The Role of Marine Mammals in Marine Ecosystems -- part II. Lisa T. Ballance SIO 133 Marine Mammal Biology Spring 2015

Transcription

1 The Role of Marine Mammals in Marine Ecosystems -- part II Lisa T. Ballance SIO 133 Marine Mammal Biology Spring 2015

2 Marine Mammals as Prey The ecological role of large whales as prey is the most controversial of the three potential food web pathways (Estes et al. 2006)

3 Marine mammals are prey for killer whales.. or humans.

4 Killer whale predation may drive: School size, acoustic behavior, distribution Baleen whale migration (Corkeron & Connor 1999) Trophic cascades (Estes et al. 1998, Corkeron and Connor 1999, Connor and Corkeron 2001, Springer et al. 2003, Williams 2006, Estes et al. 2011) Branch & Williams 2006

5 Sequential megafaunal collapse in the north Pacific ocean: an ongoing legacy of industrial whaling? Springer, Estes, vanvliet, Williams, Doak, Danner, Forney, & Pfister 2003

6 Background Abrupt decline of western stock of Steller sea lions (Eumetopias jubatus) across northern North Pacific and southern Bering Sea Widely attributed to nutritional limitation due to a) climate regime shift, and/or b) competition with fisheries However Adults and pups have better body condition, reduced foraging effort and field metabolic rates relative to other SSL stocks Abundance and trends in abundance of SSL prey incongruent with hypothesis of prey limitation as driver of SSL decline Abundance of seabird populations feeding on SSL prey species stable or increasing over same time period

7 An alternative hypothesis: killer whale predation as the cause of SSL decline Evidence: Precipitous declines in abundance of multiple marine mammal species in the north Pacific since 1980s (northern fur seal, harbor seal, sea otter) Incidental mortality from fisheries and intentional harvesting (1960s-70s) initial cause, but not continued decline Killer whales were likely responsible for sea otter decline (Estes et al. 1998) Prey switching a critical assumption Why the sudden and dramatic change in predator-prey relationships?

8 i) killer whales are predators of large whales Killer whales attack and consume all species of great whales Scars from KW teeth indicate attacks are common 20-40% of individuals in some species typical >60% of individuals in Southern Ocean sperm whales Migration of baleen whales proposed as an evolutionary response to KW predation Failure of bowhead whales in eastern Canadian Arctic to recover from commercial whaling proposed as result of KW predation

9 ii) industrial whaling depleted large whales in the north Pacific N Pacific right, bowhead, humpback, blue, gray whales depleted by early 1900s Early post-war industrial whaling focused in far western N Pacific primarily by Japan on fin, sei, sperm whales (A) Progressive spread and intensification of whaling to east by Japan and Soviet Union, as stocks to the east were depleted (B, C) By early 1970s depletion of stocks drives whaling south to exploit Bryde s and female sperm whales(d) < 500,000 whales removed from N Pacific and southern Bering Sea Combined current biomass of existing whales (1990s early 2000s) ~14% of pre-exploitation levels

10 iii) in response to depletion of their prey by industrial whaling, killer whales shifted to other species Sequential declines of pinnipeds and sea otters consistent with this hypothesis Initial decline began following collapse of industrial whaling Harbor seals & fur seals first higher energy density and/or less aggressive behavior? Ultimate shift to smaller and less energetically rich prey (sea otters) due to exhaustion of other prey species

11 Killer whales and marine mammal trends in the north Pacific a re-examination of evidence for sequential megafauna collapse and the preyswitching hypothesis we suggest that there are little data to support this hypothesis and much to contradict it. Wade and 23 others 2007

12 i. Spatial scale and geographic region is important Data presented in Springer et al. are not consistent with a regional approach, e.g., Peak of whale biomass removal differs Known KW predation patterns differ

13 ii. In all three regions, the biomass of large baleen & sperm whales is 5-24 x biomass of pinnipeds Argues against prey switching *from* large whales

14 iii. Trends in biomass do not support the SMC hypothesis, e.g., In WCNA, biomass of humpback and blue whales has increased In Gulf of Alaska, biomass of humpback whales has increased In BSAI, biomass of gray and fin whales has increased

15 Additionally, There was no large population of mammal-eating KWs that preyed primarily upon large whales. Minke whales and small cetaceans have likely always been available Unidirectional prey switching of extended duration is rare and maladaptive. The decline of pinnipeds and sea otters was not sequential.

