Bài giảng Biology - Chapter 52: Population Ecology

Chapter 52Population EcologyOverview: Earth’s Fluctuating PopulationsTo understand human population growthWe must consider the general principles of population ecologyPopulation ecology is the study of populations in relation to environmentIncluding environmental influences on population density and distribution, age structure, and variations in population sizeThe fur seal population of St. Paul Island, off the coast of AlaskaIs one that has experienced dramatic fluctuations in sizeFigure 52.1Concept 52.1: Dynamic biological processes influence population density, dispersion, and demographyA populationIs a group of individuals of a single species living in the same general areaDensity and DispersionDensityIs the number of individuals per unit area or volumeDispersionIs the pattern of spacing among individuals within the boundaries of the populationDensity: A Dynamic PerspectiveDetermining the density of natural populationsIs possible, but difficult to accomplishIn most casesIt is impractical or impossible to count all individuals in a populationDensity is the result of a dynamic interplayBetween processes that add individuals to a population and those that remove individuals from itFigure 52.2Births and immigration add individuals to a population.BirthsImmigrationPopuIationsizeEmigrationDeathsDeaths and emigration remove individuals from a population.Patterns of DispersionEnvironmental and social factorsInfluence the spacing of individuals in a populationA clumped dispersionIs one in which individuals aggregate in patchesMay be influenced by resource availability and behaviorFigure 52.3a(a) Clumped. For many animals, such as these wolves, living in groups increases the effectiveness of hunting, spreads the work of protecting and caring for young, and helps exclude other individuals from their territory.A uniform dispersionIs one in which individuals are evenly distributedMay be influenced by social interactions such as territorialityFigure 52.3b(b) Uniform. Birds nesting on small islands, such as these king penguins on South Georgia Island in the South Atlantic Ocean, often exhibit uniform spacing, maintained by aggressive interactions between neighbors.A random dispersionIs one in which the position of each individual is independent of other individualsFigure 52.3c(c) Random. Dandelions grow from windblown seeds that land at random and later germinate.DemographyDemography is the study of the vital statistics of a populationAnd how they change over timeDeath rates and birth ratesAre of particular interest to demographersLife TablesA life tableIs an age-specific summary of the survival pattern of a populationIs best constructed by following the fate of a cohortThe life table of Belding’s ground squirrelsReveals many things about this populationTable 52.1Survivorship CurvesA survivorship curveIs a graphic way of representing the data in a life tableThe survivorship curve for Belding’s ground squirrelsShows that the death rate is relatively constantFigure 52.41000100101Number of survivors (log scale)0246810Age (years)MalesFemalesSurvivorship curves can be classified into three general typesType I, Type II, and Type IIIFigure 52.5IIIIII5010001101001,000Percentage of maximum life spanNumber of survivors (log scale)Reproductive RatesA reproductive table, or fertility scheduleIs an age-specific summary of the reproductive rates in a populationA reproductive tableDescribes the reproductive patterns of a populationTable 52.2Concept 52.2: Life history traits are products of natural selectionLife history traits are evolutionary outcomesReflected in the development, physiology, and behavior of an organismLife History DiversityLife histories are very diverseSpecies that exhibit semelparity, or “big-bang” reproductionReproduce a single time and dieFigure 52.6Species that exhibit iteroparity, or repeated reproductionProduce offspring repeatedly over time“Trade-offs” and Life HistoriesOrganisms have finite resourcesFigure 52.7Researchers in the Netherlands studied the effects of parental caregiving in European kestrels over 5 years. The researchers transferred chicks among nests to produce reduced broods (three or four chicks), normal broods (five or six), and enlarged broods (seven or eight). They then measured the percentage of male and female parent birds that survived the following winter. (Both males and females provide care for chicks.)EXPERIMENTThe lower survival rates of kestrels with larger broods indicate that caring for more offspring negatively affects survival of the parents.CONCLUSION100806040200Reduced brood sizeNormal brood sizeEnlarged brood sizeParents surviving the following winter (%)MaleFemaleWhich may lead to trade-offs between survival and reproductionRESULTSSome plants produce a large number of small seedsEnsuring that at least some of them will grow and eventually reproduceFigure 52.8a(a) Most weedy plants, such as this dandelion, grow quickly and produce a large number of seeds, ensuring that at least somewill grow into plants and eventually produce seeds themselves.Other types of plants produce a moderate number of large seedsThat provide a large store of energy that will help seedlings become establishedFigure 52.8b(b) Some plants, such as this coconut palm, produce a moderate number of very large seeds. The large endosperm provides nutrients for the embryo, an adaptation that helps ensure the success of a relatively large fraction of offspring.Parental care of smaller broodsMay also facilitate survival of offspringConcept 52.3: The exponential model describes population growth in an idealized, unlimited environmentIt