Download Educator Materials Short Film Some Animals Are More Equal than

Short Film
Some Animals Are More Equal than
Others: Trophic Cascades and Keystone
Species
Educator Materials
IN-DEPTHFILMGUIDE
DESCRIPTION
TheshortfilmSomeAnimalsAreMoreEqualthanOthers:TrophicCascadesandKeystoneSpeciesopensbyaskingtwo
fundamentalquestionsinecology:“Whatdetermineshowmanyspeciesliveinagivenplace?Orhowlargecaneach
populationgrow?”ThefilmthendescribesthepioneeringexperimentsbyRobertPaineandJamesEstes,inthe1960sand
1970s,whichstartedtoaddressthem.
Paine’sexperimentsonthecoastofWashingtonstateshowedthatthestarfishisakeystonespecies,havinga
disproportionatelylargeimpactonitsecosystemrelativetoitsabundance.EstesandcolleagueJohnPalmisano
discoveredthatthekelpforestsoftheNorthPacificareindirectlyregulatedbyseaotters,whichfeedonseaurchinsthat
consumekelp.Thepresenceorabsenceofseaotterscausesacascadeofdirectandindirecteffectsdownthefoodchain,
whichinturnaffectthestructureoftheecosystem.
Theseearlyexperimentsinspiredcountlessothersonkeystonespeciesandtrophiccascadesinecosystemsthroughout
theworld.
KEYCONCEPTS
A. Keystonespecieshavedirectandindirecteffectsontheabundanceandnumberofspeciesinanecosystemthat
aredisproportionatelylargerelativetotheirownabundanceintheecosystem.
B. Notallspeciesinanecosystemhavestronginteractions.Theremovalofsomespecieshaslittleornoeffecton
others.
C. Manykeystonespeciesareapexpredators:predatorsatthetopofafoodwebthatarenotpreyedonbyothers.
D. Removaloradditionofanapexpredatorthatisakeystonespeciescauseschangesinthetypeandnumberof
species,andtheirpopulationsizes,atmultipletrophiclevels.
E. Keystonespeciesarecriticaltomaintainingthediversityandstabilityofanecosystem.
F. Identifyingtheinteractionsamongspeciesinanecosystemanddetermininghowspeciesnumbersandpopulation
sizesareregulatedrequiresexperimentsconductedoverlongtimeperiods.
CURRICULUMCONNECTIONS
Curriculum
Standards
NGSS(2013)
HS-LS2-1,HS-LS2-2,HS-LS2-6
APBiology(2012–13)
4.A.5,4.A.6,4.B,4.C.4
IBBiology(2016)
4.1,4.2,C.1,C.2,C.3,C.4,C.5
APEnvironmentalScience(2013)
II.A.EcosystemStructure
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PublishedAugust2016
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Short Film
Some Animals Are More Equal than
Others: Trophic Cascades and Keystone
Species
Educator Materials
PRIORKNOWLEDGE
Itwouldbehelpfulforstudentsto
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befamiliarwiththedefinitionsofbiologicalcommunitiesandecosystems:abiologicalcommunityisagroupof
interactingorganisms,whereastheecosystemalsoincludestheirenvironment(i.e.,abioticfactors).
befamiliarwiththeconceptsoffoodchains,foodwebs,andtrophiclevelsandhowenergyflowsfromproducers
toprimaryconsumersandthentothehighertrophiclevelsofsecondaryandtertiaryconsumers/predators
(carnivoresandomnivores).
understandthatahypothesisisapotentialexplanationforanobservationinnature.
PAUSEPOINTS
Begin End
1 0:00
ContentDescription
1:13
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2 1:14
4:05
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ReviewQuestions
Diversehabitatsaroundthe
worldaremadeupof
communitiesofplantsand
animals.
Communitiesaremadeup
ofavarietyofspecies.
Ecologistsareinterestedin
understandingwhat
controlsthenumberand
typesofspeciesinan
ecosystem.
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Priortotheearly1960s,
mostecologiststhought
thatthenumberof
producerslimitsthe
numberofherbivores,
whichinturnlimitsthe
numberofpredators.This
“bottomup”explanation
suggeststhateverytrophic
levelisregulatedbythe
levelbelowit.
Thebottom-upexplanation
didnotexplainwhy
herbivorepopulationsdo
notgrowlargeenoughto
eatalltheproducers.
•
Whywoulditbe
importanttounderstand
whatcontrolsthe
numberandtypesof
speciesinanecosystem?
Standards
NGSS(2013)
HS-LS2-6
AP(2012–13)
4.B.3;4.C.4
IB(2016)
4.1,C.1
APES(2013)
II.AEcosystem
structure
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Priortothe1960s,what
didpeoplethink
controlledthenumberof
herbivoresinafood
web?
