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    UunIR$TANDINCPOPUI,ATION$,TrI he human population of the world was about 6 billion ini396, three times its size in 1900.During thisperiod of rapidwannan opulation growth, populations of many other speciesrnte decreased dramatically. Will the human populationtnntinue to grow? Will populations of other species continueM'get smaller? Will other species continue to become extinct?An understanding of populat[ons rs crucial to answeringMesequestions.

    PROPERTIES OFPOPLILATIOIVSt lopulation is a group of organisms that belong to the samemrcies and live in a particular place at the same ime. All of the bassftnmg n a pond during a certain period of time constitute a popula-M;n because hey are isolated in the pond and do not interact withunss iving in other ponds. The boundaries of a population may bemnrFosedy a feature of the environment, such as a lake shore, orfltury-can e arbitrarily chosen to simplify a study of the population.

    ihe properties of populations differ from those of individuals.-*m.ndividualmay be born, it may reproduce, or it may die. A pop-ntnrionstudy focuses on a population as a whole-how many indi-mrfird]sare born, how many die, and so on.Fopulation Size.+*;ropulation'size is the number of individuals it contains. Size s a,mmdamental nd important population property, but it can be diffi-iflnrif o measure directly. If a population is small and composedru"""""""":nmobile organisms, such as plants, its size can be determinedmp[v by counting individuals. More often, though, individuals arero,r abundant, too widespread, or too mobile to be easily counted,;nnrdrcientists must estimate the number of individuals in the popu-jryBffr-Suppose that a scientist wants to lcrow how many oak treeslilrwen a 10 km2patch of forest. Instead of searching the entire patchenrmr-ounting all the oak trees, the scientist could count the trees in,um.aller section of the forest, such as a I kmz area, and use this

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    Explainhedifferencesetweenpopulationize,ensity,nddispersion.o

    Describehe hreemain atternsofpopulationispersion.IExplainhe mportancefapopulation'sge tructure.a

    Contrasthe hreemainypesof survivorshipurves.

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    FIGURE O-1These igratingildebeestsn EastAfrica re oonumerousndmobileto becounted.cientistsust sesamplingethodst severalocationsto monitorhangesn h epopulationsize f heanimals.

    lation size are based on certainthe potential for error.

    value to estimate he population c' :the larger area. If the small patc,contains 25 oaks, an area 10 time-*Iarger ikely would contain 10 ime-*as many oak trees, so a reasonab.:estimate or the population'ssize:s250 oak trees.A similar kind of sam-pling technique must be used t -estimate the size of the populatio:shown in Figure 20-1.

    This kind of estimate assumsthat the distribution of trees in th:forest is the same as that of th*sampled patch. If this assumptioris not accurate, the estimate rt-i;be inaccurate. Estimates of popl--key assumptions, so they all hav':

    Population DensityPopulation density measures how crowded a population isPopulation density is always expressed as the number of indivic-uals per unit of area or volume. For example, the population den-sity of humans in the United States s about 30 people per squa-r:kilometer. Table 20-1 shows the population densities for severaLcountries. These estimates are calculated for the total land areaSome areas of a country may be sparsely populated, while otherareas are very densely populated.DispersionA third population property is dispersion.Dispersion (di-SPUHR-zhis the spatial distribution of individuals within the population. In rclumped distribution, individuals are clustered together. In an evs.distribution, individuals are separated by a fairly consistent dis-tance. In a random distribution, each individual's location is ind*pendent of the locations of other individuals in the population. Ttr,*three possible patterns of dispersion are illustrated in Figure 2G2aClumped distributions often occur when resources such as food c'rIiving space are clumped. Clumped distributions may also occr::because of a species'social behavior, such as when zebras gatheinto herds or birds form flocks. Even distributions usually resutfrom social interactions, but the interactions result in individualsgetting as far away from each other as possible. For example, eacl,gannet,which are the birds shown in Figure 20-2c, takesout a smal"area on the coast and defends it from other gannets. Each gannetries to maximize its distance from all of its neighbors, resulting ):an even distribution of individuals. A random distribution usua$results from seed dispersal by the wind or by birds, as in the thircillustration in Figure 20-2a.Forests or a field of wildflowers resu:from random seed dispersal.

