Temperature and body size are two fundamental factors shaping physiological and ecological processes across all levels of biological organization. Understanding the relationship between these two factors is therefore essential to our ability to understand how species are responding to climate change, especially as one response of ectotherms to warming is a decline in body size. Declining body size in fishes can consequently lead to declining economic and recreational fisheries value, declining population productivity, declining food security, and altered size-structure function of communities. My thesis explored the physiological mechanisms and the ecological consequences of body size and life-stage specific vulnerability to warming. First, I used a meta-analysis approach to determine species- and group-specific vulnerability to warming in fishes. I evaluated how aerobic metabolism scales with body mass in fishes across temperature and how these changes in scaling may differ with species ecology and morphology. Further, to better understand the mechanisms behind temperature-modulated scaling of aerobic metabolism, I tested how metabolism and maximum heart rates change with acute temperature change and body size in a model species, barred surfperch (Amphistichus argenteus). I then reviewed aerobic swim performance in adult salmon to determine the potential ecological consequences of shifting body sizes. By combining work both across and within species, and examining ecologically relevant physiological performances and contexts, my thesis demonstrates that relationships between body size and performance can provide a mechanistic platform to identify climate change vulnerability in fishes.

Salt marshes are dynamic systems characterized by rapid and intense fluctuations in temperature. Because of this, the ectotherms residing in these environments must possess the ability to tolerate thermally variable environments. Mounting these plastic thermal responses requires energy and nutrients, which are acquired through diet. As the nutritional landscape shifts within these highly variable environments, it is critical we examine the modulating effect of diet on the capacity of ectotherms to respond to thermal variability. Using the California Killifish (F. parvipinnis), we tested the influence of fluctuating temperatures and diet quality on cardiac thermal performance through three specific hypotheses: (1) static acclimation temperature will yield a narrower thermal breadth when compared to fluctuating acclimation temperatures, (2) broad diets will yield higher cardiac thermal tolerance compared to a single-item diet and (3) upper thermal limits will vary when tested at the peak vs trough of a fluctuating acclimation cycle. F. parvipinnis were acclimated for 3 weeks at one of five ecologically relevant temperature regimes (11°C, 11-20°C, 20°C, 20-29°C, 29°C) while providing either a single-item diet or broad diet. Cardiac thermal tolerance was measured using an Arrhenius Breakpoint Temperature test. Both maximum heart rate and cardiac upper thermal limits increased with increasing acclimation temperature. Surprisingly, diet quality had a greater influence on cardiac thermal performance at colder fluctuating acclimations when compared to warmer fluctuating acclimations. Lastly, F. parvipinnis were found to exhibit rapid diel plasticity by altering thermal limits throughout a diurnal phase. This work suggests that both diet and fluctuating acclimations influence species’ upper thermal limits and outlines a clear need to incorporate more ecologically relevant temperature acclimations in lab-based research.

Most animals on the planet seem to exhibit some form of consistent individual variation in behavior. For instance, one goose might be exceptionally aggressive towards joggers its entire life, whereas another goose never harasses a jogger until the day it dies. These kinds of consistent patterns are called animal personality and they affect how individuals interact with members of their own species and other species. Aggressive water striders attack males and females so ferociously they prevent entire ponds of striders from mating. Active dragonfly larvae reduce the abundance of their prey, increasing the abundance of algae. Animal personality traits are thus functional traits with the potential to shape ecosystems. However, it remains unknown how animal personality affects complex ecosystems with numerous interacting populations. Here, I answer three key questions that reveal how and why animal personality shapes species interactions and ecosystem structure.

First, I asked whether the effect of animal personality on interactions between species depends on interactions between members of the same species. I tested whether behaviorally diverse katydids would be more likely to associate with members of their own species but reduce the survival of a potential competitor, froghoppers. Indeed, behaviorally diverse katydids tended to cluster together spatially and resulted in greater froghopper mortality. This suggests that the effect of animal personality on the interaction between these species may depend on how they interact with members of their own species.

Second, I asked whether habitat structure and predator cannibalism determine the effect of predator and prey personality on prey survival. We filled mesocosms with predators (mantis) and prey (katydid) groups and observed how their behavioral variability affected katydid survival, while half of the mesocosms were filled with additional habitat structure in the form of climbing sticks. Mantis behavioral diversity correlated with katydid survival, likely because diverse groups contained the fewest of highly lethal phenotype. However, this was only true when we added habitat structure. The effects of animal personality on interactions between these two species depend on the habitat structure available to them.

In ecosystems with hundreds of interacting species, the signal of individual behavioral traits from any one species could easily be lost. Third, I tested whether mantis behavioral diversity could affect the structure of wild ecosystems. I cut 4m2 squares into a meadow and added mantises with high and low levels of behavioral variability to the plots. After 40 days, I measured the biomass of plants and prey taxa in these plots. High mantis behaviorally diversity increased the biomass of the most dominant plant species (goldenrod), seemingly by reducing the prevalence of herbivores and predators. This demonstrates for the first time that animal personality can shape the structure of wild ecological communities.

