EncyclopedicEntry
foodweb
Forthecompleteencyclopedicentrywithmediaresources,visit:
http://education.nationalgeographic.org/encyclopedia/food-web/
Afoodwebconsistsofallthefoodchainsinasingleecosystem.Eachlivingthinginanecosystemispart
ofmultiplefoodchains.Eachfoodchainisonepossiblepaththatenergyandnutrientsmaytakeasthey
movethroughtheecosystem.Alloftheinterconnectedandoverlappingfoodchainsinanecosystem
makeupafoodweb.
TrophicLevels
Organismsinfoodwebsaregroupedintocategoriescalledtrophiclevels.Roughlyspeaking,these
levelsaredividedintoproducers(firsttrophiclevel),consumers,anddecomposers(lasttrophiclevel).
Producers
Producersmakeupthefirsttrophiclevel.Producers,alsoknownasautotrophs,maketheirownfood
anddonotdependonanyotherorganismfornutrition.Mostautotrophsuseaprocesscalled
photosynthesistocreatefood(anutrientcalledglucose)fromsunlight,carbondioxide,andwater.
Plantsarethemostfamiliartypeofautotroph,buttherearemanyotherkinds.Algae,whoselargerforms
areknownasseaweed,areautotrophic.Phytoplankton,tinyorganismsthatliveintheocean,arealso
autotrophs.Sometypesofbacteriaareautotrophs.Forexample,bacterialivinginactivevolcanoesuse
sulfur,notcarbondioxide,toproducetheirownfood.Thisprocessiscalledchemosynthesis.
Consumers
Thenexttrophiclevelsaremadeupofanimalsthateatproducers.Theseorganismsarecalled
consumers.
Primaryconsumersareherbivores.Herbivoreseatplants,algae,andotherproducers.Theyareatthe
secondtrophiclevel.Inagrasslandecosystem,deer,mice,andevenelephantsareherbivores.They
eatgrasses,shrubs,andtrees.Inadesertecosystem,amousethateatsseedsandfruitsisaprimary
consumer.
Inanoceanecosystem,manytypesoffishandturtlesareherbivoresthateatalgaeandseagrass.In
kelpforests,seaweedsknownasgiantkelpprovideshelterandfoodforanentireecosystem.Sea
urchinsarepowerfulprimaryconsumersinkelpforests.Thesesmallherbivoreseatdozensofkilograms
(pounds)ofgiantkelpeveryday.
1of11
Secondaryconsumerseatherbivores.Theyareatthethirdtrophiclevel.Inadesertecosystem,a
secondaryconsumermaybeasnakethateatsamouse.Inthekelpforest,seaottersaresecondary
consumersthathuntseaurchinsasprey.
Tertiaryconsumerseatthesecondaryconsumers.Theyareatthefourthtrophiclevel.Inthedesert
ecosystem,anowloreaglemaypreyonthesnake.
Theremaybemorelevelsofconsumersbeforeachainfinallyreachesitstoppredator.Toppredators,
alsocalledapexpredators,eatotherconsumers.Theymaybeatthefourthorfifthtrophiclevel.They
havenonaturalenemiesexceptpeople.Lionsareapexpredatorsinthegrasslandecosystem.Inthe
ocean,fishsuchasthegreatwhitesharkareapexpredators.Inthedesert,bobcatsandmountainlions
aretoppredators.
Consumerscanbecarnivores(animalsthateatotheranimals)oromnivores(animalsthateatboth
plantsandanimals).Omnivores,likepeople,consumemanytypesoffoods.Peopleeatplants,suchas
vegetablesandfruits.Wealsoeatanimalsandanimalproducts,suchasmeat,milk,andeggs.Weeat
fungi,suchasmushrooms.Wealsoeatalgae,inedibleseaweedslikenori(usedtowrapsushirolls)and
sealettuce(usedinsalads).Bearsareomnivores,too.Theyeatberriesandmushrooms,aswellas
animalssuchassalmonanddeer.
