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The Role of Marine Mammals in Marine Ecosystems
Lisa T. Ballance SIO 133 – Marine Mammal Biology
Spring 2014
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Preface: The Take-Home Message
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Sources: RL Brownell pers. comm.; synthesis by Hewitt and Lipsky 2009
~15.7 million tons of whale in the world’s marine ecosystems
Current Population Estimates
Southern Hemisphere North Pacific North Atlantic Total Mean Body Mass
(tons) Tons of Whale in the
Ecosystem
Humpback Whale 56,000 20,000 12,000 88,000 31.8 2,798,400
Right Whales 12,000 550 450 13,000 53.0 689,000
Gray Whales 20,000 --- --- 20,000 25.0 500,000
Blue Whales 4,600 2,000 2,000 8,600 69.2 595,120
Fin Whales 15,000 11,000 37,000 62,000 42.3 2,622,600
Sei Whales 10,000 10,000 12,400 32,400 19.9 644,760
Bryde's Whales 50,000 39,000 5,000 94,000 13.2 1,240,800
Minke Whales 500,000 30,000 185,000 715,000 7.0 5,005,000
Bowhead Whales --- 12,000 8,000 20,000 80.0 1,600,000
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This sheer tonnage implies that marine mammals can play significant roles in
marine ecosystems. • Cetaceans in the California Current
are estimated to consume ~12% of the ecosystem’s net primary productivity (Barlow et al. 2008).
• Benthic feeding by gray whales provides habitat for benthic scavengers, prey for seabirds, and mixes sediment, with the potential to alter primary production (Oliver and Slattery 1985, Obst and Hunt 1990, Pilskaln et al. 1998).
• Whale carcasses support 350+ species and can be havens of deep-sea biodiversity (Jones et al. 1998, Smith 2006).
• A decline of sperm whales in the tropical and subtropical Pacific may have resulted in a shift toward ecosystems dominated by squids and tunas (Essington 2006).
• Killer whales may have sequentially depleted marine mammal species in the Bering Sea (Springer et al. 2003).
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“The irony is that, although whales have become a symbol of the human capacity for greedy overharvest and a rallying point for environmental activists, we know surprisingly little about their ecological role.”
Total Number of Publications Number (%) Addressing Whale Ecology, Species
Interactions, Effects on Ecosystem Processes
Marine Ecology (1999-2003) 106 0 (0%)
Marine Mammal Science (1985-2003) 94 (focused on whales) 17 (18%)
Kareiva et al. 2006
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Marine Mammals and Ecosystems: a trophic
interactions perspective
• MMs as Consumers • MMs as Competitors • MMs as Prey
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Marine Mammals as Consumers
“The combination of great abundance, large body size, and endothermic metabolic demands make
odontocete and mysticete whales some of the most voracious consumers in the oceans. As such, they
have the potential to place extraordinary pressures on marine resources.” (Williams 2006)
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When we think of consumers in an ecosystem context, we ask questions
like:
• What is eaten? • How much? • How does this consumption
impact the ecosystem?
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Case Study: Cetaceans in the Southern Ocean1 Ecosystem
1South of the Polar Front
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Krill
E. superba, E. crystallorophias, T. macrura
“Krill-based ecosystem” in about one quarter of the ~32 million sq. km of the Southern Ocean (Siegel & Loeb 1995)
Estimated Abundance Mean Mass (tons)
Ingestion Rate/Day
(1,000 kcal)
No. Days Spent
Feeding % Krill in
Diet
Total Krill Consumed Annually (millions of
tons) Antarctic Blue Whales 2,300 83 3,708 120 100 1.1 Fin Whales 15,000 48 2,415 120 100 4.7 Sei Whales 20,000 17.5 1,096 120 80 2.3 Humpback Whales 55,000 26.5 1,517 120 100 10.8 Antarctic Minke Whales 500,000 7 535 120 100 34.5
53.4
How much krill is consumed?
