Killer whale predation on sea otters: how predation can cause major changes in an ecosystemSpecial thanks to Dr. James Estes for the images and figures

Location of the ecosystem Aleutian Islands, Alaska

Change in sea otter populations around the Aleutian Islands

Chart1

1001911

108.96646626911912

118.73690771171913

129.38341249071914

140.98453252941915

153.62586308331916

167.40067427731917

182.41059927061918

198.76638412541919

216.58870491231920

236.0090580811921

257.17073066581922

280.22985748491923

305.35657313221924

332.73626726261925

362.57095243171926

395.0807545831927

430.50553717841928

469.10667095611929

511.16896237341930

557.00275496271931

606.94621910431932

661.36784711181933

720.66917203771934

785.28773026021935

855.70028970931936

932.42636755061937

1016.03206328121938

1107.13423551841939

1206.40505329981940

1314.57695547251941

1432.44805476621942

1560.88802641911943

1700.84452480621944

1853.35017541261945

2019.53019373921946

2200.61068735491947

2397.92770235061948

2612.93708093911949

2847.2252029341950

3102.52069036021951

3380.70716155291952

3683.83712885031953

4014.14714241691954

4374.07429193341955

4766.27418790461956

5193.64055525551957

5659.32658378041958

6166.76819297281959

6719.70938288871960

7322.22985808641961

7978.7751284571962

8694.18930903681963

9473.75086080251964

10323.21153615411965

11248.83881643031966

12257.462154571967

13356.52336410621968

14554.1315262721969

15859.12282033441970

17281.12571859351971

18830.63202707041972

20519.07429603441973

22358.91017151811974

24363.71431018371975

26548.27853570411976

28928.72097563251977

31522.60498399211978

34349.06872702061979

37428.9663881761980

40785.02203424131981

44441.9972777831982

48426.87397300591983

52769.05329297091984

57500.57265700391985

62656.34210882861986

68274.40188946321987

198874405

19891989

19901990

19911991

199228811

19931993

19941994

19951995

19961996

19971997

19981998

19991999

20008742

20012001

20022002

20033311

Year

Estimated Population Size

Sheet1

198874,405110367

1989

1990

1991

199228,811

1993

1994

1995

1996

1997

1998

1999

20008,742

2001

2002

20033,311

Sheet1

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Year

Estimated Population Size

Sheet2

198874,405

1989

1990

1991

199228,811

1993

1994

1995

1996

1997

1998

1999

20008,742

2001

2002

20033,311

Aleutian Archipelago

Estimated carrying capacity

Sheet4

1911100

1912108.9664662691

1913118.7369077117

1914129.3834124907

1915140.9845325294

1916153.6258630833

1917167.4006742773

1918182.4105992706

1919198.7663841254

1920216.5887049123

1921236.009058081

1922257.1707306658

1923280.2298574849

1924305.3565731322

1925332.7362672626

1926362.5709524317

1927395.080754583

1928430.5055371784

1929469.1066709561

1930511.1689623734

1931557.0027549627

1932606.9462191043

1933661.3678471118

1934720.6691720377

1935785.2877302602

1936855.7002897093

1937932.4263675506

19381016.0320632812

19391107.1342355184

19401206.4050532998

19411314.5769554725

19421432.4480547662

19431560.8880264191

19441700.8445248062

19451853.3501754126

19462019.5301937392

19472200.6106873549

19482397.9277023506

19492612.9370809391

19502847.225202934

19513102.5206903602

19523380.7071615529

19533683.8371288503

19544014.1471424169

19554374.0742919334

19564766.2741879046

19575193.6405552555

19585659.3265837804

19596166.7681929728

19606719.7093828887

19617322.2298580864

19627978.775128457

19638694.1893090368

19649473.7508608025

196510323.2115361541

196611248.8388164303

196712257.46215457

196813356.5233641062

196914554.131526272

197015859.1228203344

197117281.1257185935

197218830.6320270704

197320519.0742960344

197422358.9101715181

197524363.7143101837

197626548.2785357041

197728928.7209756325

197831522.6049839921

197934349.0687270206

198037428.966388176

198140785.0220342413

198244441.997277783

198348426.8739730059

198452769.0532929709

198557500.5726570039

198662656.3421088286

198768274.4018894632

198874,405

1989

1990

1991

199228,811

1993

1994

1995

1996

1997

1998

1999

20008,742

2001

2002

20033,311

Sheet4

Year

Estimated Population Size

Sheet3

19110100100

19121108.9664662691

19132118.7369077117

19143129.3834124907

19154140.9845325294

19165153.6258630833

19176167.4006742773

19187182.4105992706

19198198.7663841254

19209216.5887049123

192110236.009058081

192211257.1707306658

192312280.2298574849

192413305.3565731322

192514332.7362672626

192615362.5709524317

192716395.080754583

192817430.5055371784

192918469.1066709561

193019511.1689623734

193120557.0027549627

193221606.9462191043

193322661.3678471118

193423720.6691720377

193524785.2877302602

193625855.7002897093

193726932.4263675506

1938271016.0320632812

1939281107.1342355184

1940291206.4050532998

1941301314.5769554725

1942311432.4480547662

1943321560.8880264191

1944331700.8445248062

1945341853.3501754126

1946352019.5301937392

1947362200.6106873549

1948372397.9277023506

1949382612.9370809391

1950392847.225202934

1951403102.5206903602

1952413380.7071615529

1953423683.8371288503

1954434014.1471424169

1955444374.0742919334