Whatwasmissingfrom
this“bottomup”
explanation?
Describehowthegreen
worldhypothesisdiffers
fromthebottom-up
explanation.
NGSS(2013)
HS-LS2-1,HS-LS2-2,
HS-LS2-6
AP(2012–13)
4.B.3;4.C;4.C.4
IB(2016)
4.1,4.2,C.1,C.5
APES(2013)
II.AEcosystem
structure
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Short Film
Some Animals Are More Equal than
Others: Trophic Cascades and Keystone
Species
Educator Materials
3 4:06
7:05
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Thegreenworldhypothesis
proposedthatpredators
keepherbivorepopulations
inchecksothattheydon’t
consumealltheplants.
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RobertPainetestedthe
greenworldhypothesison
therockycoastofthe
PacificOcean.Hecreateda
foodwebforthe
communityandthen
removedthepredator,
Pisastorochraceus,a
starfish,fromoneoutcrop.
Painediscoveredthatone
predatorcouldregulatethe
compositionofanentire
community.
•
Keystonespeciesare
speciesinafoodwebthat
havelargedirectand
indirecteffectsonthe
abundanceandnumberof
speciesinanecosystem.
Keystonespeciesareoften
apexpredatorsandare
instrumentalinmaintaining
thestructureofan
ecosystem.
Somespecieshavelittle
influenceonthe
distributionandabundance
ofotherspecies.
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Painenotedanother
patternintidepools:some
hadkelpwhileothers
didn’t.Thosewithsea
urchinshadlesskelp.
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4 7:06
8:44
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5 8:45 11:11
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Whywasitimportantfor
Painetorecordthe
numberofdifferent
speciesaswellasthe
overallnumberof
animalsontherock
outcrop?
Whathappenedtothe
numberofdifferent
speciesovertime?
Whatconclusiondid
Painearriveatafter
sevenyears?
NGSS(2013)
HS-LS2-1,HS-LS2-2,
HS-LS2-6
Whyisthearchshape
andthekeystoneatthe
topofthearchafitting
analogyforwhatRobert
Paineobserved?
Whatdoeshemeanby
hisclaimthatsome
speciesaremoreequal
thanothers?
Whatevidencedoeshe
havetosupportthat
claim?
NGSS(2013)
HS-LS2-2,HS-LS2-6
Whywasitimportantfor
himtoremoveurchins
fromsometidepoolsbut
notallothers?
Howdidtheobservation
thatseaurchinswere
NGSS(2013)
HS-LS2-2,HS-LS2-6
AP(2012–13)
4.C;4.B.3;4.C.4
IB(2016)
4.1,4.2,C.1,C.5
AP(2012–13)
4.B.3;4.C;4.C.4
IB(2016)
4.1,4.2,C.1,C.5
APES(2013)
II.AEcosystem
structure
AP(2012–13)
4.B.3;4.C;4.C.4
IB(2016)
4.1,4.2,C.1,C.5
APES(2013)
II.AEcosystem
structure
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Short Film
Some Animals Are More Equal than
Others: Trophic Cascades and Keystone
Species
Educator Materials
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6 11:12 14:50
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7 14:51 18:53
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Paineexperimentedby
removingtheseaurchin,
andkelpstartedgrowing.
Theurchins(herbivores)
werekeepingthekelp
(producer)population
downtoverylowlevels.
Tofurtherexplorethe
impactofpredatorson
otherspecies,Paineand
JimEstescountedthe
numberofurchinsandkelp
atsitesintheAleutian
ArchipelagoinAlaskain
areaswithandwithout
otters.Siteswithoutotters
hadlotsofurchinsandlittle
kelp.Siteswithottershad
fewurchinsbutlotsofkelp.
Theotterswereindirectly
controllingtheamountof
kelp.
Atrophiccascadeoccurs
whenremovalofone
species(usuallyapredator)
affectstheabundanceand
distributionofspeciesthat
itdoesnotdirectlyprey
upon.
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Whenorcasstartedtoeat
seaotters,itcauseda
furthertrophiccascade.
Seaotternumbers
decreased,urchin
populationsincreased,and
theamountofkelp
decreased.
Trophiccascadesand
keystonespecieshavebeen
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eatingallthekelpin
sometidepoolsviolate
predictionsfromthe
greenworldhypothesis?
APES(2016)
II.AEcosystem
structure
Howdidthetwoislands
withandwithoutotters
differintheamountof
kelp?
HowdidPayneand
Estes’observations
supportthegreenworld
hypothesis?
Whatweretheindirect
effectsoftheremovalof
seaottersthatPaine
described?
