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e num npopur, on of the worrd was obout 6 birion irt':5' three times itssize in 1g00.During this period of rapid-1an population growth,populotions of many other species__e decreoseddramotically. Witt the human populotion':tinueto grow? Wittpopulations of other speciescontinue:et smaller? wilt otherspeciescontinue b becomeextinct?- understondingof populations is crucial to answering.;e questions.

Explainhedifferenceserweenpopulationize,ensity,nddispersion.

Describehe hreemain afternsofpopulationispersion.

Explainhe mportancefapopulation'sge tructure.

Contrasthe hreemainypesofsurvivorshipurves.

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, :opulation is a group of organisms that belong to the same-:cies and rive n a particurarprice at the same ime.Alr of the bassng n a pond during a certain period of time .o.,stituie a popura- because hey are solated n the pona una do no t interact withss iving in other ponds. The boundariesof a population may be'rosed by a feature of the environment,such as a lake shore, or:y can be arbitrarily chosen o simprifya study of the popuration.Th e properties of popurationr ;tf;; from those of indrviduars._- ndividual may be born, it may."O.oau.", or it ma y die. A pop_- iuon study focuseson a population as a whole_how many indi_uals ar e born, how many die, and so on .-' .;se=+alEa:e:*==-,- popuration'ssize s the number of individuars t contains.size s aldamental and important.population froperty, but it can be diffi_lt to measure directly. ff -u p.;;;;iJ' i, small and composed- immobile organisms, uch as plants, ts size can be determinedply by counting individuals. il";; ;,;., though, individuals areo abundant, oo widespread,or too _on,l" to be easily counted,-: d scientistsmust estimate he number oi,nO,rr,Oualsin the popu_':ion' suppose that a scientist wants to know how many oak trees''ein a 10km2patch of forest. Insteadot r"u..ning the entire patch-r d countingat the oa k trees, h" r.i;;;t courd count the rrees nmaller section of the forest, ru.rr ur-u 1 km2 area, and use this

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r: ', rl-'l--1.-rr'-These igrating i ldebeestsn East: Africa re oo numerousndmobile, to becounted. cientistsust se: sampling ethodst severalocations, to monitor hangesn hepopulation' size f theanimals.

lation size are based on certainth e potential for error.F*pe:E=?*ea**aesEy

value to estimatethe poputh e larger area. f the smacontains25 oaks,an area 1larger ikely would containas many oak trees, so o re&estimate or the population250oa k rees.A similarkindpling technique must be estimate he sizeof the poshown n Figure20-1.

This kind of estimate athat the distribution of treeforest is the same as thasampledpatch. f th is assis not accurate, he est imbe inaccurate.Estimatesokey assumptions,so theY

Population density measures how crowded a populaPopulation density is always expressedas the number ofuals pe r unit of area or volume. For example, he populatsity of humans in the United States s about 30 people pekilometer. Table 20-1 shows the population densities forcountries. These estimatesare calculated fo r the total laSome areasof a country may be sparsely populated, whiareasare very densely populated.E3=p*e"**xeA third populationproperty is dispersion.Dispersion(di-Sis the spatial distribution of individualswithin the populaclumped distribution, individuals are clustered together' Indistribution, individuals are separated by a fairly consistance. In a random distribution, each individual's ocationpendent of the locations of other individuals in the populathree possible patterns of dispersion are illustrated in FiguClumpeddistributionsoften occur when resourcessuch asliving space are clumped. Clumped distributions may alsbecauseof a species'social behavior,such as when zebrainto herds or birds form flocks. Even distributions usuafrom social interactions, but the interactions result in indgetting as far away from each other as possible. For examgannet,which are the birds shown in Figure20-2c, takesouarea on the coast and defends it from other gannets.Eactries to maximize its distance from all of its neighbors, resan even distribution of individuals. A random distributionresults from seed dispersal by the wind or by birds, as in n i ted tates

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-:e dispersion pattern of a population sometimesdepends on^ - .cale at which th e population is observed.The gannetsshown' :ure 20-2careevenly distributed on a scale of a few meters' Ifr" :cale of observation is the entire island on which the gannets: 1owVer,he distribution appearsclumped because he birds

" ": , 'nl] near the shore.

