Population Ecology - ibiocideb

PopulationEcology

Outline how population size is affected by natality, immigration,mortality and emigration.Draw and label a graph showing a sigmoid (S-shaped) populationgrowth curve.Explain the reasons for the exponential growth phase, the plateauphase and the transitional phase between these two phases.List three factors that set limits to population increase.Distinguish between r-strategies and K-strategies.Discuss the environmental conditions that favour either r-strategies or K-strategies.Describe one technique used to estimate the population size of an animalspecies based on a capture–mark–release–recapture method.Describe the methods used to estimate the size of commercial fish stocks.Outline the concept of maximum sustainable yield in the conservation of fishstocks.Discuss international measures that would promote the conservation of fish.

Features of PopulationsPopulations are dynamic and exhibitattributes that are not shown by theindividuals themselves.These attributes can be measured orcalculated and include:• Population size: the total number oforganisms in the population.• Population density: the number oforganisms per unit area.• Population distribution: the locationof individuals within a specific area.

Features of PopulationsPopulation composition providesinformation relevant to the dynamicsof the population, i.e. whether thepopulation is increasing or declining.Information on population composition (orstructure) includes:• Sex ratios: the number oforganisms of each sex.• Fecundity (fertility): the reproductivecapacity of the females.• Age structure: the number oforganisms of different ages.

Population DynamicsThe study of changes in the size andcomposition of populations, and thefactors influencing these changes, ispopulation dynamics.Key factors for study include:• Population growth rate: the change inthe total population size per unit time.Population size isinfluenced by births…• Natality (birth rate): the numberof individuals born per unit time.• Mortality (death rate): the number ofindividuals dying per unit time.• Migration: the number moving into orout of the population.…anddeaths

MigrationMigration is the movement oforganisms into (immigration)and out of (emigration) apopulation. It affects populationattributes such as age and sexstructure, as well as thedynamics of a population.Wildebeest• Populations lose individualsthrough deaths and emigration.• Populations gain individualsthrough births and immigration.Canada geeseMigrating species may group together toform large mobile populations

Population DensityThe number of individuals perunit area (for terrestrialorganisms) or volume (foraquatic organisms) is termedthe population density.• At low population densities,individuals are spaced well apart.Examples: territorial, solitarymammalian species such as tigersand plant species in marginalenvironments.• At high population densities,individuals are crowded together.Examples: colonial animals, suchas rabbits, corals, and termites.Low density populationsHigh density populations

Population DistributionThe population distributiondescribes the location of individualswithin an area.• Distribution patterns are determinedby the habitat patchiness(distribution of resources) andfeatures of the organismsthemselves, such as territoriality inanimals or autotoxicity in plants.• Individuals in a population may bedistributed randomly, uniformly, orin clumps.Clumped distribution in termitesMore uniform distribution in cacti

Random DistributionA population’s distribution is consideredrandom if the position of each individualis independent of the others.Random distributions are not common;they can occur only where:• The environment is uniform andresources are equally availablethroughout the year.• There are no interactions betweenindividuals or interactions produce nopatterns of avoidance or attraction.Random distributions are seen in someinvertebrate populations, e.g. spidersand clams, and some trees.Spider populations appear to showa random distribution

Uniform DistributionUniform or regular distributionpatterns occur where individuals aremore evenly spaced than would occurby chance.Regular patterns of distribution resultfrom intraspecific competition amongstmembers of a population:• Territoriality in a relativelyhomogeneous environment.• Competition for root and crown spacein forest trees or moisture in desertand savanna plants.• Autotoxicity: chemical inhibition ofplant seedlings of the same species.Saguaro cacti compete for moistureand show a uniform distribution

Clumped DistributionClumped distributions are the mostcommon in nature; individuals areclustered together in groups.Population clusters may occur arounda a resource such as food or shelter.Clumped distributions result from theresponses of plants and animals to:• Habitat differences• Daily and seasonal changes inweather and environment• Reproductive patterns• Social behaviorSociality leads to clumped distribution

