Top–down influence of resident and overwintering Bald Eagles (Haliaeetus leucocephalus) in a model marine ecosystem

Abstract

Conservation of predators presents challenges when predators affect prey populations that provide ecosystem services. Near Puget Sound, resident and overwintering populations of Bald Eagle ( Haliaeetus leucocephalus (L., 1766)) have expanded in recent decades. We modeled the potential impact of Bald Eagles on marine food-web structure. Bald Eagles caused trophic cascade dynamics through mid-level predators (seabirds) to lower trophic levels (fishes, benthic invertebrates), particularly when seabirds were more abundant in eagle diets. Resident Bald Eagles affected food-web structure more than overwintering eagles, despite the latters’ greater abundance. Predator avoidance behavior by nearshore diving birds and herbivorous birds exacerbated trophic cascade effects, but only in a narrow range of species. Variability in the number of overwintering Bald Eagles, which come to the area to feed on salmon carcasses (primarily chum salmon, Oncorhynchus keta (Walbaum in Artedi, 1792)), had little effect on the food web. Our results indicate that Bald Eagles are important to marine food-web structure, owing to their high consumption rates and the high consumption rates of their seabird prey, but uncertainty about eagle diets limits our full understanding of their impact. In systems where Bald Eagles affect large seabird breeding colonies, their role in food-web structure is likely greater.