is useful to study population growth in an idealized situationIn order to understand the capacity of species for increase and the conditions that may facilitate this type of growthPer Capita Rate of IncreaseIf immigration and emigration are ignoredA population’s growth rate (per capita increase) equals birth rate minus death rateZero population growth Occurs when the birth rate equals the death rateThe population growth equation can be expressed asdNdtrNExponential GrowthExponential population growthIs population increase under idealized conditionsUnder these conditionsThe rate of reproduction is at its maximum, called the intrinsic rate of increaseThe equation of exponential population growth isdNdtrmaxNExponential population growthResults in a J-shaped curveFigure 52.905101505001,0001,5002,000Number of generationsPopulation size (N)dNdt1.0NdNdt0.5NThe J-shaped curve of exponential growthIs characteristic of some populations that are reboundingFigure 52.1019001920194019601980Year02,0004,0006,0008,000Elephant populationConcept 52.4: The logistic growth model includes the concept of carrying capacityExponential growthCannot be sustained for long in any populationA more realistic population modelLimits growth by incorporating carrying capacityCarrying capacity (K)Is the maximum population size the environment can supportThe Logistic Growth ModelIn the logistic population growth modelThe per capita rate of increase declines as carrying capacity is reachedWe construct the logistic model by starting with the exponential modelAnd adding an expression that reduces the per capita rate of increase as N increasesFigure 52.11MaximumPositiveNegative0N  KPopulation size (N)Per capita rate of increase (r)The logistic growth equationIncludes K, the carrying capacitydNdt(K  N)KrmaxNA hypothetical example of logistic growthTable 52.3The logistic model of population growthProduces a sigmoid (S-shaped) curveFigure 52.12dNdt1.0NExponential growthLogistic growthdNdt1.0N1,500  N1,500K  1,500 05101505001,0001,5002,000Number of generationsPopulation size (N)Figure 52.13a8006004002000Time (days)051015(a) A Paramecium population in the lab. The growth of Paramecium aurelia in small cultures (black dots) closely approximates logistic growth (red curve) if the experimenter maintains a constant environment.1,000Number of Paramecium/mlThe Logistic Model and Real PopulationsThe growth of laboratory populations of parameciaFits an S-shaped curveSome populations overshoot KBefore settling down to a relatively stable densityFigure 52.13b1801500120906030Time (days)016014012080100604020Number of Daphnia/50 ml(b) A Daphnia population in the lab. The growth of a population of Daphnia in a small laboratory culture (black dots) does not correspond well to the logistic model (red curve). This population overshoots the carrying capacity of its artificial environment and then settles down to an approximately stable population size.Some populationsFluctuate greatly around KFigure 52.13c080604020197519801985199019952000Time (years)Number of females(c) A song sparrow population in its natural habitat. The population of female song sparrows nesting on Mandarte Island, British Columbia, is periodically reduced by severe winter weather, and population growth is not well described by the logistic model.The logistic model fits few real populationsBut is useful for estimating possible growthThe Logistic Model and Life HistoriesLife history traits favored by natural selectionMay vary with population density and environmental conditionsK-selection, or density-dependent selectionSelects for life history traits that are sensitive to population densityr-selection, or density-independent selectionSelects for life history traits that maximize reproductionThe concepts of K-selection and r-selectionAre somewhat controversial and have been criticized by ecologists as oversimplificationsConcept 52.5: Populations are regulated by a complex interaction of biotic and abiotic influencesThere are two general questions we can askAbout regulation of population growthWhat environmental factors stop a population from growing?Why do some populations show radical fluctuations in size over time, while others remain stable?Population Change and Population DensityIn density-independent populationsBirth rate and death rate do not change with population densityIn density-dependent populationsBirth rates fall and death rates rise with population densityDetermining equilibrium for population densityFigure 52.14a–cDensity-dependent birth rateDensity-dependent death rateEquilibrium densityDensity-dependent birth rateDensity-independent death rateEquilibrium densityDensity-independent birth rateDensity-dependent death rateEquilibrium densityPopulation densityPopulation densityPopulation densityBirth or death rate per capita(a) Both birth rate and death rate change with population density.(b) Birth rate changes with populationdensity while death rate is constant.(c) Death rate changes with populationdensity while birht rate is constant.Density-Dependent Population RegulationDensity-dependent birth and death ratesAre an example of negative feedback that regulates population growthAre affected by many different mechanismsCompetition for ResourcesIn crowded populations, increasing population densityIntensifies intraspecific competition for resourcesFigure 52.15a,b10010010001,00010,000Average number of seeds per reproducing individual (log scale)Average clutch sizeSeeds planted per m2Density of females070102030405060802.83.03.23.43.63.84.0(a) Plantain. The number of seeds produced by plantain (Plantago major) decreases as density increases.