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, 364 (1524), 1647-58

Causes and Consequences of Marine Mammal Population Declines in Southwest Alaska: A Food-Web Perspective

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Causes and Consequences of Marine Mammal Population Declines in Southwest Alaska: A Food-Web Perspective

J A Estes et al. Philos Trans R Soc Lond B Biol Sci.

Abstract

Populations of sea otters, seals and sea lions have collapsed across much of southwest Alaska over the past several decades. The sea otter decline set off a trophic cascade in which the coastal marine ecosystem underwent a phase shift from kelp forests to deforested sea urchin barrens. This interaction in turn affected the distribution, abundance and productivity of numerous other species. Ecological consequences of the pinniped declines are largely unknown. Increased predation by transient (marine mammal-eating) killer whales probably caused the sea otter declines and may have caused the pinniped declines as well. Springer et al. proposed that killer whales, which purportedly fed extensively on great whales, expanded their diets to include a higher percentage of sea otters and pinnipeds following a sharp reduction in great whale numbers from post World War II industrial whaling. Critics of this hypothesis claim that great whales are not now and probably never were an important nutritional resource for killer whales. We used demographic/energetic analyses to evaluate whether or not a predator-prey system involving killer whales and the smaller marine mammals would be sustainable without some nutritional contribution from the great whales. Our results indicate that while such a system is possible, it could only exist under a narrow range of extreme conditions and is therefore highly unlikely.

Figures

Figure 1
Figure 1
(a) Time series (from left to right) of population change of great whales, harbour seals, northern fur seals, Steller sea lions and sea otters in the North Pacific Ocean and southern Bering Sea (from Springer et al. 2003). (b) Data are the best estimates (circles) and 95% CIs from the best available survey data (details of data sources and model fitting are in Springer et al. 2008).
Figure 2
Figure 2
Food-web relationships among selected species in the North Pacific Ocean and southern Bering Sea. The arrows represent linkages for which there are known (solid lines) or suspected (dashed lines) dynamic interactions. Black arrows represent top-down forcing and the grey arrows represent bottom-up forcing. Strong dynamic responses are known for food-web pathways that connect at least eight species (e.g. great whales→killer whales→sea otters→sea urchins→kelp→coastal fishes→gulls→bald eagles). See text for explanations.
Figure 3
Figure 3
Species and measurements used in the feasibility analyses based on their demography and energetics. (Predator: killer whales (abundance, field metabolic rate, consumption efficiency); prey: sea otter; pinnipeds; large cetaceans; small cetaceans (life history, caloric value, abundance)).
Figure 4
Figure 4
Graphical representation of sustainability analyses. The horizontal axis depicts the proportion of the maximum sustainable mortality that would have to be consumed by transient killer whales to support the number of animals depicted on the vertical axis.
Figure 5
Figure 5
Numbers of killer whales that could be supported by historical and current populations of great whales or small marine mammals. Each set of boxplots or points show distributions of estimated killer whale numbers currently (grey) or historically (red) as functions of the fraction of animals dying that are predated: (a) only young great whales and minke whales consumed, (b) young, minkes, and tongues and blubber of adults consumed, (c) small marine mammals (seals and porpoises) consumed. Horizontal lines at 400 indicate the minimum number of transient killer whales that presently occur in the system. For great whales, boxplots are shown to indicate the range of predictions over the different parameter values governing consumption and population regulation (see Doak et al. (2006) for further details); for small marine mammals, only averages of the two scenarios for historic population regulation are shown.
Figure 6
Figure 6
Fractions of food resources for killer whales estimated for each guild of prey under current population numbers. (a) Relative food resources presented by each great whale species under current population estimates and assuming that young, minkes, and tongues and blubber of adults consumed. (b) Relative food resources presented by each small marine mammal species under current conditions. (n, northern; h., harbour; s., sea; SSL, Stellar's sea lion; NF, north fur.)
Figure 7
Figure 7
Isoclines indicating the number of transient killer whales supportable with different combinations of proportional prey consumption of great whales (x-axes) and small marine mammals (y-axes). (a,b) Results for historic population estimates of each prey are shown, and assuming either (a) limited or (b) broader consumptions of great whales. (c,d) Results for current population estimates of each prey are shown, and assuming either (c) limited or (d) broader consumptions of great whales. All results assume mean food resources over the range of parameter values governing population regulation and consumption patterns, as described in the text. Note that the current minimum estimate of transient killer whales in this area is 400.

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