Sinh học - Chapter 53: Population ecology

Chapter 53Population EcologyOverview: Counting SheepA small population of Soay sheep were introduced to Hirta Island in 1932They provide an ideal opportunity to study changes in population size on an isolated island with abundant food and no predatorsFig. 53-1Population ecology is the study of populations in relation to environment, including environmental influences on density and distribution, age structure, and population sizeConcept 53.1: Dynamic biological processes influence population density, dispersion, and demographicsA population is a group of individuals of a single species living in the same general areaDensity and DispersionDensity is the number of individuals per unit area or volumeDispersion is the pattern of spacing among individuals within the boundaries of the populationDensity: A Dynamic PerspectiveIn most cases, it is impractical or impossible to count all individuals in a populationSampling techniques can be used to estimate densities and total population sizesPopulation size can be estimated by either extrapolation from small samples, an index of population size, or the mark-recapture methodFig. 53-2APPLICATIONHector’s dolphinsDensity is the result of an interplay between processes that add individuals to a population and those that remove individualsImmigration is the influx of new individuals from other areasEmigration is the movement of individuals out of a populationFig. 53-3BirthsBirths and immigrationadd individuals toa population.ImmigrationDeaths and emigrationremove individualsfrom a population.DeathsEmigrationPatterns of DispersionEnvironmental and social factors influence spacing of individuals in a populationIn a clumped dispersion, individuals aggregate in patchesA clumped dispersion may be influenced by resource availability and behaviorVideo: Flapping Geese (Clumped)Fig. 53-4(a) Clumped(b) Uniform(c) RandomFig. 53-4a(a) ClumpedA uniform dispersion is one in which individuals are evenly distributedIt may be influenced by social interactions such as territorialityVideo: Albatross Courtship (Uniform)Fig. 53-4b(b) UniformIn a random dispersion, the position of each individual is independent of other individualsIt occurs in the absence of strong attractions or repulsionsVideo: Prokaryotic Flagella (Salmonella typhimurium) (Random)Fig. 53-4c(c) RandomDemographicsDemography is the study of the vital statistics of a population and how they change over timeDeath rates and birth rates are of particular interest to demographersLife TablesA life table is an age-specific summary of the survival pattern of a populationIt is best made by following the fate of a cohort, a group of individuals of the same ageThe life table of Belding’s ground squirrels reveals many things about this populationTable 53-1Survivorship CurvesA survivorship curve is a graphic way of representing the data in a life tableThe survivorship curve for Belding’s ground squirrels shows a relatively constant death rateFig. 53-5Age (years)20486101011,000100Number of survivors (log scale)MalesFemalesSurvivorship curves can be classified into three general types:Type I: low death rates during early and middle life, then an increase among older age groupsType II: the death rate is constant over the organism’s life spanType III: high death rates for the young, then a slower death rate for survivorsFig. 53-61,000100101050100IIIIIPercentage of maximum life spanNumber of survivors (log scale)IReproductive RatesFor species with sexual reproduction, demographers often concentrate on females in a populationA reproductive table, or fertility schedule, is an age-specific summary of the reproductive rates in a populationIt describes reproductive patterns of a populationTable 53-2Concept 53.2: Life history traits are products of natural selectionAn organism’s life history comprises the traits that affect its schedule of reproduction and survival:The age at which reproduction beginsHow often the organism reproducesHow many offspring are produced during each reproductive cycleLife history traits are evolutionary outcomes reflected in the development, physiology, and behavior of an organismEvolution and Life History DiversityLife histories are very diverseSpecies that exhibit semelparity, or big-bang reproduction, reproduce once and dieSpecies that exhibit iteroparity, or repeated reproduction, produce offspring repeatedlyHighly variable or unpredictable environments likely favor big-bang reproduction, while dependable environments may favor repeated reproductionFig. 53-7“Trade-offs” and Life HistoriesOrganisms have finite resources, which may lead to trade-offs between survival and reproductionFig. 53-8MaleFemale100RESULTS806040200Reducedbrood sizeNormalbrood sizeEnlargedbrood sizeParents surviving the following winter (%)Some plants produce a large number of small seeds, ensuring that at least some of them will grow and eventually reproduceFig. 53-9(a) Dandelion(b) Coconut palmFig. 53-9a(a) DandelionOther types of plants produce a moderate number of large seeds that provide a large store of energy that will help seedlings become establishedFig. 53-9b(b) Coconut palmIn animals, parental care of smaller broods may facilitate survival of offspringConcept 53.3: The exponential model describes population growth in an idealized, unlimited environmentIt is useful to study population growth in an idealized situationIdealized situations help us understand the capacity of species to increase and the conditions that may facilitate this growthPer Capita Rate of IncreaseIf immigration and emigration are ignored, a population’s growth rate (per capita increase) equals birth rate minus death rateZero population growth occurs when the birth rate equals the death rateMost ecologists use differential calculus to express population growth as growth rate at a particular instant in time:NtrNwhere N = population size, t = time, and r = per capita rate of increase = birth – death Exponential GrowthExponential population growth is population increase under idealized conditionsUnder these conditions, the rate of reproduction is at its maximum, called the intrinsic rate of increaseEquation of exponential population growth:dNdtrmaxNExponential population growth results in a J-shaped curveFig. 