Population RegulationPopulation size is regulated by environmental factorsthat limit population growth. These may be dependentor independent of the population density.Density independentfactorsDIRECTLY OR INDIRECTLYAFFECTS FOOD SUPPLYDensity dependentfactorsPhysical factorse.g. rainfallCatastrophic eventse.g. floodFood supplyDiseaseCompetitionPredationRegardless of populationdensity, these are thesame for all individualsThe effects of thesefactors are influenced bypopulation densityPoor health or deathINCREASE IN MORTALITYChange in ability to reproduceNATALITY IS AFFECTED

Environmental FactorsEnvironmental factors may becategorized according to howmuch population densityinfluences their effect onpopulation growth:• Density independent factorshave a controlling effect onpopulation size and growth,regardless of the populationdensity.Severe fires can result in high mortality• Density dependent factorshave an increasing effect onpopulation growth as the densityof the population increases.Humans often live at high density

Density Dependent FactorsDensity dependent factors exert agreater effect on population growthat higher population densities.• Compete more for resources.Competition increasesin crowded populations• Are more easily located by predatorsand parasites.• Are more vulnerable to infection anddisease.Density dependent factors are bioticfactors such as food supply, disease,parasite infestation, competition, andpredation.Parasites can spread rapidlythrough dense populations

Density Independent FactorsPhysical (or abiotic) factors• temperature• precipitation• humidity• acidity• salinity etc.Catastrophic events• floods and tsunamis• fire• droughtThe effect of density independent factors on apopulation’s growth is not dependent on thatpopulation’s density:• earthquake and eruption

Population GrowthPopulation growth depends on thenumber of individuals added to thepopulation from births and immigration,minus the number lost through deathsand emigration.This can be expressed as a formula:Population growth =Births – Deaths + Immigration – Emigration(B) (D) (I) (E)Net migration is the difference betweenimmigration and emigration.

Calculating Population ChangeBirths, deaths, and net migrations determinethe numbers of individuals in a population

Population numbers (N)Exponential GrowthColonizing PopulationHere the number beingadded to the populationper unit time is large.They may undergo a rapidexponential (logarithmic) increasein numbers to produce a J-shapedgrowth curve.Exponential (J) curveExponential growth issustained only whenthere are no constraintsfrom the environment.In natural populations, populationgrowth rarely continues to increaseat an exponential rate.Here, the number beingadded to the populationper unit time is small.Factors in the environment, such asavailable food or space, act to slowpopulation growth.LagphaseTime

Population numbers (N)Logistic GrowthAs a population grows, its increase will slow, and it willstabilize at a level that can supported by the environment.This type of sigmoidal growth produces the logisticgrowth curve.The population encountersresistance to exponential growthas it begins to fill up theenvironment. This is calledenvironmental resistance.Established PopulationCarrying capacity (K)The population density that can besupported by the environment.Environmental resistanceincreases as the populationovershoots K.Logistic (S) curveAs the population grows,the rate of populationincrease slows, reachingan equilibrium levelaround the carryingcapacity.LagphaseIn the early phase,growth is exponential(or nearly so)Environmental resistancedecreases as the populationfalls below K.The population tends to fluctuate around an 'equilibriumlevel'. The fluctuations are caused by variations in thebirth rate and death rate as a result of the populationdensity exceeding of falling below carrying capacity.Time

Logistic GrowthExponential phasePopulation increasesas natality is higherthan mortality.Abundant resources

Logistic GrowthTransitional phaseoccurs as natalityrate starts to fall ,mortality rate startsto rise. Natality isstill higher thanmortality – less rapidrise

Logistic GrowthPlateau phase orstationary phasenatalitymortality arethe same.Population is limiteddue to shortages ofresources,predation, diseaseor parasites.

