Indirect food web interactions: sea otters and kelp forest fishes in the Aleutian archipelago

Abstract

Although trophic cascades-the effect of apex predators on progressively lower trophic level species through top-down forcing-have been demonstrated in diverse ecosystems, the broader potential influences of trophic cascades on other species and ecosystem processes are not well studied. We used the overexploitation, recovery and subsequent collapse of sea otter (Enhydra lutris) populations in the Aleutian archipelago to explore if and how the abundance and diet of kelp forest fishes are influenced by a trophic cascade linking sea otters with sea urchins and fleshy macroalgae. We measured the abundance of sea urchins (biomass density), kelp (numerical density) and fish (Catch per unit effort) at four islands in the mid-1980s (when otters were abundant at two of the islands and rare at the two others) and in 2000 (after otters had become rare at all four islands). Our fish studies focused on rock greenling (Hexagrammos lagocephalus), the numerically dominant species in this region. In the mid-1980s, the two islands with high-density otter populations supported dense kelp forests, relatively few urchins, and abundant rock greenling whereas the opposite pattern (abundant urchins, sparse kelp forests, and relatively few rock greenling) occurred at islands where otters were rare. In the 2000, the abundances of urchins, kelp and greenling were grossly unchanged at islands where otters were initially rare but had shifted to the characteristic pattern of otter-free systems at islands where otters were initially abundant. Significant changes in greenling diet occurred between the mid-1980s and the 2000 although the reasons for these changes were difficult to assess because of strong island-specific effects. Whereas urchin-dominated communities supported more diverse fish assemblages than kelp-dominated communities, this was not a simple effect of the otter-induced trophic cascade because all islands supported more diverse fish assemblages in 2000 than in the mid-1980s.

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

“…Sea otter populations declined during this period (Doroff et al 2003, as did the distribution and abundance of kelp (Estes et al 2004) and kelp-associated species (Reisewitz et al 2006). Diets of Bald Eagles during 1993-1994 vs. 2000-2002 changed dramatically, supporting our initial prediction.…”

“…For example, the growth rates of filter-feeding mussels and barnacles are enhanced 2-3 fold in otter-dominated communities because of increased production and elevated concentrations of particulate organic matter in the water column (Duggins et al 1989). In addition, fish densities in kelp forests are elevated substantially in otter-dominated systems (Reisewitz et al 2006), and the diet and foraging behavior of Glaucous-winged Gulls (Larus glaucescens) differ greatly between areas with and without sea otters (Irons et al 1986).…”

Changes to Marine Trophic Networks Caused by Fishing

“…Thus, strong fishery impacts on sea urchin predators intensified grazing and the deterioration of kelp forests, leading to marked changes in the fauna of that ecosystem (Duggins et al, 1989;Estes & Duggins, 1985;Reisewitz et al, 2005;Tegner & Dayton, 2000).…”

An Introduction to the Biocomplexity of Sanak Island, Western Gulf of Alaska

“…These data suggest that otters, with changes in both carbon and nitrogen isotopes, may have been feeding at slightly different trophic levels through time. For example, sea urchins, a favored prey item, are secondary producers in a kelp forest ecosystem while pelagic fish, also otter prey species, are not only in higher trophic positions but also have phytoplankton as the base of their food web (Estes and Duggins 1995, Watt et al 2000, Steneck et al 2002, Estes et al 2003, Bodkin et al 2004, Reisewitz et al 2006). The Although there are many more sites included in the HOI than in the faunal data, patterns do emerge.…”