AsGilbertWhite，Darwin，andothersobservedlongago，allspeciesappeartohavetheinnatecapacitytoincreasetheirnumbersfromgenerationtogeneration。Thetaskforecologistsistountangletheenvironmentalandbiologicalfactorsthatholdthisintrinsiccapacityforpopulationgrowthincheckoverthelongrun。Thegreatvarietyofdynamicbehaviorsexhibitedbydifferentpopulationmakesthistaskmoredifficult：somepopulationsremainroughlyconstantfromyeartoyear;othersexhibitregularcyclesofabundanceandscarcity;stillothersvarywildly，withoutbreaksandcrashesthatareinsomecasesplainlycorrelatedwiththeweather，andinothercasesnot。

Toimposesomeorderonthiskaleidoscopeofpatterns，oneschoolofthoughtproposesdividingpopulationsintotwogroups。Theseecologistspositthattherelativelysteadypopulationshave“density－dependent“growthparameters;thatis，ratesofbirth，death，andmigrationwhichdependstronglyonpopulationdensity。Thehighlyvaryingpopulationshave“density－independent“growthparameters，withvitalratesbuffetedbyenvironmentalevents;theseratesfluctuateinawaythatiswhollyindependentofpopulationdensity。

Thisdichotomyhasitsuses，butitcancauseproblemsiftakentooliterally。Foronething，nopopulationcanbedrivenentirelybydensity－independentfactorsallthetime。Nomatterhowseverelyorunpredictablybirth，death，andmigrationratesmaybefluctuatingaroundtheirlong－termaverages，iftherewerenodensity－dependenteffects，thepopulationwould，inthelongrun，eitherincreaseordecreasewithoutbound。Putanotherway，itmaybethatonaverage99percentofalldeathsinapopulationarisefromdensity－independentcauses，andonlyonepercentfromfactorsvaryingwithdensity。Thefactorsmakinguptheonepercentmayseemunimportant，andtheircausemaybecorrespondinglyhardtodetermine。Yet，whetherrecognizedornot，theywillusuallydeterminethelong－termaveragepopulationdensity。

Inordertounderstandthenatureoftheecologist‘sinvestigation，wemaythinkofthedensity－dependenteffectsongrowthparametersasthe“signal“ecologistsaretryingtoisolateandinterpret，onethattendstomakethepopulationincreasefromrelativelylowvaluesordecreasefromrelativelyhighones，whilethedensity－independenteffectsacttoproduce“noise“inthepopulationdynamics。Forpopulationsthatremainrelativelyconstant，orthatoscillatearoundrepeatedcycles，thesignalcanbefairlyeasilycharacterizedanditseffectsdescribed，eventhoughthecausativebiologicalmechanismmayremainunknown。Forirregularlyfluctuatingpopulations，wearelikelytohavetoofewobservationstohaveanyhopeofextractingthesignalfromtheoverwhelmingnoise。Butitnowseemsclearthatallpopulationsareregulatedbyamixtureofdensity－dependentanddensity－independenteffectsinvaryingproportions。