Sources: RL Brownell pers. comm.; synthesis by Hewitt and Lipsky 2009
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~53.4 million tons krill consumed annually by 5 species of whales
• For some perspective, annually: – 104,182 – 211,984 metric tons harvested: 2000 – 2011 (SC-CAMLR
2011) – 190 million tons consumed by baleen whales alone prior to
commercial exploitation (Laws 1985) – 250 million tons consumed by current populations of all krill predators
(whales, birds, pinnipeds, fish, squid: Miller and Hampton 1989)
• Despite much careful attention, estimates of krill consumption are invariably associated with a great degree of uncertainty (e.g., Leaper and Lavigne 2007)
• How does this consumption impact the ecosystem?*
*remember for later
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Squid • Sperm whales
– 10,000 males (Brownell, pers. comm.)
• Beaked whales – 599,300 Ziphiid whales (almost all H.
planifrons) in the Southern Ocean (Kasamatsu & Joyce 1995)
– H. planifrons – the most abundant cetacean in the Southern Ocean?
• Killer whales? (Type B)
• How much squid is consumed?*
• How does this consumption impact the ecosystem?*
*Big Unanswered Questions Durban & Pitman, unpublished data
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Other Species
• Killer whales as consumers of:
Fish, penguins, seals, whales
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Fish - Ross Sea KWs “Type C”
Krahn et al. 2008
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Penguins – Gerlache KWs “Mini Type B” Pitman & Durban 2010
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Seals - Pack Ice KW “Big Type B” Pitman & Durban 2011
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Whales – “Type A” Pitman & Ensor 2003
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How much of these species do killer whales consume?
• ~ 25,000 killer whales in the Southern Ocean (Branch & Butterworth 2001)
Prey specialization (?)
• What is the distribution and abundance of KW ecotypes?*
• How does this consumption impact the ecosystem?*
*Big Unanswered Questions
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Marine Mammals as Competitors
“…competition from any one species may profoundly affect the population dynamics
and carrying capacity of another, either through its effect on their mutual resources
or by direct interference.” (Rickleffs 1990)
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Case Study: Marine Mammals in the Southern Ocean1 Ecosystem
1South of the Polar Front
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Competitive Release: The “Krill Surplus” Hypothesis
• > 2 million whales removed from Southern Hemisphere (Clapham and Baker 2006)
• Up to 150 million tons krill/yr unconsumed (Laws
1977)
• Theoretically could support addition of 200-300
million penguins per year (Sladen 1964; Emison 1968)
• Growth in penguin populations attributed to “krill
surplus” (Sladen 1964; Emison 1968; Conroy 1975; Croxall and Kirkwood 1979; Croxall et al. 1981; Laws 1985; Rootes 1988)
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Antarctic Fur Seals: the best example of competitive release?
• Remarkable recovery from commercial exploitation at South Georgia
• Populations above pre-exploitation levels?
Hodgson and Johnston 1997
South Georgia • Range expansion to Signy, South Orkneys, due to population increase
• Numbers 78-94% > than during past 6570 (±60) radiocarbon years – exceeding range of natural variability
Boyd 1993
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Competition for krill is occurring.
• At Anvers Island, there are significant relationships between humpback whale abundance, the size-frequency distribution of krill targeted by Adélie Penguins, and penguin foraging success (1993-2001).
Humpback whales and Adélie Penguins appear to target and share similar spatio-temporal prey resources.
Friedlander et al. 2008
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This competition can result in changes in abundance (of predators and prey).
• Penguin population declining • Interpretation
– Predators reaching K – Competition for krill is
occurring – Fur seals are outcompeting
penguins
• At Bird Island, South Georgia, Antarctic fur seals and Macaroni Penguins exploit the same size and population of krill
• % krill in fur seal diet > in penguin diet
Antarctic Fur Seal
Macaroni Penguin
Macaroni Penguin
Antarctic Fur Seal
Barlow et al. 2002
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Competition can result in niche partitioning.