Whatisatrophic
cascade,andwhyisit
importanttounderstand
therelationshipsit’s
describing?
NGSS(2013)
HS-LS2-2,HS-LS2-6
AP(2012–13)
4.B.3;4.C;4.C.4
Howhadhumanactivity
influencedthetrophic
cascadeoftheAleutian
Islandskelpforest?
Whyareapexpredators
criticaltoecosystem
structure?
NGSS(2013)
HS-LS2-2,HS-LS2-6
AP(2012–13)
4.B.3;4.C;4.C.4
IB(2016)
4.1,4.2,C.1,C.2,C.3,
C.4,C.5
APES(2013)
II.AEcosystem
structure
IB(2016)
4.1,4.2,C.1,C.2,C.3,
C.4,C.5
APES(2013)
II.AEcosystem
structure
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Short Film
Some Animals Are More Equal than
Others: Trophic Cascades and Keystone
Species
Educator Materials
identifiedinmanyother
habitats.Understanding
top-downeffectsgives
conservationistsand
scientistsanewwayof
lookingatthemanagement
ofecosystemswithan
emphasisonthecritical
roleofapexpredators.
BACKGROUND
TheGreenWorldHypothesisandKeystoneSpecies
Historically,ecologistshademphasizedtheimportanceof“bottom-up”forcesoncommunitystructure.Inthisview,the
amountandavailabilityofvegetation(primaryproduction)controlsherbivorenumbers,whichthendeterminepredator
populationsizes.Anewapproach,calledthegreenworldorHSShypothesis(afterecologistsNelsonHairston,FredSmith,
andLawrenceSlobodkin,whoproposeditin1960),flippedthisidea:insomeecosystems,predatorscontrolherbivore
populations,indirectlysupportingplantcommunitiesandkeepingtheworldvisiblygreen.
RobertPainewasoneofthefirstscientiststotestthegreenworldhypothesisexperimentallyattheecosystemlevel.In
the1960s,PainedesignedexperimentsinwhichheremovedallindividualsofthestarfishspeciesPisasterochraceus,a
predator,fromexperimentalplotsalongthePacificNorthwestcoast.Atthestartofhisexperiment,mussels(Mytilus
californianus),oneofthemainfoodresourcesofthestarfish,coveredbetween1%and5%oftherockfacehabitat,and
sharedthehabitatwith17otherspecies.However,justthreeyearsafterthestarfishhadbeenremoved,onlyseven
specieswereleftontheexperimentalplotsandthemusselscoveredmostofthearea.Withoutthestarfish,themussels
crowdedoutmanyotherspeciesandalmostcompletelydisplacedfourspeciesofalgae,amajorcomponentofthe
vegetationofthiscoastalmarineecosystem.Thelossofalgaethenledtoareductioninotherspecies.
Bytheendofthestudyin1973,theplotswithoutstarfish(experimental)were“essentiallyamonoculture,”accordingto
Paine,withmusselscoveringtherockface,whereasinthecontrolplotsthetotalnumberofspecieshadremainedstable.
Thus,removingPisasterstarfishallowedmusselstoquicklydominatetherockfaces,whichinturndrovedownthe
diversity(i.e.,thenumberanddistributionofspecies)oftheentireecosystem.
Studentsmaywonderwhatthemusselsareeatingifallotherspeciesaregone.Musselsarewhat’sknownasfilterfeeders
thatliveonrockfaces.Theyfeedonplanktonfromtheoceanthatfloatsinwhenthetiderises.
In1969,PainewroteaboutthefindingsinalettertotheeditorofthejournalTheAmericanNaturalist,inwhichhe
introducedtheconceptofakeystonespecies.Heexplainedthat“[t]heseindividualpopulations[ofstarfish]arethe
keystoneofthecommunity'sstructure,andtheintegrityofthecommunityanditsunalteredpersistencethroughtime,
thatis,stability,aredeterminedbytheiractivitiesandabundances.”
InspiredbyPaine’sunprecedentedresults,manyecologistsinthe1970sbeganstudyingkeystonespeciesandpredators
andfurthertestingthegreenworldhypothesis.Forexample,JamesEstesandJohnPalmisanodemonstratedthatsea
ottersareakeystonespeciesintheNorthPacificcoastalecosystems.In1975,EstesandPalmisanoobservedthatinareas
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Short Film
Some Animals Are More Equal than
Others: Trophic Cascades and Keystone
Species
Educator Materials
whereotterswererare,seaurchinpopulationsincreasedinnumber.Theurchinsbecamesocommonthatthey
devastatedthekelpforests,overgrazingtheveryvegetationthatsustainstheirpopulations.However,inareaswherethe
otterpopulationshadrebounded,theotterssuppressedtheurchinsandenabledkelpforeststorecover.Theseresults
showthattheseaottersarethusindirectlysupportingthekelpforestsbyregulatingtheurchins,whichfeedonthekelp.