1956454766.2741879046

1957465193.6405552555

1958475659.3265837804

1959486166.7681929728

1960496719.7093828887

1961507322.2298580864

1962517978.775128457

1963528694.1893090368

1964539473.7508608025

19655410323.2115361541

19665511248.8388164303

19675612257.46215457

19685713356.5233641062

19695814554.131526272

19705915859.1228203344

19716017281.1257185935

19726118830.6320270704

19736220519.0742960344

19746322358.9101715181

19756424363.7143101837

19766526548.2785357041

19776628928.7209756325

19786731522.6049839921

19796834349.0687270206

19806937428.966388176

19817040785.0220342413

19827144441.997277783

19837248426.8739730059

19847352769.0532929709

19857457500.5726570039

19867562656.3421088286

19877668274.4018894632

198874,405

1989

1990

1991

199228,811

1993

1994

1995

1996

1997

1998

1999

20008,742

2001

2002

20033,311

WHY?

Killer whale prey in the Aleutians

Images from Estes et al. 1998

Images from Estes et al. 1998

Your turn!Make a hypothesis about what is going to happen if the sea otters continue to decline or even disappear.

Initially, a dramatic decline in sea otter populations was first noted which indicated that some sort of major shift in predation as a result of a disturbance had happened. This is the kind of figures and data that a real biologist would make. How does it compare to the bar graphs you made? Is it similar? What happened? The number of sea otters in this population is on the y axis (left) and the Year is on the x axis.What is causing this major decline!?Killer whales had begun to prey upon the sea otters very heavily. But Why? Both organisms had always been in this ecosystem and this trend had not happened before.Killer whales prey was at one point, mainly northern sea lions, whales, and some sea otters but theres been a shift away from sea lions to sea otters. Why?Make a hypothesis. Why did killer whales start eating sea otters at such a high rate? What might have happened in their ecosystem to cause this?

Heres another graph from a Biologist. What do you see? Do the different animal populations increase or decrease?Decline in whales, followed by a decline in harbor seals, followed by a decline in sea lions, and finally by a decline in sea otters. The decline in whales was due to commercial whaling that became very heavy following WW 2 and ended at the end of the 1970s. At this same time, there was also heavy commercial fishing. Because of the decreasing numbers of fish in the oceans, the harbor seals and sea lions did not have as much food as they needed. The killer whales were slowly shifting between prey as the prey became less available until they ended up eating sea otters. Sea otters have small bodies so these are not the best food for a large, killer whale. Theyd much rather use their energy to kill something large bodied that would provide lots of food and energy.That decline in animals was happening mainly out in the ocean. Change then began to happen near shore when the killer whales began eating sea otters. IN this picture, you can see a sea otter eating a sea urchin in a kelp bed.When sea otters are present, they eat a huge amount of sea urchins. This is important because the sea urchins eat the kelp. Youve seen this image before. You can see a kelp bed with otters present.This photo was taken at the exact same place as the photo above but this photo was taken AFTER the killer whales had eaten all of the sea otters that were present in this area. This image shows how the sea urchin populations have gotten out of control and have eaten all of the kelp. We call this sort of an area an urchin barrens.Here is what happened after the killer whales began focusing predation on the sea otters.Fig. 1. (A) Changes in sea otter abundance over time at several islands in the Aleutian archipelagoand concurrent changes in Sea otters declined on four different islands.(B) sea urchin biomass, The number of sea urchins increased. The number of sea urchins in 1997 was MUCH larger than in the 1980s.

(C) grazing intensity, The amount of kelp the sea urchins ate also increased dramatically in 1997 from 1990 because there were MORE sea urchins.

and (D) kelp densityThe amount of kelp decreased because the otters were gone or were fewer in number and werent eating as many sea urchins and the increasing number of sea urchins were eating many, many more kelp beds.

measured from kelp forests at Adak Island. Error bars in (B) and (C) indicate 1 SE. The proposedmechanisms of change are portrayed in the marginal cartoons, the one on the left shows how thekelp forest ecosystem was organized before the sea otters decline and the one on the right showshow this ecosystem changed with the addition of killer whales as an apex predator. Heavy arrowsrepresent strong trophic interactions; light arrows represent weak interactions.

Population trends and survival rates ofsea otters in Clam Lagoon (solid squares) andadjacent Kuluk Bay (open circles), Adak Island,Alaska. Clam Lagoon did not have killer whales.You can see that the number of sea otters in the area without killer whales had more sea otters than the area that had killer whales that were eating sea otters.