Graduate Thesis Or Dissertation


Changing Climate, Changing Contexts : Variation in Rocky Intertidal Predator-prey Interactions Seen Through an Environmental Stress Framework Public Deposited

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  • The multifaceted role of the environment in regulating the structure and dynamics of biological communities has long fascinated ecologists and motivated much debate and research. Now, in a time of accelerated global changes due to human impacts, the need to understand how the environment shapes communities has gained new urgency. The environment acts directly on communities by causing direct mortality and changes to vital rates of individuals. However, the environment can also exert indirect effects on communities by changing the nature of biotic interactions. This occurs either through changes to the physiological performance of interacting species or through shifts in the abundance of other species in the community. Much of the effort to understand how global change will influence communities has focused on direct effects of environmental conditions. However, the essential influence of biotic interactions suggests that we will need to improve our conceptual understanding of indirect environmental effects to better predict outcomes of anthropogenic change.Understanding how the interactions of predators and prey are vulnerable to environmental context may provide a useful pathway to link relatively well-resolved individual effects of climate change to a broader community context. Predators are often important in determining community structure and stability through their control of lower trophic levels. However, predators also tend to be particularly sensitive to environmental stress. As a result, environmental stress models predict that the impacts of predators will lessen as stress increases, which could weaken existing processes regulating communities. Top predators, which often have the strongest impacts, may be especially vulnerable to climate change because of their large body size, energy needs, range requirements, and dependence on prey populations. The effects of environmental change on top predators have been justifiably well-studied, yet changing contexts require a more comprehensive view of which species may be important in novel environmental contexts. Subordinate predators are often weak interactors in communities, but they may play increasingly critical roles if top predators decline. Similarly, because weak interactions are highly variable, shifting environmental contexts could lead to different outcomes with subordinate predator interactions.Communities that experience high environmental variability across short spatial and temporal scales, such as rocky intertidal communities, are particularly useful for examining effects of environmental context on predator-prey interactions. The rocky shores along the US west coast have a rich history of study, enabling us to combine new insights and existing knowledge to build a greater context for predicting the impacts of environmental changes. Due to anthropogenic climate change, communities along rockyshores in the NE Pacific are predicted to experience warmer air temperatures, intensified upwelling, and greater exposure to low pH waters. These abiotic changes are likely to influence biotic changes, such as shifting species abundances and distributions, reductions in performance, and increases in disease and mortality.This dissertation explores how three different environmental contexts – two abiotic and one biotic – influence the interactions between a predator and its prey species. My focal predators are two gastropod congeners, the whelks Nucella canaliculata and N. ostrina that feed on mussels and barnacles. Whelks are abundant predators in the mid-intertidal zone, and we know their interactions can be sensitive to environmental conditions. My aim in this dissertation is to expand our understanding of predation in the rocky intertidal and how it is affected by 1) ocean acidification, 2) existing variability in the environment, and 3) the disease-driven decline of the keystone sea star Pisaster ochraceus, which are all contexts relevant to climate change.In Chapter 2, I explore how interactions between whelks and mussel prey are affected by ocean acidification (OA), an abiotic stressor that can influence species physiology and behavior. I use two separate mesocosm experiments designed to capture mechanistic changes in the two pairwise predator-prey interactions. Specifically, I test how the feeding rate and handling time of whelk predators is influenced by elevated CO₂, which has the effect of lowering the pH and reducing the saturation state of carbonate minerals used by both whelks and mussels for building shells. The results show that whelks consume fewer mussels in elevated CO₂, and that this may be caused in part by substantially longer handling times of prey. These results are consistent with the idea that predators are more vulnerable to the stresses associated with OA than prey, at least on shorter time scales.In Chapter 3, I use a comparative experiment at eight sites in Oregon to assess how variability in the interaction between whelks and mussels is shaped by dynamic conditions in the field. Previous experiments have focused on local-scale gradients such as wave exposure and tidal elevation in testing environmental stress models. I expand on these studies to test how predation rate responds to larger-scale variability in upwelling and temperature, both of which are relevant to climate change in the intertidal. In three similar experiments that span 14 years, I observe patterns in mussel survival and assess whether the importance of environmental variables has changed through time. I find that predation by whelks is relatively consistent in the field context, but is explained in part by upwelling. In the final year of study (2013), there was evidence that variability in air temperatures decreased predation, which may point to shifting environmental influences on whelk predators.In Chapter 4, I follow the population responses and community effects of whelks after the striking decline of the keystone sea star Pisaster ochraceus along rocky shores in Oregon. Sea star wasting disease has caused declines in P. ochraceus populations by up to 80%, greatly reducing the population impact of this keystone species. Past research has demonstrated that when P. ochraceus is removed, it often results in the formation of a near-monoculture of the mussel M. californianus in the low intertidal zone. I hypothesize that whelks will be able to minimize mussel invasion following declines in P. ochraceus because whelks will be able to control sessile prey species, like barnacles, that facilitate mussel establishment. However, my field experiment provides no evidence of compensation by whelks; instead, they weakly facilitate mussel establishment. To understand whelk population responses to keystone species loss, I also monitor whelk abundance, distribution with tidal elevation, and population size structure. My results indicate the potential for a lagged whelk population response suggested by a shift of the size structure towards smaller individuals.Overall, my dissertation highlights the sensitivity of rocky intertidal predator-prey interactions to environmental contexts relevant to anthropogenic change. It also points to the need to continue studying predation in relevant environmental contexts in order to scale existing knowledge in species responses to the community level. Further, my results reveal that at both the per capita and population levels, weak interactors have variability in their interactions with other species that will likely influence their role in communities undergoing change. Even in a well-understood system, our results were often unexpected and did not necessarily match the predictions of environmental stress models and other existing frameworks. This suggests we need to build further conceptual and empirical frameworks to determine how sensitivity in species interactions will ultimately affect community structure, functioning, and the provision of ecosystem services.
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