53-10Number of generations05101505001,0001,5002,000 1.0N =dNdt 0.5N =dNdtPopulation size (N)The J-shaped curve of exponential growth characterizes some rebounding populationsFig. 53-118,0006,0004,0002,00001920194019601980YearElephant population1900Concept 53.4: The logistic model describes how a population grows more slowly as it nears its carrying capacityExponential growth cannot be sustained for long in any populationA more realistic population model limits growth by incorporating carrying capacityCarrying capacity (K) is the maximum population size the environment can supportThe Logistic Growth ModelIn the logistic population growth model, the per capita rate of increase declines as carrying capacity is reachedWe construct the logistic model by starting with the exponential model and adding an expression that reduces per capita rate of increase as N approaches KdNdt(K  N)KrmaxNTable 53-3The logistic model of population growth produces a sigmoid (S-shaped) curveFig. 53-122,0001,5001,0005000051015Number of generationsPopulation size (N)Exponentialgrowth1.0N=dNdt1.0N=dNdtK = 1,500Logistic growth1,500 – N1,500The Logistic Model and Real PopulationsThe growth of laboratory populations of paramecia fits an S-shaped curveThese organisms are grown in a constant environment lacking predators and competitorsFig. 53-131,0008006004002000051015Time (days)Number of Paramecium/mLNumber of Daphnia/50 mL0306090180150120020406080100120140160Time (days)(b) A Daphnia population in the lab(a) A Paramecium population in the labFig. 53-13a1,0008006004002000051015Time (days)Number of Paramecium/mL(a) A Paramecium population in the labSome populations overshoot K before settling down to a relatively stable densityFig. 53-13bNumber of Daphnia/50 mL0306090180150120020406080100120140160Time (days)(b) A Daphnia population in the labSome populations fluctuate greatly and make it difficult to define KSome populations show an Allee effect, in which individuals have a more difficult time surviving or reproducing if the population size is too smallThe logistic model fits few real populations but is useful for estimating possible growthFig. 53-14The Logistic Model and Life HistoriesLife history traits favored by natural selection may vary with population density and environmental conditionsK-selection, or density-dependent selection, selects for life history traits that are sensitive to population densityr-selection, or density-independent selection, selects for life history traits that maximize reproductionThe concepts of K-selection and r-selection are oversimplifications but have stimulated alternative hypotheses of life history evolutionConcept 53.5: Many factors that regulate population growth are density dependentThere are two general questions about regulation of population growth:What environmental factors stop a population from growing indefinitely?Why do some populations show radical fluctuations in size over time, while others remain stable?Population Change and Population DensityIn density-independent populations, birth rate and death rate do not change with population densityIn density-dependent populations, birth rates fall and death rates rise with population densityFig. 53-15(a) Both birth rate and death rate vary.Population densityDensity-dependentbirth rateEquilibriumdensityDensity-dependentdeath rateBirth or death rateper capita(b) Birth rate varies; death rate is constant.Population densityDensity-dependentbirth rateEquilibriumdensityDensity-independentdeath rate(c) Death rate varies; birth rate is constant.Population densityDensity-dependentdeath rateEquilibriumdensityDensity-independentbirth rateBirth or death rateper capitaDensity-Dependent Population RegulationDensity-dependent birth and death rates are an example of negative feedback that regulates population growthThey are affected by many factors, such as competition for resources, territoriality, disease, predation, toxic wastes, and intrinsic factorsCompetition for ResourcesIn crowded populations, increasing population density intensifies competition for resources and results in a lower birth rateFig. 53-16Population size100806040200200400500600300Percentage of juveniles producing lambsTerritorialityIn many vertebrates and some invertebrates, competition for territory may limit densityCheetahs are highly territorial, using chemical communication to warn other cheetahs of their boundariesFig. 53-17(a) Cheetah marking its territory(b) GannetsFig. 53-17a(a) Cheetah marking its territoryOceanic birds exhibit territoriality in nesting behaviorFig. 53-17b(b) GannetsDiseasePopulation density can influence the health and survival of organismsIn dense populations, pathogens can spread more rapidlyPredationAs a prey population builds up, predators may feed preferentially on that speciesToxic WastesAccumulation of toxic wastes can contribute to density-dependent regulation of population sizeIntrinsic FactorsFor some populations, intrinsic (physiological) factors appear to regulate population sizePopulation DynamicsThe study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in population sizeStability and FluctuationLong-term population studies have challenged the hypothesis that populations of large mammals are relatively stable over timeWeather can affect population size over timeFig. 53-182,1001,9001,7001,5001,3001,1009007005000195519651975198519952005YearNumber of sheepChanges in predation pressure can drive population fluctuationsFig. 