DetritivoresandDecomposers
Detritivoresanddecomposersmakeupthelastpartoffoodchains.Detritivoresareorganismsthateat
nonlivingplantandanimalremains.Forexample,scavengerssuchasvultureseatdeadanimals.Dung
beetleseatanimalfeces.
Decomposers,likefungiandbacteria,completethefoodchain.Decomposersturnorganicwastes,
suchasdecayingplants,intoinorganicmaterials,suchasnutrient-richsoil.Theycompletethecycleof
life,returningnutrientstothesoiloroceansforusebyautotrophs.Thisstartsawholenewseriesoffood
chains.
FoodChains
Foodwebsconnectmanydifferentfoodchains,andmanydifferenttrophiclevels.Foodwebscan
supportfoodchainsthatarelongandcomplicated,orveryshort.
Forexample,grassinaforestclearingproducesitsownfoodthroughphotosynthesis.Arabbiteatsthe
grass.Afoxeatstherabbit.Whenthefoxdies,decomposerssuchaswormsandmushroomsbreak
downitsbody,returningittothesoilwhereitprovidesnutrientsforplantslikegrass.
Thisshortfoodchainisonepartoftheforestsfoodweb.Anotherfoodchaininthesameecosystem
mightinvolvecompletelydifferentorganisms.Acaterpillarmayeattheleavesofatreeintheforest.A
birdsuchasasparrowmayeatthecaterpillar.Asnakemaythenpreyonthesparrow.Aneagle,anapex
predator,maypreyonthesnake.Ahawk,anotherapexpredator,maypreyontheeagle.Yetanother
2of11
bird,avulture,consumesthebodyofthedeadhawk.Finally,bacteriainthesoildecomposethe
remains.
Inadesertecosystem,anautotrophsuchasacactusproducesfruit.Herbivorousinsects,suchasflies,
consumethecactusfruit.Birdssuchastheroadrunnerconsumetheseinsects.Detritivoressuchas
termiteseattheroadrunnerafteritdies.Bacteriaandfungihelpdecomposetheremainingbonesofthe
roadrunner.Thecarboninthebonesenrichesthedesertsoil,helpingplantslikecactusesdevelop.
Algaeandplanktonarethemainproducersinmarineecosystems.Tinyshrimpcalledkrilleatthe
microscopicplankton.ThelargestanimalonEarth,thebluewhale,preysonthousandsoftonsofkrill
everyday.Apexpredatorssuchasorcaspreyonbluewhales.Asthebodiesoflargeanimalssuchas
whalessinktotheseafloor,detritivoressuchaswormsbreakdownthematerial.Thenutrientsreleased
bythedecayingfleshprovidechemicalsforalgaeandplanktontostartanewseriesoffoodchains.
Biomass
Foodwebsaredefinedbytheirbiomass.Biomassistheenergyinlivingorganisms.Autotrophs,the
producersinafoodweb,convertthesunsenergyintobiomass.Biomassdecreaseswitheachtrophic
level.Thereisalwaysmorebiomassinlowertrophiclevelsthaninhigherones.
Becausebiomassdecreaseswitheachtrophiclevel,therearealwaysmoreautotrophsthanherbivores
inahealthyfoodweb.Therearemoreherbivoresthancarnivores.Anecosystemcannotsupportalarge
numberofomnivoreswithoutsupportinganevenlargernumberofherbivores,andanevenlarger
numberofautotrophs.
Ahealthyfoodwebhasanabundanceofautotrophs,manyherbivores,andfewcarnivoresand
omnivores.Thisbalancehelpstheecosystemmaintainandrecyclebiomass.
Inaddition,smalleranimalsaremorenumerousthanlargerones.Tigersandantsarebothconsumersin
atropicalfoodweb.However,ittakesmuchmorebiomasstosupportatigerpopulationthanacolonyof
ants.Tigersconsumemorefoodandtakeupamuchlargerspace.Therearemanymoreantsthantigers
inthefoodwebofatropicalecosystem.
Everylinkinafoodwebisconnectedtoatleasttwoothers.Thebiomassofanecosystemdependson
howbalancedandconnecteditsfoodwebis.Whenonelinkinthefoodwebisthreatened,someorallof
thelinksareweakenedorstressed.Theecosystemsbiomassdeclines.
