| Summary: | The timing of reproduction is important for fitness, and has been used to measure the effect of widespread environmental change across ecosystems globally. Across trophic levels, species occupying higher levels of a food web are generally adjusting their timing of breeding in response to environmental change at a slower rate than their prey (Poloczanska et al., 2013; Thackeray et al., 2010). This may lead to a trophic mismatch between the energy requirements of consumers and the timing of peak availability of resources during the crucial reproductive period, potentially negatively impacting on fitness. However, the effects of environmental change have not been uniform across populations, species, or regions of the world. This makes it difficult to predict how different populations will adjust their response to environmental change and the consequences of this for fitness. Marine species are generally underrepresented in studies of environmental change, and seabirds are a group of marine organisms that may be particularly at risk. They generally occupy higher trophic levels, are long-lived, and reproduce slowly, meaning they may lack the evolutionary capacity to adapt if the timing of key resources shifts rapidly under climate change. However, the disconnected nature of previous studies of the trends and drivers of seabird breeding phenology and the effects of trophic mismatch on seabird fitness has precluded a global understanding of the extent to which seabirds will respond to climate-mediated environmental change. In this thesis, I make use of resources contributed by a global network of collaborators to first establish the global average trends in seabird breeding phenology over time and in response to sea surface temperature. I then identify which seabird populations may be at higher risk of mismatch with prey by characterising sources of variance around these phenological trends (e.g. due to differences in phylogeny, biogeographic region, or life history traits). I go on to explore the scales at which phenology is correlated across breeding North Atlantic seabird populations, to understand whether it is likely that phenology is driven by conditions experienced by populations at the breeding grounds, overwintering locations, or across multiple spatial scales. Finally, I examine the fitness consequences of trophic mismatch between the resource and consumer in two ways. I first use 30+ years of data from the long-term monitored population of European shags Phalacrocorax aristotelis on the Isle of May, Scotland, to identify the impact of trophic mismatch on population- and individual-level fitness over time and in relation to changes in SST and diet. My final data chapter expands the focus on the effects of mismatch on population level breeding success back to the global scale. In the absence of detailed information on prey availability and phenology, I develop on an existing framework that allows us to predict when phenological change may impact on population level fitness to identify whether trophic mismatch is both present in a population and getting worse over time. I use these criteria to compare relationships across populations, regions and life history traits to identify the prevalence of trophic mismatch across populations on a global scale.
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