Climate is changing at unprecedented rates and marine species worldwide will need to adapt, acclimate, or shift their ranges to persist. The majority of marine organisms have complex life cycles, in which developmental stages differ in morphology, physiology, habitat, and environmental tolerances. To better predict the impacts of climate change, we must understand how each stage responds and copes with ocean warming – especially early life stages that are most vulnerable to environmental stress. In Chapter 1, I investigate the effects of environmental conditions during development on larval survival in Mexacanthina lugubris, a whelk species shifting its range northward along the west coast of the United States in response to climate change. We found no difference in reproductive output or larval survival across shore heights, suggesting egg-laying behavior may buffer developing stages from thermal stress. In Chapter 2, I test the influence of parental environments on offspring thermal tolerances in an important foundation species, the California mussel (Mytilus californianus). We observed parental effects across one generation, where adult mussels exposed to warmer habitats yielded less tolerant offspring. We also found indicators of trade-offs in energy investment where mussels in low stress environments had higher reproductive condition and larger egg diameters. Finally, in Chapter 3, I explore the critical time windows that influence transgenerational plasticity across generations in marine invertebrates using a meta-analysis. This meta-analysis revealed that the outcomes of transgenerational plasticity vary by trait type, taxonomical groups, and life history traits. I found that gametogenesis is a key window in a parent’s life for experiencing an environmental stressor to result in transgenerational plasticity. Together these results elucidate some of the factors and coping mechanisms that will determine the persistence of marine species under climate change. My dissertation highlights the importance of understanding the impact of climate change across life stages and generations when predicting the survival of marine invertebrates.

Climate change is likely the greatest threat to global biodiversity and ecosystem functioning, and it is becoming increasingly apparent that understanding the effects of climate change requires going beyond single species or limited spatial scales. Climate change will impact species and communities through both direct and indirect effects, as mediated by species interactions, and incorporating these indirect effects can increase the ability to track shifting species distributions. I used a space-for-time approach to test if incorporating indirect effects increases predictive ability through surveys of vertical distributions of predators (sea stars) and prey species (mussels) spanning a thermal gradient along the West Coast. Prey distributions were directly influenced by temperature, but there was also a significant indirect effect of temperature, as mediated by predator distributions. Under future climate change, mussel ranges may undergo vertical shifts towards subtidal habitats, allowing for localized persistence of mussels and their associated species. However, both local and broad scale range-shifts can displace other species or alter community and ecosystem processes. Despite inherent differences between introduced and range-shifting species, impacts can occur via analogous mechanisms, and the magnitude of impacts can be similar. Risk assessments developed for invasive species provide a useful tool for assessing potential impacts of range-shifting species. Altered species interactions and communities can be observed locally in southern California intertidal communities, where the whelk Mexacanthina lugubris is undergoing a northward range shift. I assessed the impacts of Mexacanthina on local species, through long-term field surveys, coupled with manipulative experiments to assess current and future impacts on competitors. Mexacanthina is now well established and utilizes analogous resources and habitats as native whelk species. Mexacanthina can also survive at warmer temperatures than native whelks, suggesting that range-shifters may have a competitive advantage in a warming climate. The persistence of some species at the expense of others underscores the complexities of conservation in the era of climate change. My thesis research explores this dichotomy by examining how species interactions can indirectly alter distributions (Ch. 1), which traits are indicative of problematic range-shifters (Ch. 2), and how a range-shifting species is altering southern California communities (Ch. 3).

Climate change is resulting in unprecedented increases in both mean and extreme global temperatures, with widespread effects on Earth’s biota. It is difficult to predict the impacts of climate change, and resulting extreme heat events, on species with complex life cycles. Throughout their complex life cycles, life stages of marine invertebrates may experience unequal levels of environmental stress (exposure) and have differing thermal tolerances (sensitivity), which vary in space and time. A meta-analysis of differences in sensitivity across life stages of marine invertebrates revealed that, when exposed to the same levels of warming, younger life stages are more sensitive than older life stages (Chapter 1). We used field observations of environmental conditions experienced coupled with laboratory assessments of thermal tolerance to investigate whether thermal safety margins differed across three benthic life stages of the California mussel, Mytilus californianus. Our results demonstrated that thermal sensitivity decreased throughout ontogeny, and, since exposure trends were similar across life stages, sensitivity drove thermal safety margin differences across life stages and approaches (Chapter 2). Finally, we investigated how sensitivity and exposure vary across small-scale habitats in space and over four seasons for one year, calculating mortality risk for two benthic life stages of mussels. We deployed temperature loggers to document habitat-specific exposure patterns, conducted lab thermal tolerance assays to calculate sensitivity, and used field surveys to determine whether risk patterns were reflected in abundance distributions. We found that sensitivity was more variable across seasons (time) than small-scale habitats (space), resulting in high temporal variation in mortality risk (Chapter 3). Together, these results highlight the importance of incorporating life stage and seasonal dynamics when predicting impacts of climate change on the survival of marine invertebrates.

