Graduate Thesis Or Dissertation
 

Patterns of distribution, abundance, and change over time in the marine bird community of Prince William Sound, Alaska, 1989-2012

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  • Over recent decades, the marine ecosystems of Prince William Sound (PWS), Alaska have experienced the concurrent effects of a major anthropogenic disturbance, the 1989 Exxon Valdez oil spill (EVOS), and a dynamic atmospheric-oceanic environment. Studies of top marine consumers can provide insights into processes of ecosystem change. Using data collected during boat-based marine bird surveys in PWS over the period 1989-2012, I used complimentary taxon- and community-centric approaches to investigate patterns of distribution and abundance of marine birds across space and time. In doing so, I sought to provide insight into processes that may have affected both the birds themselves and the ecosystems of which they are a part. I found that the marine bird community as a whole was spatially structured along a primary onshore-offshore environmental gradient, and secondarily structured along an estuarine-marine environmental gradient. I found two patterns of community variability across time. The first temporal community-level pattern was likely related to sustained rates of change in the abundance of some genera, likely caused by demographic processes. This pattern was correlated with climate variability at time-scales of several years to a decade. The second temporal community-level pattern was characterized by fluctuations that correlated with climate variability at an annual time-scale, likely caused by movements of birds between PWS and the adjacent Gulf of Alaska. I also evaluated changes in the abundance of 18 genera of marine birds, and found that seven had declined in abundance and three had increased over the study period; all genera that declined were piscivorous or planktivorous. Furthermore, the rates of population change of genera were related to their position in the onshore-offshore gradient, with the highest rates of decline occurring in offshore-associated genera. I also investigated spatial habitat associations and temporal changes in abundance of a focal taxon, the seabird genus Brachyramphus. Two species within this piscivorous, pursuit-diving genus occur in PWS, the marbled murrelet (B. marmoratus) and the Kittlitz’s murrelet (B. brevirostris). Both are species of conservation concern, and both experienced acute mortality in PWS caused by the EVOS. Using a statistical model that accounted for variables affecting observed abundance across space and time, I found that abundance decreased by more than two-thirds over the study period. I found no evidence that rates of change differed along environmental or geographic gradients. There was also no evidence that changes in seasonal patterns of abundance occurred. These results are indicative of a widespread decline in the abundance of Brachyramphus murrelets within PWS. Taken as a whole, the results of my thesis are consistent with the hypothesis that climate change has differentially affected nearshore and offshore components of PWS food webs. This in turn has likely contributed to the failure of some taxa, including Brachyramphus murrelets, to recover from population injury caused by the EVOS.
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