(b) Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply.TerritorialityIn many vertebrates and some invertebratesTerritoriality may limit densityCheetahs are highly territorialUsing chemical communication to warn other cheetahs of their boundariesFigure 52.16Oceanic birdsExhibit territoriality in nesting behaviorFigure 52.17HealthPopulation densityCan influence the health and survival of organismsIn dense populationsPathogens can spread more rapidlyPredationAs a prey population builds upPredators may feed preferentially on that speciesToxic WastesThe accumulation of toxic wastesCan contribute to density-dependent regulation of population sizeIntrinsic FactorsFor some populationsIntrinsic (physiological) factors appear to regulate population sizePopulation DynamicsThe study of population dynamicsFocuses on the complex interactions between biotic and abiotic factors that cause variation in population sizeStability and FluctuationLong-term population studiesHave challenged the hypothesis that populations of large mammals are relatively stable over timeFigure 52.18The pattern of population dynamics observedin this isolated population indicates that various biotic and abiotic factors can result in dramatic fluctuations over time in a moose population.Researchers regularly surveyed the population of moose on Isle Royale, Michigan, from 1960 to 2003. During that time, the lake never froze over, and so the moose population was isolated from the effects of immigration and emigration.FIELD STUDYOver 43 years, this population experiencedtwo significant increases and collapses, as well as several less severe fluctuations in size.RESULTSCONCLUSION19601970198019902000YearMoose population size05001,0001,5002,0002,500Steady decline probably caused largely by wolf predationDramatic collapse caused by severe winter weather and food shortage, leading to starvation of more than 75% of the populationExtreme fluctuations in population sizeAre typically more common in invertebrates than in large mammalsFigure 52.191950196019701980Year199010,000100,000730,000Commercial catch (kg) of male crabs (log scale)Metapopulations and ImmigrationMetapopulationsAre groups of populations linked by immigration and emigrationHigh levels of immigration combined with higher survivalCan result in greater stability in populationsFigure 52.20Mandarte islandSmall islandsNumber of breeding females1988198919901991Year0102030405060Population CyclesMany populationsUndergo regular boom-and-bust cyclesFigure 52.21Year1850187519001925040801201600369Lynx population size (thousands)Hare population size (thousands)LynxSnowshoe hareBoom-and-bust cyclesAre influenced by complex interactions between biotic and abiotic factorsConcept 52.6: Human population growth has slowed after centuries of exponential increaseNo population can grow indefinitelyAnd humans are no exceptionThe Global Human PopulationThe human populationIncreased relatively slowly until about 1650 and then began to grow exponentiallyFigure 52.228000 B.C.4000 B.C.3000 B.C.2000 B.C.1000 B.C.1000 A.D.0The PlagueHuman population (billions)2000 A.D.0123456Though the global population is still growingThe rate of growth began to slow approximately 40 years agoFigure 52.2319501975200020252050Year2003Percent increase2.221.61.41.210.80.60.40.201.8Regional Patterns of Population ChangeTo maintain population stabilityA regional human population can exist in one of two configurationsZero population growth = High birth rates – High death ratesZero population growth = Low birth rates – Low death ratesThe demographic transitionIs the move from the first toward the second stateFigure 52.24504020030101750180018501900195020002050Birth rateDeath rateBirth rateDeath rateYearSwedenMexicoBirth or death rate per 1,000 peopleThe demographic transitionIs associated with various factors in developed and developing countriesAge StructureOne important demographic factor in present and future growth trendsIs a country’s age structure, the relative number of individuals at each ageAge structureIs commonly represented in pyramidsFigure 52.25Rapid growth AfghanistanSlow growth United StatesDecrease ItalyMaleFemaleMaleFemaleMaleFemaleAgeAge864202468864202468864202468Percent of populationPercent of populationPercent of population80–848575–7970–7465–6960–6455–5950–5445–4940–4435–3930–3420–2425–2910–145–90–415–1980–848575–7970–7465–6960–6455–5950–5445–4940–4435–3930–3420–2425–2910–145–90–415–19Age structure diagramsCan predict a population’s growth trendsCan illuminate social conditions and help us plan for the futureInfant Mortality and Life ExpectancyInfant mortality and life expectancy at birthVary widely among developed and developing countries but do not capture the wide range of the human conditionFigure 52.26Developed countriesDeveloping countriesDeveloped countriesDeveloping countriesInfant mortality (deaths per 1,000 births)Life expectancy (years)6050403020100806040200Global Carrying CapacityJust how many humans can the biosphere support?Estimates of Carrying CapacityThe carrying capacity of Earth for humans is uncertainEcological FootprintThe ecological footprint conceptSummarizes the aggregate land and water area needed to sustain the people of a nationIs one measure of how close we are to the carrying capacity of EarthEcological footprints for 13 countriesShow that the countries vary greatly in their footprint size and their available ecological capacityFigure 52.2716141210864200246810121416New ZealandAustraliaCanadaSwedenWorldChinaIndiaAvailable ecological capacity (ha per person)SpainUKJapanGermanyNetherlandsNorwayUSAEcological footprint (ha per person)At more than 6 billion peopleThe world is already in ecological deficit