1Population Ecologyand EcosystemsConcepts and Applications: Chapters 40 & 43Basic Concepts: Chapters 27 & 30http://darwin.bio.uci.edu/~sustain/bio65/lec16/b65lec16.htmEcology• Certain ecological principles govern the growth and sustainability of all populations • Human populations are no exceptionEcology: The study of how organisms interact with one another and with their environmentPopulationEcologyHuman Population Problems• Over 6 billion people alive• About 2 billion live in poverty• Most resources are consumed by the relatively few people in developed countries PopulationEcologyPopulation• A group of individuals of the same species occupying a given area• Can be described by demographics– Vital statistics such as size, density, distribution, and age structure PopulationEcologyPopulation Age Structure• Divide population into age categories• Population’s reproductive base includes members of the reproductive and pre-reproductive age categoriesPopulationEcologyDensity & Distribution• Number of individuals in some specified area of habitat• Crude density information is more useful if combined with distribution dataPopulationEcologyFig 40.22Determining Population Size• Direct counts are most accurate but seldom feasible• Can sample an area, then extrapolate• Capture-recapture method is used for mobile speciesMark and Recapture# marked Captured = Number releasedTotal Captured Total PopPopulationEcologyAssumptions in Capture-Recapture• Marking has no effect on mortality• Marking has no effect on likelihood to being captured• There is no immigration or emigration between sampling timesPopulationEcologyChanges in Population Size• Immigration adds individuals• Emigration subtracts individuals• Births add individuals• Deaths subtract individualsPopulationEcologyZero Population Growth- ZPG• Interval in which number of births is balanced by number of deaths• Assume no change as a result of migration• Population size remains stablePopulationEcologyPer Capita Rates• Rates per individual• Total number of events in a time interval divided by the number of individuals• Per capita birth rate per month =Number of births per month Population sizePopulationEcologyExponential Growth• Population size expands by ever increasing increments during successive intervals• The larger the population gets, the more individuals there are to reproducePopulationEcologyFig 40.43r• Net reproduction per individual per unit time• Variable combines per capita birth and death rates (assuming both constant)• Can be used to calculate rate of growth of a populationPopulationEcologyExponential Growth EquationG = rN• G is population growth per unit time• r is net reproduction per individual per unit time• N is population sizePopulationEcologyBiotic Potential• Maximum rate of increase per individual under ideal conditions• Varies between species• In nature, biotic potential is rarely reachedPopulationEcologyLimiting Factors• Any essential resource that is in short supply• All limiting factors acting on a population dictate sustainable population sizeSpaceFoodShelterTemperatureMatesPollutionDiseasePopulationEcologyCarrying Capacity (K)• Maximum number of individuals that can be sustained in a particular habitat• Logistic growth occurs when population size is limited by carrying capacityPopulationEcologyLogistic Growth EquationG = rmaxN (K-N/K)• G = population growth per unit time• rmax= maximum population growth rate per unit time• N = number of individuals• K = carrying capacityPopulationEcology4Logistic Growth• As size of the population increases, rate of reproduction decreases• When the population reaches carrying capacity, population growth ceasesPopulationEcologyOvershooting Capacity• Population may temporarilyincrease above carrying capacity• Overshoot is usually followed by a crash; dramatic increase in deathsPopulationEcologyFig 40.7Resetting the Carrying Capacity• Major changes in environment can change the carrying capacity of a local systemPopulationEcologyFig 40.6Density-Dependent Controls• Logistic growth equation deals with density-dependent controls• Limiting factors become more intense as population size increases• Disease, competition, parasites, toxic effects of waste productsPopulationEcologyDensity-Independent Controls• Factors unaffected by population density• Natural disasters or climate changes affect large and small populations alikePopulationEcologyLife History Patterns• Patterns of timing of reproduction and survivorship• Vary among species• Summarized in survivorship curves and life tablesPopulationEcology5Life Table• Tracks age-specific patterns • Population is divided into age categories• Birth rates and mortality risks are calculated for each age categoryPopulationEcologySurvivorship CurvesPopulationEcologyFig 40.8Human Population Growth • Population now exceeds 6 billion• Rates of increase vary among countries• Average annual increase is 1.26 percent• Population continues to increase exponentiallyPopulationEcologyFig. 40.9, p. 695Human Population GrowthPopulationEcologyHow Humans have Side-Stepped density dependent controls• Expanded into new habitats • Agriculture increased carrying capacity; use of fossil fuels aided increase• Hygiene and medicine lessened effects of density-dependent controlsPopulationEcologyFuture Growth• Exponential growth cannot continue forever• Breakthroughs in technology may further increase carrying capacity • Eventually, density-dependent factors will slow growthPopulation Momentum• Lowering fertility rates cannot immediately slow population growth rate. Why? • If every couple had just two children, population would still keep growing for another 60 yearsPopulationEcology6Resource Consumption• United States has 4.7 percent of the world’s population• Americans have a disproportionately large effect on the world’s resources• Per capita, Americans consume more resources and create more pollution than citizens of less developed nationsPopulationEcologyEffects of Economic Development • Total fertility rates (TFRs) are highest in developing countries, lowest in developed countries• When individuals are economically secure, they are under less pressure to have large familiesPopulationEcologySlowing Growth in China• World’s most extensive family planning program •
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