Amphibians are influenced by climate change, but we do not have a clear understanding of how changes in temperature, precipitation, or both, may influence amphibian larvae in temperate regions. Do amphibian larvae have similar developmental responses to increased temperature and increased drying rates of wetlands - both plausible effects of climate change? What influence do the interactive effects of temperature and drying have on the relationships between the larval responses themselves (e.g., survival, body size, and time to metamorphosis)? To address these questions, we studied larval responses of two amphibian species, wood frogs (<em>Lithobates sylvaticus</em>) and spring peepers (<em>Pseudacris crucifer</em>), to simulated warming and drying in experimental ponds. Over 13 weeks, we manipulated temperature and water levels in ponds to produce 4 treatments: control, drying, warming, and drying + warming. Our manipulations created warming treatments that were on average 2 ° C higher than controls, and our drying treatments decreased in water depth by 2.5 cm each week compared to warming and control treatments that held a consistent amount of water. In both species, warming treatments resulted in significantly earlier timing of metamorphosis, and drying treatments resulted in significantly reduced body size. We saw a negative relationship between body size and time to metamorphosis (i.e., individuals that metamorphosed faster generally had larger body sizes), indicating an unexpected decoupling of the typical positive relationship between time to- and size at metamorphosis. The strength of the relationship between responses also varied by treatment for wood frogs but not spring peepers, indicating that the responses of larval amphibians to climate change may vary among species. Our study reveals complex relationships among larval survival, body size, and time to metamorphosis and highlights the need for considering not only the role of interacting climate-related pressures on amphibians but also the mechanisms underlying coupling of larval responses to these pressures. We encourage future research and discussion on a better understanding of why different climate pressures caused different responses, and if these patterns may be consistent in other aquatic species. / Master of Science / Across the globe, shifts in temperatures and the availability of freshwater habitats due to climate change are presenting challenges as well as opportunities for many species, particularly those that rely on freshwater habitats to complete their life cycle. Climate change is leading to warmer water temperature and accelerated drying of wetlands and ponds. Warming and drying often occur simultaneously, yet our understanding of how warming and drying may interact and affect sensitive aquatic species is limited. Amphibians with an aquatic life stage (for example, frog tadpoles) are particularly vulnerable to the effects of climate change on wetlands and ponds because they must transform from swimming larvae to land-dwelling adults before aquatic habitats dry out. Warming and drying help amphibian larvae determine when to start that process, called metamorphosis. For this reason, amphibian larvae in aquatic habitats are especially vulnerable to shifts in water temperature and the timing of drying. In this thesis, I explore how warming and drying influence amphibian survival, body size, and time to metamorphosis. To better understand the responses of amphibian larvae to warming and drying, I tested the effects of warming and drying on three response variables: amphibian survival, body size, and time to metamorphosis. I used two different species, wood frogs and spring peepers, to determine whether frogs' responses vary among species. I created an artificial pond experiment where I filled large tanks to represent natural ponds in a controlled, outdoor setting. In these artificial ponds, I measured wood frog and spring peeper growth under experimentally increased water temperatures and accelerated drying levels over 13 weeks. I found a negative relationship between body size and time to metamorphosis, suggesting individuals who spent less time in the water as larvae were more likely to be larger than individuals who spent more time in the water as larvae. Additionally, ponds with higher larval survival were associated with larger body size and a shorter time to metamorphosis. Warmer water temperatures led to a shortened time to metamorphosis but did not always lead to higher body sizes. Accelerated drying did not lead to a shortened time to metamorphosis, but it did lead to smaller body sizes in both species compared to control and warming ponds. Overall, I found complex relationships among larval responses with the directions of responses varying between treatments and species. This highlights the need for considering the role of climate-related changes in the environment (warming and drying) as well as the interactions between specific larval responses to those environmental changes. By understanding how warming and drying influence amphibian larval success, we can make a more direct link between climate change and its effects on aquatic larvae. Incorporating the responses between survival, body size, and time to metamorphosis to gain a more complete understanding of amphibian larval responses to the changing climate is an important step toward conserving and protecting freshwater aquatic species.