53-19WolvesMoose2,5002,0001,5001,000500Number of moose0Number of wolves50403020100195519651975198519952005YearPopulation Cycles: Scientific InquirySome populations undergo regular boom-and-bust cyclesLynx populations follow the 10 year boom-and-bust cycle of hare populations Three hypotheses have been proposed to explain the hare’s 10-year intervalFig. 53-20Snowshoe hareLynxNumber of lynx(thousands)Number of hares(thousands)160120804001850187519001925Year9630Fig. 53-20aFig. 53-20bSnowshoe hareLynxNumber of lynx(thousands)Number of hares(thousands)160120804001850187519001925Year9306Hypothesis: The hare’s population cycle follows a cycle of winter food supplyIf this hypothesis is correct, then the cycles should stop if the food supply is increasedAdditional food was provided experimentally to a hare population, and the whole population increased in size but continued to cycleNo hares appeared to have died of starvationHypothesis: The hare’s population cycle is driven by pressure from other predatorsIn a study conducted by field ecologists, 90% of the hares were killed by predatorsThese data support this second hypothesisHypothesis: The hare’s population cycle is linked to sunspot cyclesSunspot activity affects light quality, which in turn affects the quality of the hares’ foodThere is good correlation between sunspot activity and hare population sizeThe results of all these experiments suggest that both predation and sunspot activity regulate hare numbers and that food availability plays a less important roleImmigration, Emigration, and MetapopulationsMetapopulations are groups of populations linked by immigration and emigrationHigh levels of immigration combined with higher survival can result in greater stability in populationsFig. 53-21AlandIslandsEUROPEOccupied patchUnoccupied patch5 km˚Concept 53.6: The human population is no longer growing exponentially but is still increasing rapidlyNo population can grow indefinitely, and humans are no exceptionThe Global Human PopulationThe human population increased relatively slowly until about 1650 and then began to grow exponentiallyFig. 53-228000B.C.E.4000B.C.E.3000B.C.E.2000B.C.E.1000B.C.E.01000C.E.2000C.E.0123456The PlagueHuman population (billions)7Though the global population is still growing, the rate of growth began to slow during the 1960sFig. 53-232005ProjecteddataAnnual percent increaseYear195019752000202520502.22.01.81.61.41.21.00.80.60.40.20Regional Patterns of Population ChangeTo maintain population stability, a regional human population can exist in one of two configurations:Zero population growth = High birth rate – High death rateZero population growth =Low birth rate – Low death rateThe demographic transition is the move from the first state toward the second stateFig. 53-24175018001900195020002050Year1850SwedenMexicoBirth rateBirth rateDeath rateDeath rate01020304050Birth or death rate per 1,000 peopleThe demographic transition is associated with an increase in the quality of health care and improved access to education, especially for womenMost of the current global population growth is concentrated in developing countriesAge StructureOne important demographic factor in present and future growth trends is a country’s age structureAge structure is the relative number of individuals at each ageFig. 53-25Rapid growthAfghanistanMaleFemaleAgeAgeMaleFemaleSlow growthUnited StatesMaleFemaleNo growthItaly85+80–8475–7970–7460–6465–6955–5950–5445–4940–4435–3930–3425–2920–2415–190–45–910–1485+80–8475–7970–7460–6465–6955–5950–5445–4940–4435–3930–3425–2920–2415–190–45–910–1410 10 886644220Percent of populationPercent of populationPercent of population664422088664422088Age structure diagrams can predict a population’s growth trendsThey can illuminate social conditions and help us plan for the futureInfant Mortality and Life ExpectancyInfant mortality and life expectancy at birth vary greatly among developed and developing countries but do not capture the wide range of the human conditionFig. 53-26Less indus-trializedcountriesIndus-trializedcountries60504030201000204080Life expectancy (years)Infant mortality (deaths per 1,000 births)Less indus-trializedcountriesIndus-trializedcountries60Global Carrying CapacityHow many humans can the biosphere support?Estimates of Carrying CapacityThe carrying capacity of Earth for humans is uncertainThe average estimate is 10–15 billionLimits on Human Population SizeThe ecological footprint concept summarizes the aggregate land and water area needed to sustain the people of a nationIt is one measure of how close we are to the carrying capacity of EarthCountries vary greatly in footprint size and available ecological capacityFig. 53-27Log (g carbon/year)13.49.85.8Not analyzedOur carrying capacity could potentially be limited by food, space, nonrenewable resources, or buildup of wastesFig. 53-UN1Patterns of dispersionClumpedUniformRandomFig. 53-UN2Number of generationsPopulation size (N)rmax NdNdt=Fig. 53-UN3Number of generationsK = carrying capacityPopulation size (N)rmax NdNdt=K – NKFig. 53-UN4Fig. 53-UN5You should now be able to:Define and distinguish between the following sets of terms: density and dispersion; clumped dispersion, uniform dispersion, and random dispersion; life table and reproductive table; Type I, Type II, and Type III survivorship curves; semelparity and iteroparity; r-selected populations and K-selected populationsExplain how ecologists may estimate the density of a speciesExplain how limited resources and trade-offs may affect life histories Compare the exponential and logistic models of population growthExplain how density-dependent and density-independent factors may affect population growthExplain how biotic and abiotic factors may work together to control a population’s growthDescribe the problems associated with estimating Earth’s carrying capacity for the human speciesDefine the demographic transition