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Direct and Indirect Effects: Interactions between Bald Eagles and Common Murres
Parrish, J.K., Marvier, M. & Paine, R.T. (2001) Ecological Applications 11(6):1858-1869
We have studied the changing demography of a colonial seabird, the Common Murre, Uria aalge, at a single site on the Washington coast. Whereas informal estimates suggest a steep population increase at Tatoosh Island from ~1975 to 1990, more rigorous censuses from 1991-1999 indicate that the population has subsequently declined at a rate of -3% per year. The primary factor causing this decline appears to be increasing numbers of Bald Eagles, Haliaeetus leucocephalus, visiting the site, where eagles inflict both direct mortality due to eagle predation on adult murres and indirect mortality due to eagle facilitation of gull and crow predation on murre eggs. To explore the contribution of eagles to the observed population decline, we used age-specific vital rates drawn from the literature to develop a wide range of plausible models of murre demography. We found that the match between the projected and observed murre numbers is generally much improved by the inclusion of eagle effects. Our study highlights three general challenges to conservation and management practitioners. First, the difficulty of multispecies management may be exacerbated by successful restoration of high-trophic-level consumers, such as eagles, which may generate subsequent declines in their prey species. Second, indirect effects are usually subtle and often difficult to observe but, as in this case, may contribute substantially to the decline of focal species. Finally, when viewed in the context of the balance between local production and mortality, population trends are difficult to interpret without an explicit understanding of immigration/emigration rates and patterns.

Key words: Bald Eagle; Common Murre; Haliaeetus leucocephalus; indirect effects; multispecies; predation; reproductive success; survivorship; Tatoosh Island, Washington (USA); Uria aalge.


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