Body temperature affects physiological processes and, consequently, has a large impact on fitness. Lizards need to thermoregulate behaviourally to maintain their body temperature within a range that maximizes performance, but there are costs associated with thermoregulation. The thermal quality of an environment is a major cost of thermoregulation because it directly affects the time and energy that must be invested by an individual to achieve and maintain an optimal body temperature for performance. According to the cost-benefit model of thermoregulation, lizards should only thermoregulate when the benefits outweigh the costs of doing so. Thus, in habitats of poor thermal quality, individuals should thermoregulate less. Using two systems, an elevational gradient and a pair of habitats that vary in the amount of solar radiation they receive, I tested the hypothesis that investment in thermoregulation by lizards is dictated by the associated costs of thermoregulating. Temperature, and thus thermal quality, decreases with elevation. I found a significant positive relationship between elevation and effectiveness of thermoregulation of Yarrow’s spiny lizards (Sceloporus jarrovii). When comparing thermoregulation of ornate tree lizards (Urosaurus ornatus) living in the thermally superior open-canopy wash habitat or the closed-canopy upland habitat, I found that habitat type was a significant predictor of accuracy of body temperature. In the poorer quality habitat, lizards had smaller deviations of body temperature from their preferred temperature range. Overall, I conclude that the thermal quality of a lizards’ environment impacts their thermoregulation in the opposite direction than predicted by the cost-benefit model of thermoregulation. This suggests that the disadvantages of thermoconformity may be greater than the costs thermoregulating as habitats become more thermally challenging.

The effect of temperature on predator and herbivore consumption is an important factor for predicting the effects of climate warming on ecosystems. The Metabolic Theory of Ecology (MTE) describes the temperature dependence of biological and ecological rates and states that metabolism is the fundamental biological mechanism that governs most observed patterns in ecology. This statement has been criticized empirically for a number of organismal traits and systematic deviations have been found. Here, a meta-analysis is performed on published temperature responses of ectotherm consumption. The mean effect of temperature on consumption was higher than the mean value predicted by proponents of the MTE and was highly variable. Some of this variation is explained by habitat type, where the consumption rates of marine organisms displayed stronger temperature dependence than for terrestrial and freshwater organisms. The frequency distribution of temperature dependencies is right skewed for consumption. Here, this skewness is explained by a methodological artefact as values close to “no effect” are more unlikely to be sampled than others when fitting the Arrhenius equation. In conclusion, the assumptions of the MTE do not hold for rates of consumption and marine organisms display a stronger temperature dependence compared to terrestrial and freshwater organisms.

Explaining spatial and temporal variation in the abundance of species is one of the primary goals of ecology. Habitat selection, the behaviour that organisms use to choose habitat patches that maximize fitness, can explain patterns in abundance between patches at small spatial scales within the dispersal capacity of the species. However, habitat selection models assume there is a reduction in individual fitness as population density increases due to increased competition between individuals. Ectotherms, which often select habitats based on temperature, a density-independent resource, may not display density-dependent responses if temperature limits energy assimilation more than finite food resources limit energy acquisition. As predicted by their dependence on environmental temperatures, some ectotherms select habitat largely independently of population density when temperatures are far from the optimal temperature for performance. But, is density-dependence prevalent in ectotherm populations when temperatures are close to the optimal temperature for performance? Habitat selection models also assume that all individuals of a population exhibit the same strategy for maximizing fitness through habitat selection. However, differences in morphology and behaviour (e.g., reproductive strategy) can modify the optimal habitat selection strategy for different phenotypes. Finally, observed patterns in habitat selection and abundance can also be modified by competition with other species. Quantifying the relative importance of these different factors that affect habitat selection behaviour will improve our ability to predict the spatial distribution and relative abundance of organisms. The objective of my thesis was to explain spatial variation in the abundance of ectotherms, using the ornate tree lizard (Urosaurus ornatus) as a study species. In chapter one, I tested whether density-dependent habitat selection explained patterns in abundance and fitness of lizards between two habitats differing in suitability. In chapter two, I tested whether density dependent habitat selection in tree lizards was caused by intraspecific competition for food that limited body size and growth. In chapter three, I tested whether variation in reproductive strategy, as indicated by throat colour phenotype, affected space use and habitat selection in male tree lizards. Finally, in chapter four, I tested whether interspecific competition with another lizard species affected habitat selection, fitness, and abundance of tree lizards. My thesis emphasizes the importance of intraspecific competition in shaping patterns of habitat selection and abundance in terrestrial ectotherms. I show that habitat selection is strongly density-dependent despite differences in thermal quality between habitats. I show that density-dependent mortality and growth lower the fitness of individuals when populations reach high densities, and this likely caused habitat selection to be density-dependent. Despite this evidence for density-dependent habitat selection, I show considerable variation between individuals in habitat selection and space use. Males with different throat colour phenotypes select habitats differently, demonstrating that variation in morphology can influence habitat selection patterns within a population. Finally, I show that interspecific competition with another lizard affects space use and how frequently tree lizards switch habitats, but this does not lead to differences in fitness or in the relative abundance of tree lizards in habitats. Therefore, intraspecific competition for resources was the dominant force shaping the relative abundance of tree lizards in different habitats.

