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Population Ecology Chapter 45

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Population Ecology Certain ecological principles govern the growth and sustainability of all populations--including human populations

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Limits to Growth A population’s growth depends on the resources of its environment Moose/Wolf study on Isle Royal

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Human 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

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Population 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

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Population Age Structure Divide population into age categories Population’s reproductive base includes members of the reproductive and pre-reproductive age categories

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Density & Distribution Number of individuals in some specified area of habitat Crude density information is more useful if combined with distribution data clumped nearly uniform random Figure 45.2 Page 808

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Determining Population Size Direct counts are most accurate but seldom feasible Can sample an area, then extrapolate Capture-recapture method is used for mobile species

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Capture-Recapture Method Capture, mark, and release individuals Return later and capture second sample Count the number of marked individuals and use this to estimate total population

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Assumptions in Capture-Recapture Assumptions 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 times

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Changes in Population Size Immigration adds individuals Emigration subtracts individuals Births add individuals Deaths subtract individuals

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Zero Population Growth Interval in which number of births is balanced by number of deaths Assume no change as a result of migration Population size remains stable

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Per Capita Rates Per 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 size

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r 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 population

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Exponential Growth Equation G = rN G is population growth per unit time r is net reproduction per individual per unit time N is population size

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Exponential Growth Population size expands by ever increasing increments during successive intervals The larger the population gets, the more individuals there are to reproduce Figure 45.4 Page 810

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Effect of Deaths Population grows exponentially as long as per capita death rates are lower than per capita birth rates 25% mortality between divisions Figure 45.5 Page 811

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Biotic Potential Maximum rate of increase per individual under ideal conditions Varies between species In nature, biotic potential is rarely reached

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Limiting Factors Any essential resource that is in short supply All limiting factors acting on a population dictate sustainable population size

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Carrying Capacity (K) Maximum number of individuals that can be sustained in a particular habitat Logistic growth occurs when population size is limited by carrying capacity

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Logistic Growth Equation G = r max N (K-N/K) G = r max N (K-N/K) G = population growth per unit time r max = maximum population growth rate per unit time N = number of individuals K = carrying capacity

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Logistic Growth Logistic Growth As size of the population increases, rate of reproduction decreases When the population reaches carrying capacity, population growth ceases

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Figure 45.6 Page 812 initial carrying capacity new carrying capacity Logistic Growth Graph

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Overshooting Capacity Population may temporarily increase above carrying capacity Overshoot is usually followed by a crash; dramatic increase in deaths Figure 45.6 Page 812 Reindeer on St. Matthew’s Island

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Density-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 products

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Density-Independent Controls Factors unaffected by population density Natural disasters or climate changes affect large and small populations alike

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Life History Patterns Patterns of timing of reproduction and survivorship Vary among species Summarized in survivorship curves and life tables

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Life Table Tracks age-specific patterns Population is divided into age categories Birth rates and mortality risks are calculated for each age category

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Survivorship Curves Survivorship Curves Graph of age-specific survivorship Figure 45.8 Page 815

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Predation and Life History Guppy populations vary in life history characteristics and morphology Differences have genetic basis Variation seems to be result of directional selection by predators

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Human Population Growth Population now exceeds 6 billion Rates of increase vary among countries Average annual increase is 1.26 percent Population continues to increase exponentially

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Side-Stepping Controls Expanded into new habitats Agriculture increased carrying capacity; use of fossil fuels aided increase Hygiene and medicine lessened effects of density-dependent controls

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Future Growth Exponential growth cannot continue forever Breakthroughs in technology may further increase carrying capacity Eventually, density-dependent factors will slow growth

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Fertility Rates Worldwide, average annual rate of increase is 1.26% Total fertility rate (TFR) is average number of children born to a woman Highest in developing countries, lowest in developed countries

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Age Structure Diagrams Show age distribution of a population Rapid Growth Slow Growth Zero Growth Negative Growth Figure 45.14 Page 821

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Population Momentum Lowering fertility rates cannot immediately slow population growth rate Why? There are already many future parents alive If every couple had just two children, population would still keep growing for another 60 years

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Slowing Growth in China World’s most extensive family planning program Government rewards small family size, penalizes larger families, provides free birth control, abortion, sterilization Since 1972, TFR down to 1.8 from 5.7

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Effects 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 families

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Population Sizes in 2001 Asia3.7 billion Europe727 million Africa816 million Latin America525 million North America316 million Oceania31 million

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Resource 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 nations

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