TheTrophicCascadeHypothesis
In1979,Painecombinedhisownresultswiththoseofothers,includingEstesandPalmisano,toproposetheideaof
trophiccascades,inwhichdirectandindirecteffectsfromaspeciesatornearthetopofthefoodchainpropagatedown
andregulatethespeciesandpopulationsizesintherestofthecommunity.
Researchcontinuedandbythe1990sitwasclearthatthetrophiccascadewasarealandmeasurablephenomenon.It
alsobecameclearthattrophiccascadescanchangeasecosystemschange.AsEstescontinuedtomonitortheotterurchin-kelpfoodchainthroughthe1990s,hediscoveredthatkillerwhaleshadstartedfeedingonotters.Estesargued
thatcollapsesinthekillerwhale’snormalfoodsources(includingStellersealionsandharborseals)inthe1970sand
1980scausedthisshiftinpredation.Asseaotterpopulationsdeclined,seaurchinnumberssoared,andthekelpforest
ecosystemagainsuffered.
Estesandhiscolleaguespublishedasummaryoftheirobservations,showninFigure1below,inthejournalSciencein
1998.
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Short Film
Some Animals Are More Equal than
Others: Trophic Cascades and Keystone
Species
Educator Materials
Figure1.InthekelpecosystemsoftheNorthPacific,thepresenceofseaottersinthe1970sand1980sdepressedsea
urchinpopulationsandallowedthekelpforeststogrow.Killerwhalesbeganfeedingonseaottersin1990infour
differentmonitoredlocations.Killerwhalepredationdrovedowntheseaotterpopulations,releasingpressureonthesea
urchinpopulations,whichinturnwereabletoincreasetheirpopulationsizesandsignificantlyreducethedensityofthe
kelpforests.(ReproducedwithpermissionfromEstes,J.A.,etal.Science282:473-476,1998.)
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Some Animals Are More Equal than
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Species
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DISCUSSIONPOINTS
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Studentsmayaskwhetherkeystonespeciesarealwaysapexpredators.Apexpredatorsandkeystonespeciesare
notnecessarilythesame.Anapexpredatorisaspeciesthatsitsattheverytopofitsfoodweb;nothingeatsan
apexpredator.Manyapexpredatorsarekeystonespecies,butnotall.Inthefilmwelearnthatseaottersarethe
keystonespeciesintheirfoodweb,butkillerwhalesaretheapexpredator.Somekeystonespecies,suchas
Africanelephantsandbeavers,arenotpredators.
Studentsmaybeconfusedaboutthedifferencebetweenbottom-upversustop-downregulation.Stressthatboth
typesofregulationplayaroleindeterminingthenumberofspeciesandpopulationsizes,it’snotjustoneorthe
other.Emphasizetothestudentsthat,beginninginthe1920swithCharlesElton’sfoodwebs,foodpyramids,and
trophiclevels,ecologistsfocusedonabottom-upviewofbiologicalcommunityregulation:theabundanceof
primaryproducers(commonlyplants)determinedthenumberofherbivoresandhencecarnivores.However,
RobertPaineandothersdemonstratedthatcommunitystructureisalsoregulatedfromthetopdown.The
presenceorabsenceofcertainspecies—keystonespecies—candirectlycontrolthepopulationsofsomespecies
andindirectlycontrolthepopulationsofotherspecies.Insomeecosystems,onetypeofeffect—eithertop-down
orbottom-up—maybemoreimportantthantheother,andinteractionswithintheecosystemsaffectthe
strengthofaparticulareffect.
StudentsmaywonderwhyRobertPaine’sworkisconsideredsogroundbreaking.UpuntiltheperiodwhenPaine
performedhisstarfishexperiments,thescienceofecologyconsistedmainlyofstudiesthatincludedcensuswork
andbehavioralobservation.Forover200years,ecologywasalmostexclusivelyadescriptivescience,and
scientistsstudyingecologywereoftenreferredtoasnaturalists.Naturalistsweremostlyinterestedindescribing
thenaturalhistoryofspeciesanddeterminingtheirdistributionsandassociationswithotherspecies.Paine
contributedtothetransitionofecologytoasciencebasedonmanipulativeexperiments.