Investigating ecological questions at the scale of individual organisms is necessary to understand and predict the biological consequences of environmental conditions. For small organisms this can be challenging because we need tools with the appropriate accuracy and resolution to record and quantify their ecological interactions. Unfortunately, many of our existing tools are only appropriate for medium to large organisms or those that are wide ranging, inhibiting our ability to investigate the ecology of small organisms at fine scales. In Chapter 1, I tested a novel workflow for recording animal movements at very fine spatial and temporal scales. The workflow combined direct observation and the mapping of locations onto high-resolution uncrewed aerial vehicle (UAV) imagery loaded on hand-held digital devices. Observers identified landscape features they recognized in the imagery and estimated positions relative to those features. I found this approach was approximately twice as accurate as consumer-grade GPS devices with a mean and median error of 0.75 m and 0.30 m, respectively. I also found that performance varied across landscape features, with accuracy highest in areas that had more visual landmarks for observers to use as reference points. In addition to sub-meter accuracy, this method was cost-effective and practical, requiring no bulky equipment and allowing observers to easily record locations away from their own location. While this workflow could be used to record locations in a variety of situations, it will be most cost-effective when also using high-resolution environmental data from a UAV. In Chapter 2, I used the workflow described in Chapter 1 to investigate blunt-nosed leopard lizard (Gambelia sila) thermoregulation at fine-scales. Recent research has suggested that the spatial distribution of temperatures is important to consider for ectotherm thermoregulation, but this work has been limited to simple artificial environments. My goal was to investigate this idea in a complex natural system for the first time. I tracked lizard movement and body temperatures at a desert site from May to July 2021. I used machine learning to combine high-resolution environmental data from a UAV with microclimate temperature data to create a model of the spatial distribution of environmental temperatures over time. I found that including information about the spatial distribution of temperatures improved the models of lizard thermoregulatory accuracy and movement rate. Because these response variables are important aspects of ectotherm energetics, this suggests that the spatial distribution of temperatures may be an important, but often overlooked, component of habitat quality. Going forward, identifying better methods to quantify the spatial distribution of temperatures would provide insights into the specific responses of ectotherms to different spatial distributions. In this work I used recent technological advances in UAVs to investigate ecological questions at the scale of a small organism. The methods developed here provide insights into the importance of the spatial distribution of temperatures for a small ectotherm. Further efforts to develop, test and utilize tools for fine-scale ecological research will advance our ability to understand species’ interactions with current conditions and predict their responses to future changes.

