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: 6.23). 07/2009; 364(1524):1647-58. DOI: 10.1098/rstb.2008.0231
Source: PubMed

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.

1 Bookmark
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: ABSTRACT: DNA was analysed from external wounds on 3 dead harbour porpoises Phocoena phocoena that were stranded in the Netherlands. Puncture wounds as well as the edges of large open wounds were sampled with sterile cotton swabs. With specific primers that target the mtDNA control region of grey seal Halichoerus grypus, a 196 bp DNA fragment was amplified from 4 puncture wounds. Sequencing of the fragments confirmed the presence of grey seal DNA in the puncture wounds. DNA sequences differed between the cases, implying that 3 individual grey seals were involved. As 8 control swabs from intact skin and the transport bag as well as 6 swabs from open wounds on the same harbour porpoises were all negative, contamination with environmental DNA is considered unlikely. The results provide a link between strandings of mutilated harbour porpoises and recent observations of grey seals attacking harbour porpoises. Ours is the first study to use forensic techniques to identify DNA in bite marks from carcasses recovered from the marine environment. This approach can be extended to identify other marine aggressors, including cases involving persons mutilated at sea.
    Marine Ecology Progress Series 10/2014; 513:277-281. DOI:10.3354/meps11004 · 2.64 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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; DOI:10.1111/mms.12182 · 2.13 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The reintroduction and recolonization of species extirpated from former ranges are key components of species conservation. Resource availability affects recolonization success and resulting distribution patterns, but top-down processes may also play a role through density-mediated or trait-mediated indirect effects that may exclude a species from otherwise suitable habitat. We predicted that the spatial distribution of recolonizing sea otters, Enhydra lutris, on the Canadian Pacific coastline was explained by resources as well as interspecific interactions – spatial segregation from pinnipeds, the preferred prey of killer whales Orcinus orca. We surveyed the summer occurrence of sea otters and pinnipeds on Vancouver Island, Canada. We quantified coastline density and bathymetry at multiple spatial scales as indices of habitat complexity and foraging habitat availability. We used generalized linear model selection to test hypotheses about sea otters’ spatial relationship to resources and heterospecifics. Pinniped presence negatively predicted sea otter presence, even after accounting for complexity and foraging habitat. Sea otters may segregate from pinnipeds due to trait-mediated indirect effects of predation, leading us to hypothesize apparent competition between sea otters and pinnipeds. Research is needed to test this hypothesis; if true, refuge from apparent competitors may be a key component of recolonization habitat for sea otters. Species distribution models should quantify resource landscapes but also species-scapes: the spatial plane of species interactions that combines with resources to drive species distributions. Conservation plans based on recolonization models that include only resources may overestimate available habitat, carrying capacity, and recolonization success.
    Biological Conservation 09/2014; 177:148–155. DOI:10.1016/j.biocon.2014.06.025 · 4.04 Impact Factor

Full-text (2 Sources)

Available from
Oct 29, 2014