InthefilmwelearnaboutanotherexperimentthatRobertPaineconductedbyremovingseaurchinsfromtheir
habitatstoseewhateffecttheywerehavingonkelp.BasedonhisexperimentandresearchbyEstesand
Palmisano,Painerealizedthattheurchinswouldconsumemostofthekelp,theirprimaryplantfoodsource,if
theirpredators(theseaotters)weregone.Askstudentsiftheycanthinkofotherexampleswherehumanshave
removedpredatorsfromanareaanddescribewhat,ifany,effectspredatorremovalhashadonthelocalplant
andanimalcommunities.Forexample,theremovalofpredatorslikewolvesandmountainlionshasledtoan
increaseinwhitetaildeerpopulationsinNorthAmerica,whichcanhavealargeimpactonplantcommunities.
Studentsmaybeundertheimpressionthattrophicinteractionsaresimpleandisolated.Makeitcleartostudents
thatinordertounderstandthemorecomplexnatureoffoodwebs,wemustfirstisolatesmallpartsofthem,a
three-orfour-memberfoodchain,forexample,andperformcontrolledexperiments.However,thesesimplefood
websarepartofmuchlargerandmorecomplexwebsofinteractions.Manyclassicfoodwebstudieswere
purposefullyconductedinareaswithlimitedspeciesrichness,suchastheArcticorremoteislands,tosimplifythe
experiments.
Studentsmayhavebeentaughttheconceptof“dominantspecies”andmaybeconfusedaboutthedifference
betweendominantandkeystonespecies.Explainthatdominantspecies,likethemusselsinRobertPaine’s
experiments,arenumericallyabundantintheirecologicalcommunities.However,removingthemmaynothave
muchofaneffectoncommunitystructureotherthanallowingasimilarspeciestoincreaseinabundanceandfilla
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Short Film
Some Animals Are More Equal than
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Species
Educator Materials
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similarniche.Ontheotherhand,akeystonespeciesisnotusuallyabundantinitscommunity,butaswesawwith
theseastars,removingakeystonespeciescansignificantlyaffectthedensityandpresenceorabsenceofmost
otherspeciesinthecommunity.
Studentsmaywonderhowtrophiccascadesarerelatedtothecarryingcapacityofanecosystemforaparticular
populationofaspecies.Thecarryingcapacityofanecosystemforapopulationisinfluencedbyavarietyof
factors.Theyincludeavailablefoodresources,water,space,andecologicalconditions.Ifpredationiseliminated,
asbyremovingthestarfish,thecarryingcapacityofanecosystemforaparticularpopulationcanincreaseand
allowthatpopulationtoincrease,ashappenedwiththemussels,whichinturnoutcompetedotherspecies.
What’sinaname:starfishorseastar?EchinodermsintheclassAsteroideaarecommonlyreferredtoasstarfish
orseastars.Whilethetermstarfishisusedinthefilm,someeducatorspreferthetermseastartoemphasizethat
theseanimalsarenotfish;theyaremorecloselyrelatedtootherechinodermslikeseaurchinsandsea
cucumbers.Thispointhighlightstheconfusionthatcanarisefromtheuseofcommonnamesinsteadof
taxonomicandbinomialnames.
Thefilmtouchesonthedifferencebetweenexperimentalscienceandobservationalscience.Inexperimental
science,treatmentsareusedtodirectlymanipulatesomeaspectofthesystem,suchastheremovalofstarfishor
theadditionofnitrogenfertilizertoafield.Inobservationalscience,carefulmonitoringisusedtotryto
understandtheresponsiblemechanisms,butnodirectinterventionsaremade.Forexample,scientistsrecording
climate-inducedchangesinplantfloweringdatesareconductinganobservationalstudy.Whenfeasible,scientists
preferexperimentalstudies,asthismakesiteasiertonarrowdownwhichvariablesareresponsiblefora
particulareffect.Inclimate-changestudies,thiscanbeaccomplished,forexample,byaresearcheraddingCO2
directlytosomeplotsofplantsandtestingtheinfluenceofhighCO2levelscomparedtocontrolplots.
StresstostudentsthatPaine’sandEstes’experimentsaddressedseveralclassicquestionsinecology:What
determineshowmanyspeciesliveinalocation?Howbigwilltheirpopulationsgrow?Theseexperiments
supportedpossiblemechanismsbutbynomeansfinishedansweringthequestions,andresearchonthese
questionscontinuestoday.
USINGTHEQUIZ
Wedesignedthequizasasummativeassessmentthatprobesstudents’understandingofthekeyconceptsaddressedin
thefilm.Someteachersusethequizbeforeandduringthefilmtoassessstudents’priorknowledgeandtoguidestudents
astheywatchthefilm.Weencourageyoutochoosetheusethatbestfitsyourlearningobjectivesandyourstudents’
needs.Moreover,becausethevocabularyandconceptsarecomplex,weencourageyoutomodifythequiz(e.g.,onlyask
someofthequestions,explaincomplicatedvocabularyforEnglishlearnerstudents)asneeded.
