Bald Eagles and Sea Otters in the Aleutian Archipelago: Indirect Effects of Trophic Cascades

Abstract

Abstract. Because sea otters (Enhydra lutris) exert a wide array of direct and indirect effects on coastal marine ecosystems throughout their geographic range, we investigated the potential influence of sea otters on the ecology of Bald Eagles (Haliaeetus leucocephalus) in the Aleutian Islands, Alaska, USA. We studied the diets, productivity, and density of breeding Bald Eagles on four islands during 1993-1994 and 2000-2002, when sea otters were abundant and scarce, respectively. Bald Eagles depend on nearshore marine communities for most of their prey in this ecosystem, so we predicted that the recent decline in otter populations would have an indirect negative effect on diets and demography of Bald Eagles. Contrary to our predictions, we found no effects on density of breeding pairs on four islands from 1993-1994 to 2000-2002. In contrast, diets and diet diversity of Bald Eagles changed considerably between the two time periods, likely reflecting a change in prey availability resulting from the increase and subsequent decline in sea otter populations. The frequency of sea otter pups, rock greenling (Hexagammus lagocephalus), and smooth lumpsuckers (Aptocyclus ventricosus) in the eagle's diet declined with corresponding increases in Rock Ptarmigan (Lagopus mutus), Glaucous-winged Gulls (Larus glaucescens), Atka mackerel (Pleurogrammus monopterygius), and various species of seabirds during the period of the recent otter population decline. Breeding success and productivity of Bald Eagles also increased during this time period, which may be due to the higher nutritional quality of avian prey consumed in later years. Our results provide further evidence of the wide-ranging indirect effects of sea otter predation on nearshore marine communities and another apex predator, the Bald Eagle. Although the indirect effects of sea otters are widely known, this example is unique because the food-web pathway transcended five species and several trophic levels in linking one apex predator to another.

Gonadal maturation of the female smooth lumpsucker (Aptocyclus ventricosus)

“…It is a common species that is ecologically relevant to local food webs throughout its distribution. It is a major consumer of gelatinous planktonic animals, such as jellyfishes and comb jellies (Yoshida and Yamaguchi, 1985) and an important source of food for some fish species, marine mammals, and birds (Kato, 1982;Orlov, 1997;Zeppelin and Ream, 2006;Anthony et al, 2008;Sinclair et al, 2008).…”

Indirect effects of sea otters on rockfish (Sebastes spp.) in giant kelp forests

“…Since Hairston et al (1960) proposed that regulation of herbivores by predators maintains plant populations and primary productivity, trophic cascades have become a central theme in ecology (Shurin et al 2002, Terborgh andEstes 2010). The collateral effects of trophic cascades on biodiversity and ecosystem properties remain poorly understood and difficult to predict (Pringle et al 2007, Anthony et al 2008, Beschta and Ripple 2009). The ''otter-urchin-kelp'' trophic cascade is a classic example of an apex predator with strong indirect effects on habitat-forming primary producers (Estes and Palmisano 1974, Simenstad et al 1978, Duggins et al 1989, Estes et al 1998, Reisewitz et al 2006, Anthony et al 2008, Hughes et al 2013.…”

“…The collateral effects of trophic cascades on biodiversity and ecosystem properties remain poorly understood and difficult to predict (Pringle et al 2007, Anthony et al 2008, Beschta and Ripple 2009). The ''otter-urchin-kelp'' trophic cascade is a classic example of an apex predator with strong indirect effects on habitat-forming primary producers (Estes and Palmisano 1974, Simenstad et al 1978, Duggins et al 1989, Estes et al 1998, Reisewitz et al 2006, Anthony et al 2008, Hughes et al 2013. Although factors such as wave disturbance and nutrient availability certainly affect kelp productivity (Dayton 1985, otters exert strong top-down control over kelps via urchins at high latitudes (Watson and Estes 2011).…”

Nest use dynamics of an undisturbed population of bald eagles

“…Some bald eagle recovery plans mandate that chick productivity >1 chick per nest used for reproduction (‘occupied’ in Grier, Elder, Garamlich, Mathesen, & Mattsson, 1983), and this rate is often observed in expanding (Saalfeld, Conway, Maxey, Gregory, & Ortego, 2009; Smith et al., 2016; Watts et al., 2008), or dynamic (Anthony, Estes, Ricca, Miles, & Forsman, 2008) populations. Our observed estimate of chick productivity <1 per used nest, and stable occupancy dynamics are in line with populations elsewhere in Alaska (e.g., Steidl, Kozie, & Anthony, 1997; Zwiefelhofer, 2007) where density dependence may play a larger role (Elliott, Elliott, Wilson, Jones, & Stenerson, 2011).…”