Thelossofplantlifeusuallyleadstoadeclineintheherbivorepopulation,forinstance.Plantlifecan
declineduetodrought,disease,orhumanactivity.Forestsarecutdowntoprovidelumberfor
construction.Grasslandsarepavedoverforshoppingmallsorparkinglots.
Asthenumberofplantsandotherautotrophsisreduced,therestofthefoodwebisforcedtoadaptor
die.Deerhavefewerplantstoeat.Butterfliesandbeeshavefewerflowerstopollinate.Inturn,predators
3of11
likemountainlionshavefewerdeertoconsume.Birdslikekingfishershavefewerinsectstoeat.
Thelossofbiomassonthesecondorthirdtrophiclevelcanalsoputafoodweboutofbalance.Consider
whatmayhappenifasalmonrunisdiverted.Asalmonrunisariverwheresalmonswim.Salmonruns
canbedivertedbylandslidesandearthquakes,aswellastheconstructionofdamsandlevees.
Biomassislostassalmonarecutoutoftheriversmanyfoodchains,whichmakeuptheecosystems
foodweb.Unabletoeatsalmon,omnivoreslikebearsareforcedtorelymoreheavilyonotherfood
sources,suchasants.Theareasantpopulationshrinks.Antsareusuallyscavengersanddetritivores,
sofewernutrientsarebrokendowninthesoil.Thesoilisunabletosupportasmanyautotrophs,so
biomassislost.Salmonthemselvesarepredatorsofinsectlarvaeandsmallerfish.Withoutsalmonto
keeptheirpopulationincheck,aquaticinsectsmaydevastatelocalplantcommunities.Fewerplants
survive,andbiomassislost.
Alossoforganismsonhighertrophiclevels,suchascarnivores,canalsodisruptafoodchain.Inthe
kelpforest,seaurchinsaretheprimaryconsumerofkelp.Seaotterspreyonurchins.Iftheseaotter
populationshrinksduetodiseaseorhunting,urchinsdevastatethekelpforest.Lackingacommunityof
producers,biomassplummets.Theentirekelpforestdisappears.Suchareasarecalledurchinbarrens.
Humanactivitycanreducethenumberofpredators.In1986,officialsinVenezueladammedtheCaroni
River,creatinganenormouslakeabouttwicethesizeofRhodeIsland.About1,000hilltopsturnedinto
islandsinthislake.Withtheirhabitatsreducedtotinyislands,manyterrestrialpredators,suchas
jaguars,armadillos,andweasels,werentabletofindenoughfood.Asaresult,preyanimalslikehowler
monkeys,leaf-cutterants,andiguanasflourished.Theantsbecamesonumerousthattheydestroyed
therainforest,killingallthetreesandotherplants.ThefoodwebsurroundingtheCaroniRiverwas
destroyed.
Bioaccumulation
Biomassdeclinesasyoumoveupthroughthetrophiclevels.However,sometypesofmaterials,
especiallytoxicchemicals,increasewitheachtrophiclevelinthefoodweb.Thesechemicalsusually
collectinthefatofanimals.
Whenanherbivoreeatsaplantorotherautotrophthatiscoveredinpesticides,forexample,those
pesticidesarestoredintheanimalsfat.Whenacarnivoreeatsseveraloftheseherbivores,ittakesinthe
pesticidechemicalsstoredinitsprey.Thisprocessiscalledbioaccumulation.
Bioaccumulationhappensinaquaticecosystems,too.Runofffromurbanareasorfarmscanbefullof
pollutants.Tinyproducerssuchasalgae,bacteria,andseagrassabsorbminuteamountsofthese
pollutants.Primaryconsumers,suchasseaturtlesandfish,eattheseagrass.Theyusetheenergyand
nutrientsprovidedbytheplants,butstorethechemicalsintheirfattytissue.Predatorsonthethird
trophiclevel,suchassharksortuna,eatthefish.Bythetimethetunaisconsumedbypeople,itmaybe
storingaremarkableamountofbioaccumulatedtoxins.
