Reisewitz SE, Estes JA, and Simenstad SA. Indirect food web interactions: sea otters and kelp forest fishes in the Aleutian archipelago. Oecologia

Institute of Marine Sciences, Long Marine Laboratory, 100 Shaffer Road, Santa Cruz, CA 95060, USA.
Oecologia (Impact Factor: 3.09). 02/2006; 146(4):623-31. DOI: 10.1007/s00442-005-0230-1
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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.

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Available from: Charles Simenstad, Sep 28, 2015
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    • "Several species, in particular, depend on kelp forests for finding suitable feeding and nursery areas and protection from predators (e.g. Norderhaug et al. 2005; Reisewitz, Estes & Simenstad 2006; Rosenfeld et al. 2014), leading to the hypothesis that their abundances would be drastically affected by changes in patterns of distribution and density of habitat-forming kelps (e.g. O'Connor & Anderson 2010). "
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    • "The loss of sea otters released herbivorous sea urchins (the otter's preferred prey) from limitation by predation, causing an increase in their rate of herbivory and a decline of coastal kelp forests (Estes et al. 1998). Based on evidence obtained from the Aleutian archipelago and elsewhere in the North Pacific Ocean, kelp forest declines led to reductions in primary productivity, coastal fish populations, and a decrease in the marine sequestration of C (Reisewitz et al. 2006; Markell 2011; Wilmers et al. 2012). Follow-on effects from the pinniped declines are less well known; pinnipeds in the North Pacific Ocean prey on finfish such as cod (Gadus spp), which themselves prey on smaller forage fish and neritic and benthic crustaceans. "
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