Estes_et_al_1998

Estes_et_al_1998 - Killer Whale Predation on Sea Otters...

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Unformatted text preview: Killer Whale Predation on Sea Otters Linking Oceanic and Nearshore Ecosystems J. A. Estes,* M. T. Tinker, T. M. Williams, D. F. Doak After nearly a century of recovery from overhunting, sea otter populations are in abrupt decline over large areas of western Alaska. Increased killer whale predation is the likely cause of these declines. Elevated sea urchin density and the consequent deforestation of kelp beds in the nearshore community dem- onstrate that the otter’s keystone role has been reduced or eliminated. This chain of interactions was probably initiated by anthropogenic changes in the offshore oceanic ecosystem. Apex predators often initiate forces that cas- cade across successively lower trophic levels, sometimes reaching the base of the food web ( 1 ). Plant-herbivore interactions vary predict- ably with trophic complexity in such systems, being weak or strong when the number of trophic levels is odd or even, respectively ( 2 ). Sea otters ( Enhydra lutris ) and kelp forests provide a well-known example of this pattern ( 3 ). After being protected from overhunting, recovering otter populations transformed nearshore reefs from two- to three-trophic- level systems by limiting the distribution and abundance of herbivorous sea urchins, there- by promoting kelp forest development ( 4 ). Sea otters abounded across the North Pacif- ic rim until unregulated exploitation in the mar- itime fur trade reduced the species to near- extinction by the early 20th century ( 5 ). Popu- lation regrowth began when protection was af- forded under the International Fur Seal Treaty. A geographically discordant recovery pattern ensued because of the fragmented distribution of surviving colonies, the discontinuous nature of their habitat, and the otter’s limited dispersal ability ( 5 , 6 ). Consequently, by the 1970s otter populations had recovered to near maximum densities in some areas of their historic range, were growing rapidly in others, and remained absent from still others ( 7 ). The sea otter’s predatory role in kelp forest ecosystems was discovered by contrasting inhabited with unin- habited areas ( 8 ) and by observing changes over time as the uninhabited areas were recol- onized and their founding populations grew ( 4 , 9 ). In addition to showing the influence of sea otters on North Pacific kelp forests, this ap- proach has demonstrated a breadth of indirect effects on coastal ecosystems ( 10 ). The sea otter’s reputation as a keystone species ( 11 ) is based on these interactions and processes. Recently, sea otter populations have de- clined precipitously and unexpectedly over large areas of western Alaska. We first detected this decline through population surveys at Adak Island in the central Aleutian archipelago, which indicated that the otter population de- creased ; 25% per year through the 1990s, resulting in nearly an order-of-magnitude over- all reduction by 1997 (Fig. 1). Additional sur- veys of Little Kiska, Amchitka, and Kagalaska Islands all show population declines of similar...
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Estes_et_al_1998 - Killer Whale Predation on Sea Otters...

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