捕食者捕集循环预期如何影响猎物动物种群答案并不像思想tpo33套音频tpo27阅读.pdf

TPO-27-3

Predator-PreyCycles

Howdopredetorsaffectpopulationsofthepreyanimals?Theanswerisnotassimpleasmightbe

thought.MoosereachedIsleRoyaleinLakeSuperiorbycrossingoverwintericeandmultipliedfreelythere

inisolationwithoutpredators.Whenwolveslaterreachedtheislanturalistswidelyassumedthatthe

wolveswouldplayakeyroleincontrollingthemoosepopulation.Carefulstudieshavedemonstrated,

however,thatthisisnotthecase.Thewolveseatmostlyoldordiseasedanimalsthatwouldnotsurvive

longanyway.Ingeneral,themoosepopulationiscontrolledbyfoodavailability,disease,andotherfactors

ratherthanbywolves.

Whenexperimentalpopulationsaresetupundersimplelaboratoryconditions,thepredatoroften

exterminatesitspreyandthenbecometinctitself,havingnothinglefttoeat.However,ifsafeareaslike

thosepreyanimalshaveinthewildareprovided,thepreypopulationdropstolowlevelsbutnotto

extinction.Lowpreypopulationlevelsthenprovideinadequatefoodforthepredators,causingthe

predatorpopulationtodecrease.Whenthisoccurs,thepreypopulationcanrebound.Inthissituationthe

predatorandpreypopulationsmaycontinueinthiscyclicalpatternforsometime.

Populationcyclesarecharacteristicofsmallmammals,andtheysometimesappeartobebrought

aboutbypredators.EcologistsstudyingharepopulationshavefoundthattheNorthAmericansnowshoe

harefollowsaroughlyten-yearcycle.Itsnumbersfalltenfoldtothirtyinatypicalcycle,andahundredfold

changecanoccur.Twofactorsappeartobegeneratingthecycle:foodplantsandpredators.

Thepreferredfoodsofsnowshoeharesarewillowandbirchtwigs.Asharedensityincreases,the

quantityofthesetwigsdecreases,forcingtheharestofeedonlow-quality,high-fiberfood.Lowerbirth

rates,lowjuve