The“balanceofnature”isnotanemptyphrase.Natureprovidesapopulationtooccupyasuitableenvironmentandcutsdownsurpluspopulationtofittheavailablefoodsupply.Onemeansofreducingsurpluspopulationispreda
The“balanceofnature”isnotanemptyphrase.Natureprovidesapopulationtooccupyasuitableenvironmentandcutsdownsurpluspopulationtofittheavailablefoodsupply.Onemeansofreducingsurpluspopulationispredators;othersareparasiticdiseases.Alsopopulationdensityproducesnervousdisordersandevendrivesanimalstomassmigrations(移民群)likethelemmingsofNorwaywhoplunge（冲入）intothesea.
Thatpredatorypopulationincreasetocontrolotheranimalshaslongbeenknown.ManyyearsagotheHudson’sBayCompanyrecordsrevealedthatthefoxpopulationwentupanddownaboutayearafterrabbitpopulationhadgoneupanddown.
Sometimesasituationoccursinwhichthepredatorypopulationisreducedtoalevelbelowwhichnaturecanreadilyreplace.OnValcourIslandinLakeChamplainacostlycampaignresultedintheelimination（消除）ofpredatoryanimalsonlytohavebirdsandsmallanimalsincreaseforfouryearsafterward.Thenlackingpredatorcontrolnatureresortedtodiseasetocutdownthesepopulations.
Jamaicahadanexampleofnature’spersistenceinprovidinganimalsforexistinghabits.Sugarplantersabout75yearsagoimportedmongooses（狐猴）tocontrolrats.Themongooseskilledofftheratskidspuppiesandwildlife.Eventuallyfoodbecamescarceandthemongoose’spopulationdeclines.
1.Thefactthatthenumberofpredatorshasmuchtodowiththatofotheranimals .
A.isnotrecentknowledge B.hasjustbeenlearned
C.hasn’tbeenprovedyet D.isnotimportantnow
2.Whenpredatorcontrolfailsnaturebringsanimalpopulationintobalanceby .
A.eliminatingthenumberofpredators
B.seekingthehelpofdiseasetoreduceotheranimals
C.increasingthenumberofotheranimals
D.replacingthenumberofpredatorsimmediately
3.Whenthemongooseskilledofftheratsthey .
A.becameproblemsthemselvesB.atethesugarcrop
C.attackedhumans D.hadnothingtoeat
4.Itisimpliedinthispassagethat .
A.sugarplanterimportedmongoosestocontrolrats
B.manshouldneverkillanimals
C.manhascompletecontrolovernature
D.toupsetthebalanceofnaturecan