overwintering insects. I measured energy use and thermal sensitivity of beetle aggregations across two ecologically relevant temperatures, and measured locomotor activity as one possible driver of group effects on energy use. Metabolic rates per beetle decreased with increasing aggregation size and scaled hypometrically with mass at both temperatures tested, with responses more pronounced at low temperatures. Activity decreased with aggregation size, but only at low temperatures. These results suggest that individuals within aggregations enter a deeper metabolically inactive state that single individual beetles cannot achieve, which is partly but not completely explained by a reduction in locomotor activity. This behavioral strategy for energy conservation may provide an additional selective advantage for the evolution of large overwintering aggregations. Chapter 2 explores the impact that variation in predictable and stochastic environments has on an energy allocation trade-off between somatic maintenance and future reproduction. Seasonality can modulate selection of energy use strategies, but we currently lack a theoretical framework to generate predictions on energy allocation strategies under changing stochastic conditions. Stochastic extreme events, like heat waves or cold snaps, are increasing in frequency and can have major impacts on population fitness. Overwintering organisms can prepare for harsh conditions by investing energy into stress tolerance (somatic maintenance). However, energy reserves at the end of winter can also influence future reproductive fitness, depending on the relative importance of stored versus recently acquired reserves for reproduction across the capital to income breeding continuum. In this chapter, I develop a general theoretical model that assesses fitness under increasing probability of stochastic lethal events, temporal variation in probability of stochastic events through winter, and how a stochastic end of winter affects optimal allocation strategies in income and capital breeders. Increasing probability of extreme events modulates allocation strategies in capital breeders. Temporal variation in extreme events and stochastic end of the season can alter optimal timing of investments, offering insights on the mismatch costs that changing environments may impose on overwintering organisms. These results highlight the role of end reserves in seasonal transitions and identify capital breeders to be especially susceptible to climate change. Chapter 3 tests the predictions from Chapter 2 by applying it to a case study of a capital breeder, the beetle Chrysomela aeneicollis, and answering how variation in snow cover can affect a trade-off between winter survival and future reproduction in realistic scenarios. Variation in snow cover modulates stochastic cold events and baseline energetic costs for organisms overwintering underground. Winters with deep snow cover have high baseline winter costs while having a low risk of lethal cold events. On the other hand, winters with little or no snow cover have low baseline winter costs but organisms are exposed to a high risk of lethal cold. In this chapter, I build on Chapter 2 by incorporating organism-specific parameters for energy reserves, rates of energy use, and microclimate conditions. I develop a model to systematically explore how cold risk, baseline winter costs, and their interaction under realistic winter scenarios affect energy use, survival, and allocation strategies of the willow leaf beetle (Chrysomela aeneicollis). My results support conclusions from Chapter 2, as increased cold risk and baseline costs can drive allocation strategies, but the response is strongly dependent on starting reserves. Increased cold risk affects future reproduction of beetles starting winter with high reserves, while increased baseline costs affect both reproduction and survival for lean beetles. Under realistic winter conditions, snow cover can modulate the trade-off between winter survival and future reproduction. Overwintering organisms under deep snow cover show high survival at the cost of reproduction, while winters with low snow favored reproduction at the cost of survival. These results show that both starting reserves and winter conditions, in the form of cold risk and baseline costs, drive distinct energy allocation strategies, and offer new insights into population dynamics and predictions under climate change.

Freshwater streams are incredibly variable habitats where abiotic conditions change daily and seasonally. The species that inhabit streams must have the physiological capacity to deal with that variation. Variation that extends outside of the limits of an organism’s tolerance thresholds can cause sub-lethal or lethal stress and dictate which habitats are suitable to the organism. In addition to variation in abiotic conditions, organisms must fit into the ecological interaction web of their habitat and handle variation in biotic conditions. In my dissertation, I focus on two pathways for organisms to encounter new sources of environmental variation: invasion of novel habitats and climate change. Understanding how organisms deal with environmental variation is critical to predicting where they can live now and in the future.

In Chapter 1, I aimed to identify the temperature and oxygen conditions that limit respiration and locomotion in the highly invasive New Zealand mud snail, Potamopyrgus antipodarum. To accomplish this, I generated thermal performance curves for resting respiration rate and voluntary locomotor behaviors under normoxia and hypoxia to find the conditions that limited each performance. I found that extremely high and low temperatures limited respiration and activity, but respiration rate was more sensitive to changes in oxygen at low temperatures. Hypoxia did not significantly decrease activity. Under hypoxic conditions, activity was less thermally sensitive, increased under high temperatures, and may be fueled by anaerobic metabolism. Relying on anaerobic energy is a time-limited survival strategy, so further warming and deoxygenation of freshwater systems may limit the spread of this invasive species.

In Chapter 2, I focused on finding the abiotic and biotic conditions that explain variation in Potamopyrgus antipodarum population density in San Francisco Bay Area streams. P. antipodarum is a prolific invader of freshwater streams in California and worldwide, with the ability to form extremely dense populations that dominate the invaded habitat. To strengthen our ability to predict where this species can thrive, it is critical to understand which abiotic and biotic conditions explain variation in population density. I approached this need by measuring 25 conditions over one year in three streams with varying densities of P. antipodarum populations. The different streams could be distinguished by their water chemistry profiles. Conductivity and filamentous green algae cover were the factors most correlated to population density, but there was only marginal linear statistical support for the response to algae. Overall, P. antipodarum is robust to considerable variation in many ecological factors, and each factor may have a small effect in controlling their population sizes.

In Chapter 3, I focused on the effect that past environmental variation, specifically thermal history, had on the response to different warming regimes in the larval caddisfly Dicosmoecus gilvipes. To accomplish this, I tested the molecular physiology responses of three populations of field-acclimatized D. gilvipes from different eco-regions (mountain, valley, coast) to current and future warming scenarios. I hypothesized that distinct thermal histories could differentiate populations at baseline levels of gene expression and gene expression changes in response to daily warming and heat shock. Population-specific responses were apparent under the control and daily warming conditions, while responses to heat shock were similar across populations. In addition, underlying gene expression patterns in the daily warming and heat shock treatments were different. These results highlight the importance and limitations of measuring the stress response of wild-caught organisms in their natural environment.