Ecosystem processes occur at the scale of populations with numerous interacting species. These studies show how and why animal personality can affect species interactions and thereby ecosystems at a population level, whereas previous work focuses on the individual level. I thus show that animal personality can affect population-level processes in nature for the first time, opening the door to endless new questions.

Human activities threaten aquatic ecosystems and their biodiversity to the extent that we are now living in an era known as the Anthropocene. Fish respond to environmental threats (in part) by changing their physiology, which is reflected across many levels of biological organization. My research focused on the physiology of fishes in vulnerable systems to gain insights into the mechanisms underlying fish responses to human-induced stressors. Specifically, I worked in two threatened systems – coral reef fishes and Pacific salmon – which, although seemingly different, are highly productive and closely tied to the daily lives of humans. These systems are also similarly threatened by warming and fishing activities. In this thesis, I assessed: 1) the nutrient contribution of fish feces to coral reefs, 2) the acclimation capacity and thermal tolerance of a coral reef fish during simulated marine heatwaves, 3) population-specific thermal tolerance of pre-spawning Chinook salmon, and 4) the importance of the heart for aerobic performance in coho salmon. From community-level nutrient inputs down to the cellular function of the heart, my thesis demonstrates that we must consider performance across biological scales when thinking about the conservation and resilience of aquatic ecosystems.

Temperate fishes thrive in dynamic thermal environments, where water temperature can vary dramatically across days, seasons, and years. To survive in these variable environments, they must acquire essential energy and nutrients to support their physiological responses to temperature change. However, climate change is expected to impact the nutritional landscape (prey options, quality, and quantity) for many marine animals, which may limit their ability to respond effectively to co-occurring environmental changes, like temperature rise. In my PhD thesis, I studied how an abundant kelp forest fish, opaleye (Girella nigricans), is affected by simultaneous changes in their diet and environmental temperature.

One way opaleye may respond to temperature change is by adjusting their diet selection or ingestion rate to modify their nutritional status. However, my results demonstrate that diet and temperature have inconsistent effects on the thermal sensitivity of ecologically important traits and that opaleye are prone to making diet choices that have negative impacts on their performance in warm water. I also explored the untested hypothesis that nutritional status can affect the rate of thermal plasticity but found that opaleye’s rate of plasticity was insensitive to diet. However, their capacity for plasticity was diet-sensitive and could be partially explained by their fatty acid assimilation. Finally, I found that opaleye energetics and thermal tolerance are impacted by the combined effects of food restriction and dietary fat composition, where the quality of the fish’s diet can rescue them from the harmful effects of food restriction. Overall, this dissertation demonstrates that diet mediates thermal plasticity in ectotherms, like opaleye.

Freshwater fishes face escalating pressures as a result of habitat alteration from global climate change paired with intensified land use by humans. Habitat fragmentation, disturbances like drought and wildfire, and water pollution all contribute to reductions in the quality and quantity of available freshwater fish habitat. When a fish species inhabits a broad geographic range, however, genetically distinct populations that experience vastly different conditions can exhibit variability in tolerance to stressors such as elevated temperature. Elucidating the distributions of such fishes and the extent of the ability of individual populations to tolerate or adapt to environmental stressors is essential for understanding their vulnerability to extirpation and/or extinction from climate change.

My PhD research makes use of environmental DNA (eDNA) and streamside physiology to assess the distribution and thermal tolerance, respectively, of fishes inhabiting streams in California and Oregon. Using eDNA analysis, observed differences in biodiversity between Southern California river basins were primarily attributed to the presence of exotic species. Using streamside physiology, thermal tolerance and vulnerability to warming differed between populations of an endangered salmonid, steelhead trout (Oncorhynchus mykiss). Populations with historically warmer environmental temperatures had higher thermal tolerance but were often living in temperatures closer to their upper thermal limits compared to those from cooler locations. This information allows managers to focus conservation efforts on areas that support native fish biodiversity and on specific populations of species of concern that are most vulnerable to extirpation.