    Populationdensity(individuals/Country km2)Japan 330United 240KingdomKenya 50Mexico 50United tates 30Russia 10

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    CLUMPED

    i l[ CTUMPED ISPERSION (c ) EVEN ISPERSION

    TTledispersion pattern of a population sometimes depends ontlhr scale at which the population is observed. The gannets shownnr gure 20-2c are evenly distributed on a scale of a few meters. Iftiln* cale of observation is the entire island on which the gannetsilflrme-owever, the distribution appears clumped because the birdslffilimenly near the shore.

    POPLILATION DYNAMICS,Wt[;urpulations re dynamic-they change n size and compositionrmey time. To understand these changes, more knowledge isilMrsced bout the population than its size, density, and dispersion.tttifrurr*mportant measure is the birth rate, the number of birthsrm;rring in a period of time. In the United States, for example,'fure are about 4 million births per year.A second important mea-;mures the death rate, or mortality rate, which is the number offfimr:rs n a period of time. The death rate for the United States s

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    FIGURE O-2lllustratedn(a)are he hree is-persionatterns-clumped,ven,and andom.urtlesommonlyclumpogethero baskn hesun.Birds ften reobservedn even is-persionssa result f socialnterac-tions. forestsanexamplefrandomispersion.lose p, hefishesn(b)may ppearo be n aneven istribution,ut urther way,they an eseenobeclumped.The irdsn(c)areevenlyistrib-uted, utat a great istanceheyappearo beclumped.

    r 2.4 million deaths per year.Another important statistic is life, or how long on average an individual is expected tolllillM"n the United States n 1996, he life expectancy for a man was

    ffi lnears,and for a woman it was 79 years.

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    FIGUREO-3I Thesewo diagramshow heageI structureygender f wo countries.I A comparisonndicatesha tCountryI ha sa higher ercentagefyoung eo-I pl ean da lower ercentagef elderlyI peopleha nCountry does.

    80g, 608ro20

    Humans ave Type survivorshipurve.Some peciesf birds ave Typelsurvivorshipurve. ome peciesf ishareexamplesf a Typell survivorship.

    1. Explainow wo populat ionsanbe hesamesizebut havedifferent ensities.2. Explain hy evendistributionssuallyesult romsocialnteractionsetweenndividuals.3. Explain ow t is possibleo concluderomFigure 0-3 hat he if eexpectancyf individualsin Country sgreaterhan hat of individual sin Country .

    7. 5 5.0 2.5 A 2.5 5.0 7.5 5. 0 2. 5Percentage of Population 0 2.5 5.0

    Age StructureThe distribution of individualsamong differentages n a populati, ris called age structure. Age structures are often presented rtgraphs, as in Figure 20-3. Many important population procs-s:":vary with age. In many species, ncluding humans, very old in:1"viduals do not reproduce. Populations with a high percentageyoung individuals have a greater potential for rapid growth.Patterns of MortalityThe mortality rate data of different species end to conform to one :three curves on a graph, as shown in Figure 204. These curvs &r:called survivorship curves because hey show the likeliho,':of survival at different ages throughout the lifetime of "r--l:organism. n humans or elephants, or instance, he likeliho:,:p,^^ t , of dying is small until late in life, when mortality increa-..,:F'"" %*

    .""'-'"0.,+,,\.:r,:,, rapidly. This pattern of mortality produces the Type I s-:

    ,$ rrp" / ff i rype I ff i Type t teristic of animalssuch as oysters, salmon. and manv insec:.

    i - E . ; rapidly.This pattern of mortality produces he Type I s :t "G ru , :il:l'Tli"1r;1?.1"t1:':'slnisms,,suchas::mefi , lU =\\* . ; birds, heprobability f dyingdoesno tchangehroughout.: .il ' I \ \ givinga linear. r Type I,survivorship urve.Manyorganis: -x.2|*areVerV| ikelvtodiewhenvoung. I faninr l iv ic l t la lst l rv ivest -^[email protected] ikelytodiewhenyoung.I fanindividualsurvivest :_l.{ Age (as fraction of lifespan) early period. however. t has a good chance of surviving to ,. ,

    "-"....,.."..-.".tge.This type of survivorship curve, called Type III, s char".

    4. HowdoesFigure 0-3 ndicate hich ountry'sopu-lationhas hegreatest otentialor rapidgrowth?5. Explain hy natural election ight avorahigh eproductionate n organisms ith Type llsurvivorshipurves.6. CRITICAI HINKINGExplainwo difficulties necologistmighthave n counting population fmigratory irds.Develop ndexplain methodfor estimatinghe size f such population.