QUIZANDANSWERS
1. (KeyConceptsA&B)True/False.Allmembersofafoodwebareequalinabundanceandintheirrelativeeffects
ononeanother.
False.Somemembersofafoodwebcanhaveeffectsthataredisproportionatetotheirabundance.Forexample,
keystonespecieshavesmallpopulations,butbecauseofwhatandhowmuchtheyconsume,theirabsencefromthe
foodwebcouldhaveverylargeeffectsonthestructureoftheecosystem.
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2. (KeyConceptsA&B)ExplainthereasoningorevidenceyouusedtoanswerQuestion1.
Afterwatchingthefilm,studentsshouldrealizethatsomespecies,likethestarfishinRobertPaine’sfirstexperiment,
whichwerenotasabundantassomeotherspecies,canhavelargeeffectsonthestructureoftheirecosystems.When
thestarfishwereabsent,populationsoftheirprimarypreyspecies(mussels)increasedsoquicklythattheycrowded
outnearlyalloftheotherspeciesintheecosystem.Studentsmayalsomentionevidencefromthepresenceor
absenceofseaottersinthekelp-urchin-seaotterfoodchains.
3. (KeyConceptC)True/False.Everymemberofafoodwebisthepreyofanothermemberofthefoodweb.
False.Apexpredatorsareatthetopoffoodwebsanddonothavepredators.
4. (KeyConceptC)ExplainthereasoningorevidenceyouusedtoanswerQuestion3.
Thediagramofthefoodchaininthefilmshowsthatthestarfishisthetoppredatoranddoesnotitselfhave
predatorsinthetidepools.Also,thefilmdoesnotexplainthis,buttheorcasdon’thaveanypredatorseither.Their
populationsareregulatedfromthebottomupbytheavailabilityoffood.
5. (KeyConceptD)WhichstatementbelowexplainswhythemusselsinMukkawBaywereabletoquicklycoverthe
rockfaceinPaine’sexperiment?
A. Thestarfishtookupmostoftheroomontherocks,andwhenthestarfishwereremoved,themussels
occupiedtheemptyspaces.
B. Paineaddedmoremusselstotherocks,causingthestarfishtomovetootherhabitats.
C. Thestarfishwerecompetingdirectlywiththemusselsforfood,andremovingthestarfishallowedthe
musselsmoreaccesstothefood.
D. Starfishfeedonmussels,sowhenthestarfishwereremovedthemusselsnolongerhadapredatorand
theirpopulationsgrewunchecked.
6. (KeyConceptsE&F)Inthefilm,Painerecallsthatayearafterthestarfishhadbeenremoved,thenumberof
speciesdecreasedfrom15toeight,afterthreeyearsthenumberwentdowntoseven,andafteranotherseven
yearsitwasalmostallmussels.Inthecontrolplotsthenumberanddiversityofspecieswasbasicallyunchanged.
Whichstatement(s)bestexplain(s)theseresults?
I.Keystonespeciesarecriticaltothediversityandstabilityofanecosystem.
II.Whenapredatorisremoved,thepreyofthatpredatoralwaysincreasesandspeciesnoteatenbythepredator
alwaysdecrease.
III.Thedisappearanceofproducersfromanecosystemcancausethenumberofpredatorstoincrease.
A. Ionly
B. IandIIonly
C. IIandIIIonly
D. I,II,andIII
7. (KeyConceptsD&F)Beforethe1960s,mostecologiststhoughtthatthenumberofproducersinanecosystem
wastheonlyvariablethatlimitsthenumberofherbivores.Theideawasthateverylevelwasregulatedbythe
amountoffoodfromthetrophiclevelbelowit.
A.Howdidthegreenworldhypothesisdifferfromthis“bottom-up”view?Thegreenworldhypothesisexplained
thatthenumberofherbivoresisalsolimitedbypredatorsfromthetopdown.
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Some Animals Are More Equal than
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B.Imagineasimplefoodchain:Grass->Grasshoppers->Mice.Ifsnakesthateatmiceareaddedtothis
ecosystem,howwouldyouredrawthefoodchaintorepresentthischange?
Grass->Grasshoppers->Mice->Snakes
C.Afterthesnakesareadded,wouldyouexpecttheamountofgrasstoincreaseordecrease?Explainyour
reasoning.
Answerswillvary,butstudentsshouldexpecttheamountofgrasstodecrease.Thesnakeswillcauseareduction
inmousepopulations,leadingtoanincreaseingrasshoppersthatwilltheneatthegrass.