4of11
Becauseofbioaccumulation,organismsinsomepollutedecosystemsareunsafeandnotallowedtobe
harvested.OystersintheharborofNewYorkCity,forinstance,areunsafetoeat.Thepollutantsinthe
harboraccumulateinoysters,afilterfeeder.
Inthe1940sand1950s,apesticidecalledDDT(dichloro-diphenyl-trichloroethane)waswidelyusedto
killinsectsthatspreaddiseases.DuringWorldWarII,theAlliesusedDDTtoeliminatetyphusinEurope,
andtocontrolmalariaintheSouthPacific.Scientistsbelievedtheyhaddiscoveredamiracledrug.Inthe
1950s,DDTwaslargelyresponsibleforeliminatingmalariainplaceslikeTaiwan,theCaribbean,andthe
Balkans.
Sadly,DDTbioaccumulatesinanecosystemandcausesdamagetotheenvironment.DDTaccumulates
insoilandwater.SomeformsofDDTmaynotdecomposefor20years.Worms,grasses,algae,andfish
accumulateDDT.Apexpredators,suchaseagles,hadthehighestamountofDDTintheirbodies,
accumulatedfromthefishandsmallmammalstheypreyon.
BirdswithhighamountsofDDTintheirbodieslayeggswithextremelythinshells.Theseshellswould
oftenbreakbeforethebabybirdswerereadytohatch.Thinshellsalsomadeiteasierforpredatorssuch
assnakesandotherbirdstoconsumetheeggsandembryosinside.
DDTwasamajorreasonforthedeclineofthebaldeagle,anapexpredatorthatfeedsprimarilyonfish
andsmallrodents.Today,theuseofDDThasbeenrestricted.Thefoodwebsofwhichitisaparthave
recoveredinmostpartsofthecountry.
Vocabulary
Term
abundance
accumulate
adapt
algae
Allies
apexpredator
aquatic
Partof
Speech
noun
verb
verb
plural
noun
Definition
largeamount.
togatherorcollect.
toadjusttonewsurroundingsoranewsituation.
(singular:alga)diversegroupofaquaticorganisms,thelargestofwhichare
seaweeds.
noun
allianceofcountriesthatopposedtheAxisduringWorldWarII.TheAllies
wereledbytheU.S.,theUnitedKingdom,andtheSovietUnion.
noun
speciesatthetopofthefoodchain,withnopredatorsofitsown.Alsocalled
analphapredatorortoppredator.
adjectivehavingtodowithwater.
5of11
Term
autotroph
Partof
Speech
noun
Definition
organismthatcanproduceitsownfoodandnutrientsfromchemicalsinthe
atmosphere,usuallythroughphotosynthesisorchemosynthesis.
(singular:bacterium)single-celledorganismsfoundineveryecosystemon
Earth.
bacteria
plural
noun
Balkan
adjectivehavingtodowiththeBalkanPeninsulainsoutheasternEurope.
bioaccumulation noun
processbywhichchemicalsareabsorbedbyanorganism,eitherfrom
exposuretoasubstancewiththechemicalorbyconsumptionoffood
containingthechemical.
biomass
livingorganisms,andtheenergycontainedwithinthem.
carbon
noun
noun
chemicalelementwiththesymbolC,whichformsthebasisofallknownlife.
carbondioxide noun
greenhousegasproducedbyanimalsduringrespirationandusedbyplants
duringphotosynthesis.Carbondioxideisalsothebyproductofburning
fossilfuels.
carnivore
organismthateatsmeat.
noun
chemosynthesis noun
processbywhichsomemicrobesturncarbondioxideandwaterinto
carbohydratesusingenergyobtainedfrominorganicchemicalreactions.
colony
groupofonespeciesoforganismlivingclosetogether.
consist
consumer
dam
DDT
decay
decline
decomposer
desert
noun
verb
noun
noun
noun
verb
verb
noun
noun
tobemadeof.
organismonthefoodchainthatdependsonautotrophs(producers)or
otherconsumersforfood,nutrition,andenergy.
structurebuiltacrossariverorotherwaterwaytocontroltheflowofwater.
