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Population Ecology Chapter 27

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Population Ecology Certain ecological principles govern the growth and sustainability of all populations Human populations are no exception

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Limits to Growth A population’s growth depends on the resources of its environment Deer introduced to Angel Island –Population outstripped resources

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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

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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 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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population growth per unit time net population growth rate per individual per unit time number of individuals =X in-text, p. 688

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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 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

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Effect of Deaths Population will grow exponentially as long as per capita death rates are lower than per capita birth rates 25% mortality between divisions

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

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animation Click to view animation.

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Overshooting Capacity Population may temporarily increase above carrying capacity Overshoot is usually followed by a crash; dramatic increase in deaths

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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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Fig. 40.6, p. 690 initial carrying capacity new carrying capacity

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