Biotic interactions shift across temperature and ontogeny in an intertidal barnacle

Abstract

Natural systems provide beneficial goods and services to humanity. To maintain these benefits, modern science is tasked with predicting how natural systems respond to global change. Understanding the ecological processes that underly natural systems, such as interactions between organisms, can help improve such predictions and is thus a research priority. In this dissertation, I present novel findings on how ecological interactions vary across biotic and abiotic factors, and some potential consequences thereof. The hallmark of global change is warming. Warming is important because temperature controls biology at all levels of organization. Many organisms are also shrinking. This is important because body size is a fundamental driver of ecological processes. From survival and growth to reproduction, organismal vital rates vary with ontogeny. Research over the past two decades emphasize that knowledge of the direct effects of temperature and body size on organisms is inadequate to forecast community trajectories. Organisms do not exist in vacuum; they interact with neighbors and with predators. To improve understanding of how natural systems respond to global change, knowledge of the indirect effects – how biotic interactions change with temperature and body size – is required. The rocky intertidal zone provides a valuable system in which to study the ecological impacts of global change. Sessile ectotherms in particular have been used to elucidate principles that now pervade ecology. For example, the acorn barnacle Balanus glandula, though mostly mute and wholly illiterate, tells excellent ecological stories. Seemingly shy in its “squatter’s nutshell” , a barnacle can reveal much about competition and facilitation given the right coaxing. Then there is the whelk Nucella ostrina. All who know them admire these “tiny wet beings straining calcium from the water and spinning it into polished dreams on their backs” . All except perhaps the barnacles, whom the whelks eat. Using organisms in the rocky intertidal zone, this dissertation examines how biotic interactions shift across temperature and ontogeny. In Chapter 1, I examine how multiple physical drivers concurrently affect a consumer-resource interaction. Evidence suggests widespread non-additive effects between multiple drivers; most of this evidence, however, is based on species-level responses, which is problematic because community responses to environmental change also depend on species interactions. To address this knowledge gap, I experimentally manipulated two physical drivers fundamental in intertidal systems – air and water temperatures – and examined the responses of the predator–prey interaction between B. glandula and N. ostrina. I obtained two key findings. First, air and water warming non-additively affected interaction strength: warm water mitigated a decrease in mean whelk feeding rate caused by warm air. Second, air warming had contrasting effects on individual growth rates of predator and prey. While whelk growth decreased in warm air, barnacle growth increased. These findings suggest that combined air and water warming will benefit barnacle populations more than their whelk predators. This study highlights the value of integrating species performances and interactions to understand how multiple physical drivers may affect community structure. In Chapter 2, I examine how environmental context governs trait-rate relationships. Trait and environmental controls on individual vital rates have been well documented, but the potential interactive effects between these controls have been less studied. This is problematic because global change often alters traits and environmental factors simultaneously; to predict the responses of systems, the interaction effects have to be understood. To investigate the links between traits, environmental factors, and demographic processes, I tested how the vital rates of B. glandula respond to the interactive effects of body size, temperature, and crowding. I found that body size and crowding interact to set barnacle survival and reproduction. In the field, crowding increased survival, an effect which became stronger with increasing body size. This effect was likely due to a shift in the balance of size-dependent competition and temperature-dependent facilitation. In mesocosms, crowding increased probability of being reproductive for smaller barnacles but decreased it for larger barnacles. I also found that warmer low tide temperatures decreased barnacle survival (field and mesocosms) and growth (mesocosms) regardless of body size. These findings suggest that incorporating environmental context is necessary to predicting vital rates from traits in barnacles. In Chapter 3, I examine how ontogenetic shifts in intraspecific interactions affect population dynamics. Compared to consumer-interactions, there is a dearth of information on how ontogenetic shifts in interactions within a trophic level affect population dynamics. This is problematic because interactions within a trophic level (e.g., competition and facilitation) are critical determinants of community structure. I used population models to investigate the conditions under which ontogenetic shifts in intraspecific interactions affect the population dynamics of B. glandula. I obtained two key findings. First, crowding modified the effect of body size in an important way: the direction of intraspecific interactions on barnacle growth shifted from negative to positive across size. At a small initial size, high crowding decreased barnacle growth. At a large initial size, however, crowding increased barnacle growth slightly. Second, populations modeled with theoretical crowding distributions showed differences in population growth rates consistent with the observed ontogenetic shifts. Populations with low crowding in established individuals and high crowding in recruits had the lowest population growth rate. Conversely, populations with high crowding in established individuals and low crowding in recruits had the highest population growth rate. This difference in population growth rate was mediated by changes in the population size distribution. These findings highlight the importance of considering how ontogenetic shifts in competition and facilitation alter population size structure and dynamics when predicting the responses of sessile ectotherms to global change. The firmament of ecology is vast. Although this dissertation focuses on a small subset of ecology, it crosses levels of organization, drawing concepts from physiological, population, and community ecology. It also combines fieldwork in the spirit of my academic ancestors (including G.E. Hutchinson and R.T. Paine) with modern techniques in statistical analysis and mathematical modeling. Together, this dissertation contributes basic knowledge on how biotic interactions shift across temperature and ontogeny, and how the latter shift may scale up to population dynamics. In doing so, it bolsters the foundation on which science builds to address global change.