Causes and consequences of marine mammal population declines in southwest Alaska: A food-web perspective

Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, CA 95060, USA.
Philosophical Transactions of The Royal Society B Biological Sciences (Impact Factor: 7.06). 07/2009; 364(1524):1647-58. DOI: 10.1098/rstb.2008.0231
Source: PubMed


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.

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Available from: Terrie M Williams, Oct 29, 2014
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    • "Open rectangles (n = 11) indicate historically used foraging locations, areas where otters were regularly seen but have not been observed since the early 1990s evaluated invertebrate prey using three 20-m transects and fish prey were evaluated using one 50-m × 2-m × 2-m transect oriented at random compass headings and secured to a haphazardly placed anchor at each site. Our target sampling depth contour was 5–15 m based on mean sea otter foraging depths (Bodkin et al. 2004) and the observation that sea otter prey distribution patterns in the Aleutians are generally similar across this depth range (Estes et al. 2009). We sampled invertebrate prey using three 0.25-m 2 quadrats placed randomly along each transect (n = 9 quadrats per site), inside which we counted all potential invertebrate prey, identified them to species, and measured them to the nearest millimeter (test or shell maximum linear length). "
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    ABSTRACT: Sea otters (Enhydra lutris) inhabiting the Aleutian Islands have stabilized at low abundance levels following a decline and currently exhibit restricted habitat-utilization patterns. Possible explanations for restricted habitat use by sea otters can be classified into two fundamentally different processes, bottom-up and top-down forcing. Bottom-up hypotheses argue that changes in the availability or nutritional quality of prey resources have led to the selective use of habitats that support the highest quality prey. In contrast, top-down hypotheses argue that increases in predation pressure from killer whales have led to the selective use of habitats that provide the most effective refuge from killer whale predation. A third hypothesis suggests that current restricted habitat use is based on a need for protection from storms. We tested all three hypotheses for restricted habitat use by comparing currently used and historically used sea otter foraging locations for: (1) prey availability and quality, (2) structural habitat complexity, and (3) exposure to prevailing storms. Our findings suggest that current use is based on physical habitat complexity and not on prey availability, prey quality, or protection from storms, providing further evidence for killer whale predation as a cause for restricted sea otter habitat use in the Aleutian Islands.
    Oecologia 11/2014; 177(3). DOI:10.1007/s00442-014-3149-6 · 3.09 Impact Factor
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    • "Research on killer whales only really began after many large whale populations had already been pushed to the brink of extinction by the 1960s and early 1970s (Baird 2002, Roman et al. 2014). By then, any killer whales that had preyed extensively on large whale calves would already have been forced to either starve and die, or switch to other, presumably smaller prey (the SMC hypothesis; Estes et al. 2009). Either way, it is likely that those killer whale populations would have declined or perhaps even became extirpated in some areas. "
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    ABSTRACT: Reports of killer whales (Orcinus orca) preying on large whales have been relatively rare, and the ecological significance of these attacks is controversial. Here we report on numerous observations of killer whales preying on neonate humpback whales (Megaptera novaeangliae) off Western Australia (WA) based on reports we compiled and our own observations. Attacking killer whales included at least 19 individuals from three stable social groupings in a highly connected local population; 22 separate attacks with known outcomes resulted in at least 14 (64%) kills of humpback calves. We satellite-tagged an adult female killer whale and followed her group on the water for 20.3 h over six separate days. During that time, they attacked eight humpback calves, and from the seven known outcomes, at least three calves (43%) were killed. Overall, our observations suggest that humpback calves are a predictable, plentiful, and readily taken prey source for killer whales and scavenging sharks off WA for at least 5 mo/yr. Humpback “escorts” vigorously assisted mothers in protecting their calves from attacking killer whales (and a white shark, Carcharodon carcharias). This expands the purported role of escorts in humpback whale social interactions, although it is not clear how this behavior is adaptive for the escorts.
    Marine Mammal Science 11/2014; 31(2). DOI:10.1111/mms.12182 · 1.94 Impact Factor
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    • "Approximately 10% of the estimated 50 000 killer whales worldwide today appear to feed exclusively on marine mammals (Forney and Wade 2006; Reeves et al. 2006). Following the depletion of great whales, their killer whale predators must have either declined in abundance or expanded their diet to include other prey species (Estes et al. 2009). In the North Pacific Ocean, killer whales purportedly began to feed more extensively on smaller marine mammals, such as harbor seals (Phoca vitulina), Steller sea lions (Eumetopias jubatus), and sea otters (Enhydra lutris), with consequent decreases in these populations (Springer et al. 2003, 2008). "
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    ABSTRACT: Baleen and sperm whales, known collectively as the great whales, include the largest animals in the history of life on Earth. With high metabolic demands and large populations, whales probably had a strong influence on marine ecosystems before the advent of industrial whaling: as consumers of fish and invertebrates; as prey to other large-bodied predators; as reservoirs and vertical and horizontal vectors for nutrients; and as detrital sources of energy and habitat in the deep sea. The decline in great whale numbers, estimated to be at least 66% and perhaps as high as 90%, has likely altered the structure and function of the oceans, but recovery is possible and in many cases is already underway. Future changes in the structure and function of the world’s oceans can be expected with the restoration of great whale populations.
    Frontiers in Ecology and the Environment 09/2014; 12(7). DOI:10.1890/130220 · 7.44 Impact Factor
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