Figure1.Since1972,Dr.JimEsteshadbeenstudyingafoodchainofkelp->urchins->seaotters,andthenintheearly
1990sorcasbeganeatingtheseaurchins.ThedatacollectedbyDr.Estesareshown.PanelAshowsseaotterabundance
aroundfourdifferentislandsfrom1972to1997.PanelBshowstheamountofseaurchins(seaurchinbiomass)in1987
and1997.PanelCshowstheamountofkelpthatseaurchinsateovera24-hourperiod(grazingintensity)in1991and
1997.PanelDshowsthenumberofkelpplantswithinaspecificarea(densityofkelp)in1987and1997.Thethicknessof
thearrowsillustratesthestrengthoftheeffectonespecieshasonthespeciesbelowitinthefoodweb.(Reproducedwith
permissionfromEstes,J.A.,etal.Science282:473-476,1998.)
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RefertoFigure1forquestions8through11below.
8. (KeyConceptsA&C)In1997,whichspeciesistheapexpredatorinthefoodchain?
A. Killerwhales
B. Seaotters
C. Seaurchins
D. Kelp
9. (KeyConceptsD&F)WhichofthefollowingstatementsdescribesthedatainFigure1?
A.Anincreaseinseaurchinbiomassisassociatedwithmoreintensegrazing.
B.Anincreaseinseaurchinbiomassisassociatedwithgreaterkelpdensity.
C.Predationofseaottersbykillerwhalesisassociatedwithgreaterkelpdensity.
D.Seaotterabundancewasrelativelystablefrom1972to1997.
10. (KeyConceptsD&F)Completethefollowingsentence.Figure1illustratesthatwhenorcasstartedeatingsea
otters,theseaotterpopulation___________________,theurchinpopulation______________________,and
thekelppopulation_____________________________.
A. Decreased,decreased,decreased
B. Decreased,increased,decreased
C. Increased,decreased,increased
D. Increased,increased,increased
11. (KeyConceptsA,C,&D)ThearrowsontheleftandrightsidesofFigure1showtheeffectsofonespeciesonthe
speciesthatareonlowertrophiclevels.Thickerarrowsindicatealargeeffectandthinarrowsasmallereffect.
Thearrowsontheleftshowasysteminwhichtherearealotofseaotters.Thearrowsontherightshowasystem
inwhichtherearefewotters.Explainwhythedown-pointingarrowsontheleftsideofthefigurelookdifferent
fromthearrowsontherightsideofthefigure.
Ontheleft,theseaottershavealargeeffectontheseaurchinsintheabsenceofthekillerwhales.Theseaurchins
haveasmalleffectonthekelpbecausetheirnumbersareheldincheckbytheseaotters.Ontheright,thekiller
whaleshavealargeeffectontheseaotters,whosepopulationsthendecrease.Theeffecttheseaottershaveonthe
seaurchinsisthusweaker,allowingseaurchinpopulationstogrow,andtheyinturnhavealargeeffectonthekelp.
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Inthe1990s,ecologistsDeborahLetourneauandLeeDyerstudiedatropicalforestshrubcalledthepiperplantand
thevariousspeciesofinsectsthatliveonandneartheshrub.Aspeciesofantusesthepiperplantasahomeby
hollowingoutsomeofitsbranchesandbuildingcoloniesinsidethehollowbranchcores.Theantsdonoteatthe
plant’sleaves.Instead,theleavesareconsumedmostlybycaterpillars.Whentheantsencountercaterpillarsor
caterpillareggsontheplant’sleaves,theyeithereatthemorkickthemoff.LetourneauandDyeraddedbeetlesthat
eatantstosomeplantsandthenmeasuredtheireffect.
Figure2showstheresultsofoneofLetourneauandDyer’sexperimentsinwhichtheycomparedtheleafareaofpiper
plantsincontrolplotstothatofexperimentalplotstowhichtheyhadaddedbeetlesthateatants.
MeanLeafAreaperPlantOver18Months
2500
2000
LeafAreaperPlant(cm2)
1500
1000
500
0
2
7
10
18
MonthsAfterStartofExperiment
Withoutbeetle
Withbeetle
Figure2.Meanleafareapertree.Initialmeasurementsweretakenbefore(0to2months)andafter(7to18months)
beetleswereaddedto40of80plants.Thelightgrayroundmarkersrepresentmeasurementstakenofthecontrolplots,
towhichbeetleswerenotadded.Theblacksquaremarkersrepresentmeasurementstakenoftheexperimentalplots,to
whichbeetleswereadded.Measurementsweremadeonallleavestocalculatethemeanleafareaperplant.Errorbars
representstandarderrorofthemean.