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    frInAIURINcPOPtlIATIONSCharles Darwin calculated thot a singte pair of elephantsrould increase o a population of 19 million indiuidualswithin 750years. The fact that the world is not ouerrun withelephonts is euidence that some factor or factors restrain theptpulation growth of elephants. In this section you will studylww populotions grow and what factors imit their growth.

    POPLILATION GRO\A/THRATEilIogruoners, scientists ho studypopulation ynamics,efinethe growth rate of a population as the amount by which a popula-tlon's size changes n a given time.Whether a population grows, shrinks, or remains the same sizedependson four processes: birth, death, emigration, and immigra-tion. Immigration (im-uh-GRAY-shuhn)s the movement of individualshto a population, and emigration (em-i-GRAY-shuhn)s the movementof individuals out of the population. Two of these processes-birthand immigration-add individuals to a population, while the other,rrN-orocesses-death and emigration-subtract individuals fromthe population. Fo r simplicity's sake, demographers usually as -sume that immigration and emigration are zero when calculatinga population's growth rate. By making this simple assumption, apopulation's growth rate can be described mathematically and canbe graphed.

    It is customary for demographers to divide large populationsuntogroups of 1,000and to present data per capita, meaning perrndividual. Birth rates, death rates, and growth rates for a largepopulation are usually expressed per capita. For example, if thereare 52 births and 14 deaths per 1,000 ndividuals in a large popula-fion in one year, the per capita birth rate would be szl,ooo,or 0.052b{rths per individual per year. The per capita death rate would be{ - rn.n,r 0.014deaths per individual per year.The per capita growthrate can be found by the following simple equation:

    SC TOPIC:actorsfiecting- 7 /NKS populationrowth.6 f;3rlfrflil,.*'-i!ff0"

    OBJICTI lr ISDescribeheexponentialodelofpopulationrowth.

    aComparehe imilarit iesnddifferencesetweenhe ogisticmodel ndheexponentialodel.

    IDistinguishetweendensity-dependentnd ensity-independentegulatoryactors.

    aList hreeeasonshysmallopulationsremorevulnerableo extinction.

    birth rate - death rate : growth rate

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    FIGURE O-5The raph f exponentialopulationgrowth as characteristicshape.The xponentialodelndicatesinfinitg onstantlyncreasingopula-tiongrowth.

    FIGURE 0-6The opulationncreasef bacterianthe aboratoryroducescharacteristicgraph f exponentialrowth. ith hiskind fgraph,hesize f hepopula-tionof bacteriat any utureime anbepredictedf theculturesprovidedwithunlimitedesources,uch s ood.

    Using the same example, we can calculate the per capita gro'*trate as follows:0052birthsffiu1?,:]".nHliffi:;tserapita)To find the number of new individuals that will be added to th=population in a year, simply multiply the per capita growth rate b--the number of individuals in the population. If the population i: .

    ou r example numbers 50,000,he population will increaseby 1,9ir,individuals in one year.0.038x50,000:1,900

    If the growth rate is a positive number, the population is increa*sing. If it is a negative number, the population is shrinking.

    TFIE gXPG1xgru?A,&L fuA$#ilg,The exponential (trs-poh-NEN-shuhl)odel of population grornl:describesa population that increases apidly after only a few gener-ations; the larger the population gets, the faster it grows. This lscalled exponential growth. In constructing an exponential model c;population growth, it is assumed that birth rates and death rate:remain constant, however largethe population becomes.Fredictions Based sx: ghe ExpolaeaetiaE ftodelOne way to understand predictions based on the exponential mode-is to look at a graph of population size over time. Population growtl-according to the exponential model, follows the characteiristiiJ-shaped curve shown in Figure 20-5.As you can see on the grapl-the population grows slowly when it is small, but its growth speed,up as more and more individuals are added to the population. \\ tcan predict that the population will grow indefinitely and at a:_increasinglyrapid rate based on the exponential model. Figure 20-,:shows the exponential growth of bacteria in a laboratory culture.Limitations of the Expomesatia3McdenThe ultimate test of an exponential model is how well it matche.sthe growth pattern of a real population. Do populations grow expc-nentially? The answer is yes, but only under rare conditions ancfo r short periods of time. For example,populations of bacteria aniother microorganisms can grow exponentially in the laboratory

    DataTableTime

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    Bacteriacount I,000 2,000 4,000 8,000 16,000