1.Theprimarypurposeofthepassageistoargueagainstaconventionalexplanationfortheextinctionofcertainsalmonpopulationsandsuggestanalternativecorrectacommonmisunderstandingaboutthebehaviorofsalmoninresponsetoenvironmentaldegradationcausedbyhumanactivitycomparetheeffectsofhumanactivityonsalmonpopulationswiththeeffectsofnaturaldisturbancesonsalmonpopulationsdifferentiatetheparticulareffectsofvarioushumanactivitiesonsalmonhabitatsdescribehowenvironmentaldegradationcancausechangesinsalmonpopulationsthatextendbeyondanumericalreduction
2.Itcanbeinferredfromthepassagethattheoccasionalfailureofsomesalmontoreturntotheirnatalstreamsinordertospawnprovidesamechanismbywhich pristinestreamsthatarenearpollutedstreamsepollutedthemselvestheparticularadaptationsofapollutedstream’ssalmonpopulationcanbepreservedwithoutdilutionthenumberofsalmoninpristinehabitatsdecreasesrelativetothenumberinpollutedstreamsanenvironmentallydegradedstreamcouldberecolonizedbynewsalmonpopulationsshouldthestreamrecovertheextinctionofthesalmonpopulationsthatspawninpollutedstreamsisaccelerated
3.Accordingtothepassage,humanactivityhashadwhichofthefollowingeffectsonsalmonpopulations?AnincreaseinthesizeofsalmonpopulationsinsomepreviouslypollutedriversAdeclineinthenumberofsalmoninsomeriversAdecreaseinthenumberstrayingsalmoninsomeriversAdecreaseinthegeneflowbetweensalmonpopulationsthatspawninpollutedstreamsandpopulationsthatspawninpristinestreamsAdeclineinthevulnerabilityofsomesalmonpopulationstotheeffectsofnaturallyoccurringhabitatdestructionWeixin:zhbfree赵洪波/zhaohongbo
4.Theauthormentionsthe“aftermathoftheMountSaintHelenseruption”mostlikelyinordertoprovideanexampleoftheprocessthatallowstherepopulationofriverswhoseindigenoussalmonp