(dichloro-diphenyl-trichloroethane)toxicchemicalusedasaninsecticide
butillegalformostusesintheU.S.since1972.
torotordecompose.
toreduceorgodowninnumber.
organismthatbreaksdowndeadorganicmaterial.
areaoflandthatreceivesnomorethan25centimeters(10inches)of
precipitationayear.
6of11
Term
detritivore
devastate
divert
drought
earthquake
ecosystem
edible
eliminate
energy
enormous
farm
fat
feces
filterfeeder
foodchain
foodweb
forest
fruit
fungi
glucose
grassland
Partof
Speech
noun
verb
verb
noun
noun
noun
Definition
organismthatconsumesdeadplantmaterial.
todestroy.
todirectawayfromafamiliarpath.
periodofgreatlyreducedprecipitation.
thesuddenshakingofEarth'scrustcausedbythereleaseofenergyalong
faultlinesorfromvolcanicactivity.
communityandinteractionsoflivingandnonlivingthingsinanarea.
adjectiveabletobeeatenanddigested.
verb
noun
toremove.
capacitytodowork.
adjectiveverylarge.
noun
noun
noun
noun
noun
noun
noun
noun
plural
noun
noun
noun
landcultivatedforcrops,livestock,orboth.
materialfoundinorganismsthatiscolorlessandodorlessandmaybesolid
orliquidatroomtemperature.
wastematerialproducedbythelivingbodyofanorganism.
aquaticanimalthatstrainsnutrientsfromwater.
groupoforganismslinkedinorderofthefoodtheyeat,fromproducersto
consumers,andfromprey,predators,scavengers,anddecomposers.
allrelatedfoodchainsinanecosystem.Alsocalledafoodcycle.
ecosystemfilledwithtreesandunderbrush.
ediblepartofaplantthatgrowsfromaflower.
(singular:fungus)organismsthatsurvivebydecomposingandabsorbing
nutrientsinorganicmaterialsuchassoilordeadorganisms.
"simplesugar"chemicalproducedbymanyplantsduringphotosynthesis.
ecosystemwithlarge,flatareasofgrasses.
7of11
Term
harbor
harvest
herbivore
inorganic
kelp
kelpforest
krill
landslide
larva
levee
lumber
malaria
Partof
Speech
noun
noun
noun
Definition
partofabodyofwaterdeepenoughforshipstodock.
thegatheringandcollectionofcrops,includingbothplantsandanimals.
organismthateatsmainlyplants.
adjectivecomposedofmaterialthatisnotliving,andneverwas,suchasrock.
noun
noun
noun
noun
noun
noun
noun
noun
typeofseaweed.
underwaterhabitatfilledwithtallseaweedsknownaskelp.
smallmarinecrustacean,similartoshrimp.
thefallofrocks,soil,andothermaterialsfromamountain,hill,orslope.
aneworimmatureinsectorothertypeofinvertebrate.
bankofariver,raisedeithernaturallyorconstructedbypeople.
preciselycutpiecesofwoodsuchasboardsorplanks.
infectiousdiseasecausedbyaparasitecarriedbymosquitoes.
marine
adjectivehavingtodowiththeocean.
multiple
adjectivemany.
nori
nutrient
omnivore
organicwaste
oyster
pesticide
noun
noun
redalgaethatisoftendriedandusedtowrapsushi.
substanceanorganismneedsforenergy,growth,andlife.
noun
organismthateatsavarietyoforganisms,includingplants,animals,and
fungi.
noun
usedorexcessmaterialthatmightbebrokendowntobeusedagain.Also
calledbiodegradablewaste.
noun
noun
typeofmarineanimal(mollusk).
naturalormanufacturedsubstanceusedtokillorganismsthatthreaten
agricultureorareundesirable.Pesticidescanbefungicides(whichkill
harmfulfungi),insecticides(whichkillharmfulinsects),herbicides(which
killharmfulplants),orrodenticides(whichkillharmfulrodents.)