Number ofsurvivors (logscale)Survivorship CurvesThe age structure of a population can represented with asurvivorship curve. Survivorship curves use a semi-log plot of the number ofindividuals surviving per 1000 in the population, against age.Because they are standardized (as number of survivors per 1000),species with different life expectancies can be easily compared.Type I: latelosslargemammalsType II: constant losssmall mammals,songbirdsType III: earlylossoysters,barnaclesRelative

Population numbers (N)‘r’ and ‘K’ SelectionTwo parameters govern the logistic growth of populations.The intrinsic rate of natural increase or biotic potential. This is themaximum reproductive potential of an organism, symbolized bythe letter r.The saturation density orcarrying capacity of theenvironment, representedby the letter, K.We can characterizespecies by the relativeimportance of r and Kin their life cycles.K-selected speciesThese species exist nearasymptotic density (K) formost of the time. Competitionand effective use ofresources are important.r-selected speciesThese species rarely reachcarrying capacity (K). Theirpopulations are in nearlyexponential growth phases formuch of the year. Early growth,rapid development, and fastpopulation growth are important.Time

-Selected SpeciesCorrelates of r-selected speciesSpecies with a high intrinsic capacityfor population increase are called r-selected or opportunistic species.• These species show certain lifehistory features and, to survive,must continually invade new areasto compensate for being displacedby more competitive species.• Opportunists include algae,bacteria, rodents, many insects,and most annual plants.ClimateMortalitySurvivorshipPopulationsizeCompetitionSelectionfavorsLength of lifeLeads to:Variable and/orunpredictableDensity-independentOften type III(early loss)Fluctuates wildly. Oftenbelow K.Variable, often lax.Generalist niche.Rapid development, highrm, early reproduction,small body size, singlereproduction (annual)Short, usually less thanone yearProductivity

K-Selected SpeciesCorrelates of K-selected speciesSpecies that are K-selectedexist under strong competitionand are pushed to use availableresources more efficiently.• These species have feweroffspring and longer lives.They put their energy intonurturing their young toreproductive age.• K-selected species includemost large mammals, birds ofprey, and large, long-livedplants.ClimateMortalitySurvivorshipPopulation sizeCompetitionSelection favorsLength of lifeLeads to:Fairly constant and/orpredictableDensity-dependentUsually types I and II(late or constant loss)Fairly constant in time.Near equilibrium with theenvironment.Usually keen.Specialist niche.Slower development,larger body size, greatercompetitive ability,delayed reproduction,repeated reproductionsLonger (> one year)Efficiency

Mark and RecaptureMark and recapture is used to determine the total populationdensity for highly mobile species in a certain area.First captureReleaseSecond captureIn the first capture, a random sampleof animals from the population isselected. Each selected animal ismarked in a distinctive way.The marked animals from the firstcapture are released back into thenatural population and left to mixwith the unmarked individuals.The population is sampled again; onlya proportion of the second capturesample will have animals that weremarked in the previous capture.

The Lincoln lndexThis equation is used to estimate the size of theoverall population.The Lincoln IndexNo. of animals in 1st sample X Total no. of animals in 2nd sampleTotal population =Number of marked animals in the second sample (recaptured)1. The population is sampled by capturing as many ofthe individuals as possible and practical.2. Each animal in the sample is marked to distinguish itfrom unmarked animals.3. Animals are returned to their habitat and left to mixwith the rest of the population.4. The population is sampled again (this need not bethe same sample size as the first, but it must belarge enough to be valid).5. The numbers of marked to unmarked animals in thissecond sample is determined. The Lincoln Index isused to estimate overall population size.

The Lincoln lndexThe accuracy of using Lincoln index based on data from amark-recapture investigation depends on several factors:•The population is closed, the population needs to have very low natality,mortality, immigration and emigration.•The method of capturing needs to have little effect on the animal.•All animals have the same chance of being caught in a sample (i.e., must be arandom sample)•Marking animals does not effect their catchability, do not lose marks betweenthe two sampling periods

Sampleinitial capture of 50 individualssecond capture of 100 individuals, 10 of the 100 were markedfrom the first captureestimated population size = ?