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Population ecology

Population ecology. Cohort life table = life table based on individuals of same age (i.e. a cohort) followed from birth to death. Life tables used to study demography. Survivorship (l x ): proportion of individuals that survive from birth to age x. l x = N x /N 0.

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Population ecology

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  1. Population ecology • Cohortlifetable = life table based on individuals of same age (i.e. a cohort) followed from birth to death • Life tables used to study demography • Survivorship (lx): proportion of individuals that survive from birth to age x • lx= Nx/N0 Age (x) Number Alive (Nx) Survivorship (lx) 0 843 =843/843 = 1.000 1 722 =722/843 = 0.856 2 527 =527/843 = 0.625 3 316 =316/843 = 0.375 Where have all the sea otters gone?

  2. Population ecology • Mortality (Mx): Proportion of individuals that die between ages x and x+1 • Life tables • Mx= (Nx-Nx+1)/Nx Age (x) Number Alive (Nx) Mortality (Mx) 0 843 =(843-722)/843= 0.143 1 722 =(722-527)/722 = 0.270 2 527 =(527-316)/527 = 0.400 3 316 Where have all the sea otters gone?

  3. Population ecology • Survivorship curves show change in survivorship with age • Survivorship vs. age • y-axis on log scale (makes constant survivorship into a straight line) Where have all the sea otters gone?

  4. Population ecology • Three general patterns • Type I: high survivorship early, low survivorship late in life • Survivorship curves • Type II: constant survivorship throughout life • Type III: low survivorship early, high survivorship late Where have all the sea otters gone?

  5. Population ecology • Life tables summarize life-history traits • Life-historytrait = traits associated with organism’s life cycle • age at first reproduction (maturity) • number of offspring (fecundity) • number of reproductive bouts (parity) • Semelparity = reproduce once in lifetime • Iteroparity = reproduce multiple times during lifetime • lifespan (aging) Where have all the sea otters gone?

  6. Population ecology • Life histories involve trade-offs • Resources limited; if resources used for one life-history trait then fewer resources available for another life-history trait • Life-history traits • Increased fecundity in birds correlates with higher mortality Where have all the sea otters gone?

  7. Population ecology • Many other life history trade-offs observed • Early vs. late fecundity • Life-history traits • Number and size of offspring • Nestling weight is smaller when clutch sizes are larger in Great tits • Uta lizards with surgically reduced clutch sizes produce larger eggs Where have all the sea otters gone?

  8. Population ecology • Understanding population growth an important part of demography • What causes population growth? • Increases: births and immigration • Decreases:deaths and emigration • Births and deaths usually measured as rates • Birth rate = # births per year • Birth rate = # births per 1,000 individuals per year • Birth rate = # births per individual per year Where have all the sea otters gone?

  9. Population size Per capita growth rate Population growth rate Population ecology • Growth rates measured in two ways: • Population growth rate = change in population size per unit time • Population growth • Per capita growth rate (r) = birth rate -death rate per individual (also called intrinsic rate of natural increase) • Exponential growth model • Population grows without limits (assume r constant) (total number of individuals in population) (contribution of each individual to population growth) (change in population size over time) Where have all the sea otters gone?

  10. Population growth rate (dN/dt) is slope Population ecology • Exponential growth • Population growth rate increases as population size increases • Population growth Population size (N) Time (t) Where have all the sea otters gone?

  11. Population ecology • Example of exponential population growth • Population growth Where have all the sea otters gone?

  12. Population ecology • Does population growth continue without limits? • Number of resources usually prevent populations from growing exponentially • Population growth • Carryingcapacity (K) = maximum number of individual that an environment can support • Population growth rate = 0 when population reaches carrying capacity • At carrying capacity, population birth rate = population death rate Where have all the sea otters gone?

  13. Adjustment for limited resources Population size Per capita growth rate Population growth rate Population ecology • Logistic growth model • Population growth rates decreases as population approaches its carrying capacity • Population growth Where have all the sea otters gone?

  14. Population ecology • Logistic growth model • Logistic growth produces S-shaped curve; population growth rate decreases as N approaches K • Population growth Population size (N) K Time (t) Where have all the sea otters gone?

  15. Population ecology • Examples of logistic growth • Population growth Where have all the sea otters gone?

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