Elucidating the factors that drive variation in the abundance and distribution of organisms is central to ecology. Variables that explain the spatial variation in the abundance of organisms primarily include environmental (e.g., temperature and precipitation) and biotic factors (e.g., competition, predation, and parasitism). An important mechanism influencing the spatial distribution of organisms, at least at small spatial scales, is habitat selection. Traditionally, habitat selection theory has assumed that animals select habitat based on their ability to acquire depletable resources within that habitat, especially food. Ectotherms, however, may instead select habitat based on their ability to process food within the habitat, given the strong dependence of body temperature (and performance) on environmental temperature in this group. The major objective of my thesis was to determine whether energy gain, habitat selection, and population density were driven primarily by food availability or by temperature in ectotherms. I used Sceloporus jarrovii lizards as a study species because these lizards occur at high densities and in similar habitat across a broad altitudinal range. In Chapter 1, I tested the prediction, central to the thermal coadaptation hypothesis, that juvenile lizards prefer body temperatures that maximize their net energy gain. I also tested whether lizards shifted their preferred body temperatures to correspond to the optimal temperature for different energetic states, as per Huey’s (1982) energetics model. In Chapter 2, I determined whether the home range size and density of lizards shifted in response to manipulations of food availability and/or thermal quality within a site. In Chapters 3 and 4, I determined whether mean body condition, individual growth rate, and population density were driven by food availability or thermal quality. In Chapter 3, I visited 32 study sites over a 1,550 m altitudinal range within a year; whereas in Chapter 4, I food-supplemented five out of 10 study sites where I performed mark-recapture over a period of three years. Overall, my thesis demonstrates that both food availability and thermal quality of the habitat drive energy gain, habitat selection, and population density. Juvenile S. jarrovii preferred body temperatures that maximized net energy gain, regardless of energetic state. Although they did not shift their preferred body temperature range depending on energetic state, the difference in the optimal temperature for net energy gain between states (0.4°C), may have been too small to warrant a change in behaviour. Within a site, S. jarrovii increased their home range size and occurred at higher densities as natural food availability increased, and decreased their home range size and occurred at lower densities as the thermal quality under the rocks increased. This suggests that S. jarrovii respond to food availability and thermal quality at different scales, selecting territories based on thermal quality and home ranges based on food availability. Over 32 sites, the abundance of S. jarrovii increased with food availability, whereas the mean body condition increased and the rate at which lizards attained their maximum body size decreased with elevation (at lower thermal quality). In the three-year study, mean body condition and individual growth rate decreased and population density increased with thermal quality, but the strength of the relationship depended on natural food availability. Overall, both food availability and thermal quality of the habitat drive energy gain, habitat selection, and population density; however, thermal quality is often the stronger driver. Thus, improvements to habitat selection models should incorporate habitat thermal quality to improve predictions on how ectotherms distribute themselves on a landscape.

Knowledge of the thermal ecology of a species can improve model predictions for temperature-induced population collapse, which in light of climate change is increasingly important for species with limited distributions. Here, we use a multi-faceted approach to quantify and integrate the thermal ecology, properties of the thermal habitat, and past and present distribution of the diurnal, xeric-adapted, and active-foraging Namibian lizard Pedioplanis husabensis (Sauria: Lacertidae) to model its local extinction risk under future climate change scenarios. We asked whether climatic conditions in various regions of its range are already so extreme that local extirpations of P. husabensis have already occurred, or whether this micro-endemic species is adapted to these extreme conditions and uses behavior to mitigate the environmental challenges. To address this, we collected thermoregulation and climate data at a micro-scale level and combined it with micro-and macroclimate data across the species' range to model extinction risk. We found that P. husabensis inhabits a thermally harsh environment, but also has high thermal preference. In cooler parts of its range, individuals are capable of leaving thermally favorable conditions-based on the species' thermal preference-unused during the day, probably to maintain low metabolic rates. Furthermore, during the summer, we observed that individuals regulate at body temperatures below the species' high thermal preference to avoid body temperatures approaching the critical thermal maximum. We find that populations of this species are currently persisting even at the hottest localities within the species' geographic distribution. We found no evidence of range shifts since the 1960s despite a documented increase in air temperatures. Nevertheless, P. husabensis only has a small safety margin between the upper limit of its thermal preference and the critical thermal maximum and might undergo range reductions in the near future under even the most moderate climate change scenarios.