Climate change is directly impacting the services humans derive from the sea at an accelerated rate. Ocean warming and acidification (i.e., a decrease in ocean pH) are leading to modifications in population sizes and ecosystem functioning. The observed shifts in these higher order processes are a direct result of individuals’ responses (i.e., physiology, including metabolism, growth, calcification, and survival) occurring within communities. Natural variation in past environmental exposure experienced by individuals may lead to greater population resilience, or it may push individuals past physiological thresholds leading to increased sensitivity and vulnerability to climate change. Thus, we need to determine how individual-level physiological responses to climate change scale up to influence marine ecosystems. Rocky intertidal habitats are an ideal study system for evaluating the relationships between individual physiological responses, ecosystem functioning, and climate change. Tide pools possess unique thermal and pH environments and can be monitored under natural conditions or manipulated with field-experiments over daily and seasonal time scales, creating natural “experimental mesocosms”. In addition, many species within rocky intertidal habitats are exposed to environmental conditions close to their tolerance limits, increasing their potential vulnerability to climate change. In Chapter 1, by utilizing the unique thermal environments of tide pools, I showed that across small spatial scales (pools), thermal history influences thermal sensitivity of marine invertebrates for short-term time intervals (1-week and 1-day) and that this relationship differs seasonally and between species with differing traits, including mobility. This suggests that variability in thermal responses among individuals may allow for a natural buffer at a population level in response to climate change. Multiple stressors may affect individuals independently or interactively, amplifying or mitigating effects. Thus, to determine the impacts of climate change, in Chapter 2, I used a 6-month long field manipulation of ocean warming and acidification in tide pools. I examined the combined effects of warming and acidification on the shell structure (shell thickness and corrosion) and functional properties (shell strength) of the ecologically critical species, the Pacific blue mussel (Mytilus trossulus). Acidification led to thinner, weaker, and more corroded shells whereas combined warming and acidification resulted in an increase in shell strength. My results suggest that to some degree, warming may mitigate the negative impacts of acidification on this mollusk species. Lastly, in Chapter 3, I characterize how warming and acidification, individually and interactively, impact net ecosystem calcification and the individual and population-level mechanisms driving impacts on net ecosystem calcification. Net ecosystem calcification tended to increase during the day and decrease at night; however, addition of CO2 during the hottest months led to decreased net ecosystem calcification and increased dissolution during both day and night. I found that individual mussel metabolic rates increased significantly in the presence of elevated CO2 and increased daily maximum of pool temperatures. Through this individual-level pathway, pH and temperature had a strong impact on the metabolic rates of individuals ultimately resulting in changes in net ecosystem calcification. On the other hand, greater mussel abundance was associated with increased net ecosystem calcification. Yet, with the addition of CO2, calcification decreased even in pools with the highest abundance of mussels, indicating that there are other pathways by which changes in pH can drive alterations in net ecosystem calcification. My dissertation reveals how species’ traits and natural thermal variation from short-term to seasonal time scales influence metabolic sensitivity to future warming among individuals (Ch. 1), independent climate stressors can negatively impact shellfish in situ, whereas the combined interactive effects between multiple stressors can lead to mitigation of the negative impacts of a single stressor alone (Ch. 2), and that ecosystem-level consequences of climate change are mediated by the abundance of dominant calcifiers and that this effect is dependent on the magnitude of acidification and warming (Ch. 3).

Global change is impacting ecosystems worldwide, driving widespread biodiversity loss and disrupting a broad spectrum of ecological processes. Ecosystems are organized by interactions among species, which dictate everything from species composition to how biomass and energy flow through the system. Only by studying the effects of global change in the context of existing ecological structures and species interactions can the impacts be fully understood. My dissertation research has focused on the ways in which climate change affects the ecological roles of species: whether by altering the relative importance of common species in ecosystems, directly impacting ecosystem function via species loss, or accelerating the proliferation of invasive species. In my first chapter, I consider the potential for an abundant marine producer to engineer habitat for other species and how that role may shift with climate change. I found that this dominant alga raised pH when in isolation but not in the context of a tide pool community, suggesting that the most abundant species do not necessarily affect the impacts of global change in coastal ecosystems. My second chapter focuses on the contributions of dominant species to ecosystem multifunctionality in coastal areas, as well as the potential effects of dominant species loss on ecosystem multifunctionality. I found that a dominant producer and a dominant consumer had largely opposite effects on ecosystem function and that the loss of the producer altered the functional impact of the consumer, suggesting that species loss may impact ecosystem multifunctionality beyond the functional footprint of the individual species. In my third chapter, I identified the ways that climate change is likely to influence factors that historically limited species invasion in high-latitude, marine ecosystems. Each of these invasion barriers is likely to become increasingly porous, potentially increasing invasions in high-latitude areas in decades to come. The results of this research could be incorporated into (1) conservation plans, sharpening the focus on species which have the greatest ecological impact, either by stabilizing environmental conditions or by driving ecosystem function, and (2) invasive species management strategies, by highlighting the increasing vulnerability of high-latitude ecosystems to species invasion under warming conditions and stressing the importance of international cooperation in monitoring programs.

High-latitude marine ecosystems have experienced fewer species invasions than temperate marine ecosystems, a discrepancy that may be attributed to barriers such as low propagule pressure, extreme and seasonal abiotic conditions, and biotic resistance of relatively intact communities. Each of these barriers is being affected by climate change and increasing human activity in high-latitude (>55º N) areas. We reviewed the evidence for each of these barriers limiting species invasion in high-latitude areas in the northern hemisphere. Based on records from government documents of high‐latitude countries, non-native species appear to be increasing in number (in Denmark and the United States) although there remains a paucity of data on invasive species establishment for high-latitude regions. Future study is needed to identify the drivers and impacts of invasions at high latitudes so that managers looking to prevent invasions can focus their efforts on bolstering barriers to invasion in these unique ecosystems.