8of11
Term
Partof
Speech
Definition
photosynthesis noun
processbywhichplantsturnwater,sunlight,andcarbondioxideintowater,
oxygen,andsimplesugars.
phytoplankton
noun
microscopicorganismthatlivesintheoceanandcanconvertlightenergy
tochemicalenergythroughphotosynthesis.
noun
organismthatproducesitsownfoodthroughphotosynthesisandwhose
cellshavewalls.
plant
plummet
pollinate
pollutant
prey
primary
consumer
producer
rainforest
recycle
reduce
verb
verb
noun
noun
noun
noun
noun
verb
verb
tofallsharply.
totransferpollenfromonepartofaflower(theanther)toanother(the
stigma).
chemicalorothersubstancethatharmsanaturalresource.
animalthatishuntedandeatenbyotheranimals.
organismthateatsplantsorotherautotrophs.
organismonthefoodchainthatcanproduceitsownenergyandnutrients.
Alsocalledanautotroph.
areaoftall,mostlyevergreentreesandahighamountofrainfall.
tocleanorprocessinordertomakesuitableforreuse.
tolowerorlessen.
remains
noun
remarkable
adjectiveunusualanddramatic.
runoff
salmonrun
scavenger
seagrass
sealettuce
seaurchin
noun
noun
noun
noun
noun
noun
materialsleftfromadeadorabsentorganism.
overflowoffluidfromafarmorindustrialfactory.
areaorpathofariverwheresalmonreturntospawneveryseason.
organismthateatsdeadorrottingbiomass,suchasanimalfleshorplant
material.
typeofplantthatgrowsintheocean.
seaweedwithlarge,flatleaves.
marineanimal(echinoderm)withacircular,spinyshell.
9of11
Term
seaweed
secondary
consumer
seed
shrub
stress
sulfur
sunlight
survive
sushi
terrestrial
tertiary
consumer
toppredator
toxic
Partof
Speech
noun
noun
noun
noun
verb
noun
noun
verb
noun
Definition
marinealgae.Seaweedcanbecomposedofbrown,green,orredalgae,as
wellas"blue-greenalgae,"whichisactuallybacteria.
organismthateatsmeat.
partofaplantfromwhichanewplantgrows.
typeofplant,smallerthanatreebuthavingwoodybranches.
tostrainorputpressureon.
chemicalelementwiththesymbolS.
visibleradiationfromthesun.
tolive.
bite-sizedrollsorballsofstickyricetoppedwithseafoodorvegetables.
adjectivehavingtodowiththeEarthordryland.
noun
noun
carnivorethatmostlyeatsothercarnivores.
speciesatthetopofthefoodchain,withnopredatorsofitsown.Alsocalled
analphapredatororapexpredator.
adjectivepoisonous.
oneofthreepositionsonthefoodchain:autotrophs(first),herbivores
(second),andcarnivoresandomnivores(third).
trophiclevel
noun
tropical
existinginthetropics,thelatitudesbetweentheTropicofCancerinthe
adjectivenorthandtheTropicofCapricorninthesouth.
typhus
urbanarea
urchinbarren
vegetable
noun
highlyinfectiousandsometimesdeadlydiseasewithsymptomsofitching
soresandsevereheadache,causedbylice.
noun
developed,denselypopulatedareawheremostinhabitantshave
nonagriculturaljobs.
noun
noun
siteofaformerkelpforestthathasbeendestroyedbyseaurchins.
plantthatisgrownorharvestedforfood.
10of11
Term
volcano
WorldWarII
Partof
Speech
Definition
anopeningintheEarth'scrust,throughwhichlava,ash,andgaseserupt,
andalsotheconebuiltbyeruptions.
noun
(1939-1945)armedconflictbetweentheAllies(representedbytheUnited
States,theUnitedKingdom,andtheSovietUnion)andtheAxis
(representedbyGermany,Italy,andJapan.)
noun
Articles&Profiles
USDA:TheSoilFoodWeb
Images
EnvironmentalProtectionAgency:GreatLakesMonitoring—FishIndicators
Websites
NationalGeographic:BioBlitz
NationalGeographicOcean:MarineFoodChain
©1996–2016NationalGeographicSociety.Allrightsreserved.
11of11