Among species, populations, and individuals, there exists a tremendous amount of variation in how hosts respond to, and are thus protected from parasites. Such variation inevitably affects host-parasite dynamics and ultimately how parasites will move through and evolve in communities. A likely factor in the diversity of immune responses seen in nature are the costs associated with activation of the immune system upon exposure to parasites. Costs can manifest in many ways, including changes in resource usage or metabolism, self-damage from inflammatory reactions, lost opportunities (e.g., foraging reproduction), and often as tradeoffs with other physiological processes. However, we do not yet fully understand the factors that influence costs of immune activation across ecological scales, nor the relationship between immune costs and protection from parasites, despite the common assumption that greater costs equates to better protection. For my dissertation, I have investigated large-scale drivers of immune costs, specifically whether life history and/or body mass influence costs of immune activation (Chapter 1), whether magnitude of parasite exposure affects immune activation costs at the population level (Chapter 2), and the relationship between costs of immune activation and benefit in terms of parasite protection (Chapter 3). Across taxa, immune costs are likely to be affected by host life history traits such as longevity and reproductive scheduling such that long-lived and slow to mature species (i.e., slow-paced) should experience lower costs than animals that die comparatively early (i.e., fast-paced). By reducing immune costs, slow-paced species could reduce the accumulation of damage associated with repeated activations of the immune response that could reduce successful reproduction over a long life. Likewise, body mass should also be an important determinant of immune costs as physical size should affect the extent to which hosts are exposed to parasites as well as their ability to combat infection. For the first chapter, I used meta-analysis to determine whether life history traits (lifespan and time to maturity) and/or body mass affected functional costs of immune activation (e.g., changes in performance, food intake, growth, mass, reproductive effort/success, survival) across taxa. The results of this study showed that, in general, animals incur costs of immune activation and that costs are influenced by life history and body mass such that species that are relatively long-lived experience greater costs of immune activation than species that are relatively short-lived. We also found that small species experienced relatively greater costs than large species. Such patterns may arise because long-lived species may have been selected to endure high costs of immune activation to develop more robust adaptive responses in order to extend lifespan. In addition, small animals may experience greater costs than large animals because of a higher cell turnover rate potentially resulting in greater self- damage (via oxidative damage). While costs of immune activation can be broadly predicted by lifespan and body mass, smaller- scale factors, such as magnitude of exposure to parasites are also likely important drivers of immune costs within populations. Indeed, if costs increase linearly with magnitude of exposure, then at a certain level of parasite burden, costs may become too great for the host and selection may favor hosts that tolerate, rather than eliminate infection. In the second chapter, I investigated how exposure to multiple concentrations of Salmonella lipopolysaccharide (LPS), a immunogenic component of Gram negative bacteria cell walls, affected immune costs in brown anole lizards (Anolis sagrei). To quantify costs, I examined allocation of leucine, a critical amino acid to immune tissue (liver) and reproductive tissue (gonads). I predicted that immune costs would increase with exposure, however, because females often suffer decrements in immune function during reproduction, I expected their immune costs to be less pronounced than males. I also hypothesized that leucine allocation to reproductive tissue would decrease with increasing magnitude of LPS exposure because immune activation often results as tradeoffs as competing physiological systems vie for limited resources. I found that costs of immune activation increased with magnitude of LPS exposure, and that costs differed between the sexes as males allocated more leucine to the liver at high LPS doses, but did not shift allocation from gonads to livers. Females, however, tended to bias resources to livers and away from gonads with increasing LPS doses. Interestingly, I also found that cost of immune activation increased linearly with magnitude of exposure, indicating that this species may tolerate high levels of infection with Salmonella bacteria as the cost of resisting infection is likely to become unmanageable. Such a finding suggests that at the population level, brown anoles may be particularly important contributors to the spread of Salmonella within populations, or even communities. Although costs of immune activation may drive a host’s response to infection by limiting the magnitude of its response when costs of a strong response are too great, we cannot predict how immune costs may be driving parasite prevalence and selection on hosts unless we understand the relationship between costs of immune activation and protection. While it is commonly assumed that high costs of immune activation are indicative of a stronger immune response and ultimately better protection for the host, the relationship between immune costs and benefits remains unstudied. In the