; GFE L, g ru=*gF='==' .l %,__ pulations are dynamic-they change n size and composition, .: t ime. To understand these changes, more knowledge isrir::rd bout th e population than its size,density,and dispersion."= important measure is the birth rate, the number of births

--rring in a period of time. In the United States, or example,' '""::":&rabout 4 million births per year'A second mportant mea-r-"-=s the death rate, or mortality rate, which is the number ofL,::.rSr a period of time. The death rate for the United States s. . ,:2.4million deathsper year.Another important statistic s life,m;ectancy, or how long on average an individual is expected to' In the United States n 1996, he life expectancy or a ma n wa s

,%@: tUMPED ISPERSION

*L E*&e%R# sa&^ -* @. ff i&%e& - --ti".:,irf f

I :*'f **ii}" I

l l lustratedn (a )are he hree is-' persion atterns-clumPed,ven,and andom.urtlesommonlyclumpogethero baskn hesun.' Birds ften reobservedn even is-r persionssa result f socialnterac-; tions. forestsanexamplefrandom ispersion.lose p, hefishesn (b )ma yappearo be n aneven istribution,ut urther way,they anbeseeno beclumped.Th e irdsn (c )ar eevenly istrib-I uted, utat a great istancehe Yi appearo beclumped.

(c) EVEN ISPERSIoN

PoPULATtoN@p

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a " ' ' ' - - - -" 'Thesewo diagramshow heag ei structureygender f two countries., A comparisonndicatesha tCountry: ha sa higher ercentagef young eo -i plean da lower ercentagef elderly: peopleha nCountry does.

' .t!,1$., ,l' ,.' .:,] Humansave Type survivorshipurvej Some peciesf birds ave Typel' survivorshipurve. ome peciesf fishar eexamplesf a Typell survivorship.

0Age (as fraction of lifespan)

?pe / ffi ?pe // ffi Type II

7.s 5.0 2.5 0 2.s 5.0 7.5 5.0 2.5Percentageof PopulationAg* Se"aa*aaveThe distribution of individualsamongdifferentages n a popis called age structure. Age structures are often presegraphs, as in Figure 20-3.Many important population provary with age. In many species, ncluding humans,very oviduals do no t reproduce. Populationswith a high percenyoung individuals have a greater potential fo r rapid growthFa*s:a-asS M*vatiyThe mortality rate data of different species end to conform tothree curves on a graph, as shown in Figure 204. These cur

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called survivorship curves because hey show the likof survival at different ages throughout the lifetimeorganism. n humans or elephants, or instance, he likof dying is small until late in life, when mortality inrapidly. This pattern of mortality produces the Typvivorship clirve. For other organisms,such as some spbirds, the probability of dying doesnot change hroughgiving a linear,or Type II, survivorship curve. Many orarevery likely to die when young. If an individual surviearly period, however, t has a good chanceof survivinage.This type of survivorship curve, called Type III, steristic of animalssuch as oysters, salmon,and many

80". 60s,,20 .5

r00IOb)s's\t4os)oa.

1. Explain ow wo populationsanbe he samesizebut havedifferent ensities.2. Explain hy evendistributionssuallyesult romsocialnteractionsetweenndividuals.3. Explain ow t is possibleo concluderomFigure 0-3 hat he ife expectancyf individualsin Country sgreaterhan hat of individualsin Country .

4. HowdoesFigure 0-3 ndicate hichcountrylationhas hegreatest otentialor rapidgrow5. Explain hy natural election ight avorahigh eproductionate n organisms ithTy

survivorshipurves.6. tPi;rie,f1i ' i", ll 'Ji{il irlf xplainwo difficulecologistmighthave n counting populamigratory irds.Develop ndexplain met