The Channel Islands foxes are a popular conservation success story after managers, scientists, and zookeepers worked together to remove non-native predators, restore native habitat, and facilitate on-island captive breeding programs. However, although other island fox populations have recovered well, this success story may have been told too soon for the San Miguel Island (SMI) fox. This population experienced a decline of more than 70% between 2015 and 2018 and has been slow to recover since. This decline is attributed, in large part, to parasitism. Notably, a recently identified acanthocephalan parasite, Pachysentis canicola, has drawn considerable attention from Channel Islands National Park (CINP) managers. This parasite has not been detected on other California Channel Islands (CCI) or in mainland California canids, and little is known about the ecology of other helminth species that infect the SMI fox. Here, we sought to 1) describe the helminth assemblage of SMI foxes and the biogeographical patterns of parasitism across the CCI; 2) compare traditional methods of detecting infected foxes with new molecular DNA metabarcoding techniques; and 3) determine the spatial and temporal distribution of fox parasites. In addition to recording seven species of fox parasites, we successfully detected parasite DNA as well as DNA from prey items that may facilitate transmission using scat metabarcoding. Further, we established a baseline for the effects of temperature, rainfall, and habitat type on the prevalence and densities of parasite eggs across the island. The CINP needs a conservation plan to prevent parasite-induced extinction of the San Miguel Island fox, a vulnerable and charismatic species that exists nowhere else in the world. My dissertation provides a foundation for combining traditional and novel tools for managing wildlife threatened by emerging infectious diseases.

Sandy beaches and their associated surf zones support productive food webs that are structured by subsidies from adjacent marine ecosystems. Located at the interface of land and sea, beach ecosystems are characterized by highly dynamic intertidal zonation of mobile macroinvertebrates that varies across time scales ranging from daily and semilunar shifts with tides, to strongly seasonal patterns of beach condition and productivity. Sandy beaches provide an ideal system to examine temporal variation in resource dynamics, an important but understudied dimension of food webs. The barred surfperch, Amphistichus argenteus, is an abundant and widespread surf zone fish in central and southern California that represents an important sport fishery. This live-bearing fish forages on sandy beaches and has been reported to rely primarily upon the intertidal sand crab, Emerita analoga, as prey. However, little is known about the breadth of prey types used by this fish, especially across temporal variation in the supply and accessibility of prey resources on sandy beaches. To address this gap, we investigated variation in the diet of barred surfperch on two temporal scales, season and tide phase, that are known to affect the dynamic distribution of their intertidal prey resources on sandy beaches. Beaches exhibit strong seasonal variation in habitat characteristics that are reflected in populations of intertidal prey for surfperch. We hypothesized that the diet of barred surfperch would respond to seasonal shifts in beach condition and the availability of intertidal macroinvertebrate prey. We also expected that diet could differ with life stage of the fish. To evaluate these questions, we investigated the effect of season on the diet of juvenile and adult barred surfperch, macroinvertebrate prey composition and availability, and relevant environmental variables for two years at Refugio beach. Community composition and the availability of key intertidal prey taxa varied significantly with season. The lower beach crab, Emerita analoga, was important in the diet of barred surfperch and dominated the diet of adult fish in every season, accounting for 92% of their diet. In contrast, juvenile barred surfperch demonstrated strong seasonality in their diet and foraged on prey resources from a wider range of microhabitats, extending from subtidal to upper intertidal zones. The prey resources used by juvenile fish varied with seasonal changes in beach physical characteristics, the population dynamics of key prey species, and species richness of the intertidal macroinvertebrate community. Seasonality in the diet of juvenile fish reveals the dynamic and diverse food web resources supporting populations of barred surfperch. Semilunar tide phases greatly affect the inundation of intertidal habitats and the distribution of invertebrates on beaches. We hypothesized that the diet of barred surfperch would differ among semilunar tide phases. Specifically, we expected a greater diversity of beach invertebrates would be present in surfperch diet during spring tides that sweep more of the intertidal beach face than during neap tides. To answer this question, we investigated the diet of barred surfperch and the availability of beach prey across semilunar shifts in tidal inundation to explore how fish access prey from a range of intertidal zones at R Beach. Community composition of intertidal invertebrates did not vary with tide phase. Overall, species richness in surfperch diet was positively correlated with species richness of prey on the beach. Increased access to intertidal zones during spring tides increased the diversity of prey used by adult and juvenile barred surfperch, suggesting semi lunar tidal inundation is a mechanism of trophic connectivity across sandy beaches and surf zones. During spring tides, barred surfperch diet included intertidal clams, upper intertidal talitrid amphipods, and a reduced dominance of sand crabs. Diet composition varied significantly with fish life stage through shifts in the use of key prey taxa, supporting our finding of seasonal ontogenetic shifts in diet and underscoring the role of life stage in the food web dynamics of live bearing surf zone fish like surfperch. Barred surfperch and the intertidal communities that support them reside at a narrow intersection of human impacts and encroaching seas. Sandy beach ecosystems are highly vulnerable to the effects of sea level rise and other manifestations of climate change. These growing threats to beaches and the food webs they support are already being compounded by anthropogenic disturbance and infrastructure at many urban beaches, impacting intertidal biodiversity and beach habitat quality and quantity. Our findings provide new information on the range of prey resources and the dynamics of the intertidal food webs that support populations of this highly valued surf zone fish. Increased understanding of these trophic connections between beaches and surf zones is needed for the challenges facing coastlines in the coming decades.