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Population Ecology

Population Ecology. Chapter 27. Population Ecology. Certain ecological principles govern the growth and sustainability of all populations Human populations are no exception. Limits to Growth. A population’s growth depends on the resources of its environment

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Population Ecology

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  1. Population Ecology Chapter 27

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

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

  4. 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

  5. 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

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

  7. 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

  8. 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

  9. 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

  10. 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

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

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

  13. 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

  14. 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

  15. 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

  16. 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

  17. Exponential Growth • Population size expands by ever increasing increments during successive intervals • The larger the population gets, the more individuals there are to reproduce

  18. 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

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

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

  21. 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

  22. Logistic Growth Equation G = rmaxN (K-N/K) • G = population growth per unit time • rmax = maximum population growth rate per unit time • N = number of individuals • K = carrying capacity

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

  24. Click to view animation. animation

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

  26. 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

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

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

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