third chapter, I examined host- and parasite-mediated costs of parasite exposure and the relationship between costs and benefits of immunity by exposing brown anole lizards to live or killed malaria parasites and following the course of infection over 7 weeks. I measured costs by quantifying glucose oxidation during the 12-36 hours following exposure to determine whether and how cost of initial exposure related to protection from malaria parasites. As glucose is the primary fuel source for malaria parasites, we predicted that hosts that oxidized little glucose would also experience lower parasite burdens. We found that lizards infected with killed parasites oxidized less glucose than control-infected animals, however lizards infected with live parasites did not differ from either the control or killed parasite group, indicating the possibility of a parasite-mediated mechanism to slow host-driven glucose sequestration. Importantly, we also found that the cost of exposure appeared to come with a benefit: lizards infected with live parasites that experienced the lowest glucose oxidation also had lower parasite burdens over the 7 weeks following exposure. The results from this study show that brown anoles that experience greater costs of malaria exposure also experience greater protection, contributing to our understanding of how individual-level processes drive disease dynamics within communities. Altogether, my dissertation has identified that broad characteristics such as life history and body mass can be helpful in predicting costs of immune activation and has additionally demonstrated the importance of smaller-scale processes such as the relationship between immune costs and magnitude of exposure at the population level and the relationship between immune costs and benefits at the individual level. These studies have expanded our understanding of the factors that drive variation in immunity at multiple levels and give the field of ecoimmunology a more holistic picture of the species, population and individual-level processes that affect host-parasite interactions within communities.

Density dependence is a common phenomenon in nature, and the intensity of density dependence is driven by competition over depletable resources. Habitat selection patterns are often density-dependent, and are driven by decreasing population mean fitness in a habitat as population density increases in that habitat. Yet not all resources are depletable, and non-depletable resources may sometimes be most important in dictating patterns of habitat selection. Ectotherms, for example, are defined by their dependence on environmental temperature to regulate body temperature, and temperature is often the most important resource for ectotherms. Is density dependence an important mechanism in ectotherms, especially when temperature is a limiting factor? In this thesis, I examine density dependence of fitness and habitat selection by ectotherms using red flour beetles and common gartersnakes. In chapter one and three, I test whether density-dependent habitat selection occurs when habitats differ in both temperature and food availability with red flour beetles and common gartersnakes, respectively. In chapter two, I modify the isodar model of habitat selection to account for the effect of temperature on ectotherms, derive predictions from the modified model, and test these predictions with controlled experiments with red flour beetles selecting between habitats that differ in food quantity and temperature. Finally, in chapter four, I examine the effect of density on metrics of fitness and habitat selection with common gartersnakes. Red flour beetles exhibited strong density dependence in both habitat selection and fitness at their optimal temperature, but density dependence weakened at lower temperatures. Common gartersnakes exhibited mostly density-independent habitat selection with a strong preference for warm field habitat over cool forest habitat, but exhibited some density dependence in habitat selection within field habitat. Overall, my thesis demonstrates that ectotherms have variable density-dependent responses, and that these responses are strongly modulated by temperature.

Recent models argue that thermal environments are the major cause of ectotherm life-history clines. However, elevational clines in body size in the mountain dusky salamander Desmognathus ocoee (family Plethodontidae) shift from positive at hatching, to negative at metamorphosis to positive again as adults, and so are not consistent with this explanation. The clinal shift from hatching to metamorphosis was investigated by examining the clinal and seasonal feeding patterns of larval salamanders at high and low elevation sites in rockface and woodland habitats. Repeated cohort sampling was also used to examine clinal and seasonal patterns in body size and to estimate average growth rates. Larval growth in both rockface and woodland habitats was tightly correlated with feeding activity. Although temperature was found to vary between high and low elevation sites, the greatest growth occurred in a cold woodland habitat with a high elevation, and the lowest growth occurred in an adjacent rockface habitat. Because this difference in growth cannot be attributed to thermal differences, we conclude that local food resource levels are the predominant source of local differences in growth. These findings, clinal patterns of variation in other predatory salamanders, and experimental analyses in which both food and temperature are orthogonally manipulated, indicate that general models that single out temperature as the principle cause of ectotherm life-history clines should be viewed with caution.