• Near the South Shetland Islands baleen whales show a krill size-dependent relationship with krill abundance hotspots
Santora et al. 2010
Krill Hotspots Whale Hotspots
Smal
l M
ediu
m
Larg
e
Fin
Min
ke
Hum
pbac
k
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More on competition
• Clapham, Phillip J., and R. L. Brownell. 1996.
The potential for interspecific competition in baleen whales. REPORT-INTERNATIONAL WHALING COMMISSION 46: 361-370.
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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)
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Whales are prey for killer whales ….
…. or humans.
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Killer whale predation may drive:
• Baleen whale migration (Corkeron & Connor 1999)
• Trophic cascades: prey
switching due to loss of large whales (Branch & Williams 2006)
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Sequential megafaunal collapse in the north Pacific ocean: an ongoing legacy of industrial whaling?
Springer, Estes, vanVliet, Williams, Doak, Danner, Forney, & Pfister 2003
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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
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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)
Why the sudden and dramatic change in
predator-prey relationships?
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Sequential megafaunal collapse: 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
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Sequential megafaunal collapse: 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)
• 1949-1969 minimum of half a million whales removed from N Pacific and southern Bering Sea
• Combined current biomass of existing whales (1990s – early 2000s) ~14% of pre-exploitation levels
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Sequential megafaunal collapse: 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
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Killer whales and marine mammal trends in the north Pacific – a re-examination of evidence for sequential megafauna collapse and the prey-switching hypothesis
“…we suggest that there are little data to support this hypothesis and much to contradict it.”
Wade and 23 others 2007
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• 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
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• 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
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• 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
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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
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The sequential megafaunal collapse hypothesis: testing with existing data
DeMaster et al. 2006
“… statistical tests of the timing of the declines do not support the assumption that pinniped declines were sequential”
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Assumption 3: Pinniped declines in the 1970s and 1980s in the BSAI were sequential
Three populations of pinnipeds from same geographic region began to decline simultaneously
Springer et al. 2003
DeMaster et al. 2007
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This debate continues
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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)
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Trophic cascades 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
• ~Feb – Silverfish abundance declines – Penguin foraging duration
and distance increases – Minke whales leave to forage
elsewhere – Grazing pressure on
phytoplakton relaxes
Ainley et al. 2006 and references therein
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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
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• Let us not forget, marine ecosystems are highly perturbed marine ecosystem – Many marine mammal populations are recovering
from commercial exploitation – The planet is warming – Fisheries are removing krill and fish (and whales)
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The Big Unanswered Questions • Which is more important: top-down or bottom-up forcing
– *Both* are important – As MMs recover, top-down forcing deserves more attention
• Trophic linkages: which direction and how strong?
– Squids: what are their trophic links with MMs and their importance?
– What is the distribution and abundance of killer whale ecotypes and to what extent do the specialize in (or switch) prey?
– What are the indirect effects of food web restructuring?
• Are MMs a year-round component of the ecosystem?
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Durban and Pitman 2011
Satellite-tracked adult female Type B killer whale 13 Feb, 2010 for 109 days 30° S; 22.4° C North of 60, swim velocity ≥ 5-10 km/hr Returned 1 June, 2010
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Reading • Ballance, L.T., R.L. Pitman, R.P. Hewitt, D.B. Siniff, W.Z.
Trivelpiece, P.J. Clapham, and R.L. Brownell, Jr. 2006. The removal of large whales from the Southern Ocean: evidence for long-term ecosystem effects? Pp. 215-230 in: Whales, whaling, and ocean ecosystems. J.A. Estes, D.P. DeMaster, D.F. Doak, T.M. Williams, and R.L. Brownell, Jr. (eds.). University of California Press.
• Springer, Estes, vanVliet, Williams, Doak, Danner, Forney, & Pfister. 2003. Sequential megafaunal collapse in the north Pacific ocean: an ongoing legacy of industrial whaling? Proceedings of the National Academy of Science www.pnas.orgcgidoi10.1073pnas.1635156100