16 The sequential megafaunal collapse hypothesis: testing with existing data statistical tests of the timing of the declines do not support the assumption that pinniped declines were sequential DeMaster et al. 2006

17 SMC Assumption 3: Pinniped declines in the 1970s and 1980s in the BSAI were sequential Springer et al Three populations of pinnipeds from same geographic region began to decline simultaneously DeMaster et al. 2007

18 This debate continues

19 The Ecosystem Consequences of Consuming, Competing, and Predation: Marine Mammals as Drivers of Food Web Restructuring ( Top-Down Forcing ) Despite its popularity among Southern Ocean ecologists 25 years ago, [the] top-down hypothesis (consumer effects on resources) has been largely replaced by an emphasis on bottom-up explanations (Ainley et al. 2009)

20 Example 1 Food web restructuring in the western Ross Sea ~Nov Adelie penguins (incubating eggs) feed on crystal krill A. silverfish feed on c. krill ~Dec Penguin chicks hatch; adults feed themselves and chicks on krill minke whales arrive and feed on krill ~Jan Krill abundance declines Penguins and minke whales shift to silverfish Older silverfish become cannibalistic Ainley et al and references therein ~Feb Silverfish abundance declines Penguin foraging duration and distance increases Minke whales leave to forage elsewhere Grazing pressure on phytoplakton relaxes

21 Example 2 Food web restructuring through killer whale predation in western Australia Humpback whales recovering from industrial whaling 568 (1963) 20,000 33,000+ (2012) Now at (or over) pre-exploitation levels Largest breeding population of HB in the world s oceans 2 known KW attacks prior to 2006 (1951, 1952) 33 KW-HB interactions % of 22 attacks with known outcomes resulted in kills Pitman et al. 2014

22 Impact on HB abundance 1 satellite-tagged female 6 days of tracks 3 humpbacks killed in 20.3 hr foraging Extrapolating to KW population off WA during ~5 mos of HB presence = as a minimum, dozens of calves are taken there annually Pitman et al. 2014

23 Food web restructuring Large numbers of sharks reported among KWs attacking HBs Remains of ~9m humpback (yearling?) <60 h after kill - dozens of sharks All of this largesse ultimately derives from the Southern Ocean where Antarctic krill (Euphausia superba) is shapeshifted into Area IV humpbacks and injected annually into the relatively oligotrphic, littoral marine ecosystem of Western Australia, where killer whales do their part in making it available to local consumers. Pitman et al. 2014

24 So, how do marine mammals impact their ecosystems? They remove a huge amount of prey (krill, fish, and maybe a huge amount of squid, penguins, and other marine mammals) They impact other predators of these prey through competition They may drive evolutionary responses of their prey They have the potential to restructure food webs

25 Let us not forget, marine ecosystems are highly perturbed Many marine mammal populations are recovering from commercial exploitation; food webs are restructuring as a consequence The planet is warming Fisheries are removing krill and fish (and whales)

26 Reading Springer, Estes, vanvliet, Williams, Doak, Danner, Forney, & Pfister Sequential megafaunal collapse in the north Pacific ocean: an ongoing legacy of industrial whaling? Proceedings of the National Academy of Science Roman, J., J. Estes, L. Morissette, C. Smith, D. Costa, J. McCarthy, J.B. Nation, S. Nicol, A. Pershing, V. Smetacek Whales as marine ecosystem engineers. Frontiers in Ecology and the Environment 12(7): additional roles of marine mammals in ecosystems