Direct and indirect effects: Interactions between Bald Eagles and Common Murres

We have studied the changing demography of a colonial seabird, the Common M urre, Uria aalge, at a single site on the Washington coast. Whereas informa l estimates suggest a steep population increase at Tatoosh Island from simi lar to 1975 to 1990, more rigorous censuses from 1991-1999 indicate that th e population has subsequently declined at a rate of similar to3% per year. The primary factor causing this decline appears to be increasing numbers of Bald Eagles, Haliaeetus leucocephalus, visiting the site, where eagles inf lict both direct mortality due to eagle predation on adult murres and indir ect mortality due to eagle facilitation of gull and crow predation on murre eggs. To explore the contribution of eagles to the observed population dec line, 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 ma tch between the projected and observed murre numbers is generally much impr oved by the inclusion of eagle effects. Our study highlights three general challenges to conservation and management practitioners, First, the difficu lty of multispecies management may be exacerbated by successful restoration of high-trophic-level consumers, such as eagles, which may generate subseq uent declines in their prey species. Second, indirect effects are usually s ubtle and often difficult to observe but, as in this case, may contribute s ubstantially to the decline of focal species. Finally, when viewed in the c ontext of the balance between local production and mortality, population tr ends are difficult to interpret without an explicit understanding of immigr ation/emigration rates and patterns.