Refertothefiguretoanswerquestions12through17.
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12. (KeyConceptF)Forboththeplotswiththebeetlesaddedandthecontrolplots(nobeetlesadded),estimatethe
meantreeleafareaperplotthatthescientistsrecordedafterrunningtheexperimentfor18months.
Studentsshouldanswerabout2,00cm2forthecontrolplotsand1,100cm2fortheplotswiththebeetlesadded.
13. (KeyConceptF)Comparethetrendsinmeantreeleafareaperplotforboththeplotswiththebeetlesaddedand
thecontrolplotsoverthe18monthsoftheexperiment.
Studentsshoulddescribethattheleafareaforthecontrolplotswithoutbeetlesincreasedoverthe18-monthperiod,
butdecreasedfortheplotswherethebeetlelarvaewereadded.Acompleteanswerwillbeoneinwhichstudents
alsoexplainthatthemeanleafareasweresimilarthroughaboutthefirst7monthsoftheexperimentbutafterthat,
thedifferencesbetweenthemeansmaybesignificant.(Astatisticaltest,likethestudent’st-test,isneededto
determinewhetherthedifferenceisindeedstatisticallysignificant.)
14. (KeyConceptsD&F)Drawtwodiagramsthatshowthefoodchainsforthecontrolandexperimentalplots.Include
interactionsamongpredatorybeetles(ifpresent),ants,caterpillars,andpiperplants.
Control
piperplantsàcaterpillarsàants
Experimental
piperplantsàcaterpillarsàantsàbeetles
15. (KeyConceptsD&F)Describetheimpactofaddingthebeetlesoneachspeciesinthefoodchainabove.
Thebeetleseattheants.Withfewerantstherearemorecaterpillars.Thecaterpillarseatmoreleaves.
16. (KeyConceptsD&G)WhichstatementdoLetourneauandDyer’sresultssupport?
A. Addingbeetlesreducedantnumbersandtriggeredatrophiccascadethatincreasedthemeanleafarea
leftonplants.
B. Addingbeetleshadlittleeffectonthisecosystem,showingthatitisprimarilyregulatedfromthebottom
up.
C. Addingbeetlesreducedantnumbersandtriggeredatrophiccascadethatdecreasedthemeanleafarea
leftonplants.
D. Addingbeetlesreducedantnumbersandincreasedthecaterpillarpopulationsize,provingthatthe
caterpillarsareakeystonespeciesinthishabitat.
17. (KeyConceptD)DotheresultsoftheLetourneauandDyerexperimentsupportorrefutethegreenworld
hypothesis?Explainyouranswer.
Theexperimentsupportsthegreenworldhypothesisbecausechangesinplantleafareawerecausedbypredators
controllingherbivorenumbersfromthetopdown.Addingbeetlesdecreasedantpredationoncaterpillarsandledto
increasedcaterpillarherbivoryanddecreasedplantleafarea.
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Some Animals Are More Equal than
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REFERENCES
Campbell,N.A.,J.B.Reece,L.A.Urry,M.L.Cain,S.A.Wasserman,P.V.Minorsky,&R.B.Jackson.(2008).Biology.
Benjamin/Cummings,SanFrancisco,CA.
Carroll,S.B.(2016).TheSerengetiRules.PrincetonUniversityPress,Princeton,NJ.
Dyer,L.A.andD.K.Letourneau.(1999).Trophiccascadesinacomplex,terrestrialcommunity.Proceedingsofthe
NationalAcademyofSciences96:5072-5076.
Estes,J.A.,M.T.Tinker,T.M.Williams,&D.F.Doak.(1998).Killerwhalepredationonseaotterslinkingoceanicand
nearshoreecosystems.Science282:473-476.
Paine,R.T.(1966).Foodwebcomplexityandspeciesdiversity.TheAmericanNaturalist100:65-75.
Paine,R.T.(1969).Anoteontrophiccomplexityandcommunitystability.TheAmericanNaturalist103:91-93.
Paine,R.T.(1974).Intertidalcommunitystructure:Experimentalstudiesontherelationshipbetweenadominant
competitoranditsprincipalpredator.Oecologia15:93-120.
Paine,R.T.(1980).Foodwebs:linkage,interactionstrengthandcommunityinfrastructure.JournalofAnimalEcology
49:667-685.
AUTHORS
WrittenbyPaulStrode,PhD,FairviewHighSchool;KimParfitt,CheyenneCentralHighSchool;andLauraBonetta,PhD,HHMI
ReviewedbyPaulBeardsley,PhD,CalPolyPonoma;BridgetConneely,HHMI;andAlineWaguespack-Claytor,PhD,consultant
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