Demography: life tables, age structure and what they predict · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Life tables: reading the future of a population

Science · Ecology & Conservation · ages 20-21
Name ______________________   Date ____________

  1. Which pair shows a Type I survivorship curve?

    • Humans and elephants
    • Fish and orchids
    • Small birds at steady risk

  2. What group does a cohort life table follow?

    • Many mixed generations at once
    • One cohort born together
    • Only the oldest adults

  3. With fixed age-specific survival and reproduction and no migration, a net reproductive rate above 1 predicts long-term population growth.

    Circle one:   True   False

  4. A bird population has the same probability of death in every equal age interval. Which survivorship type fits?

    • Type III: high early risk
    • Type I: high risk late
    • Type II: constant risk

  5. A cohort starts with 1000 newborns. At age two, 400 survive. What proportion of the starting cohort survives to age two?

    Answer: ______________

  6. Many marine invertebrates produce many small offspring with little care, while elephants usually raise one calf at a time for years. Which life-history tradeoff does this illustrate?

    • More offspring versus more care
    • Same offspring survival at all ages
    • Same number of births per parent

  7. Two populations each hold 500 animals. One is mostly young and one mostly old. With no data on survival, births or migration, which will grow faster?

    • Equal size guarantees equal growth
    • The young population must grow faster
    • Age alone cannot tell us which grows

  8. A female life table has (l_1, m_1) = (0.5, 1) and (l_2, m_2) = (0.1, 2), with no births at other ages. A student adds 1 + 2 to get R0 = 3. What is the correct R0?

    • 0.7 daughters per newborn female
    • 3.0 daughters per newborn female
    • 0.6 daughters per newborn female
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Life tables: reading the future of a population W1-mt_raWfdLLACU-s1

  1. Humans and elephants · Humans and elephants often illustrate Type I: survival is high early in life, with mortality rising at older ages.
  2. One cohort born together · A cohort life table tracks individuals born together across the ages.
  3. True · R0 above 1 means more than one expected lifetime daughter per newborn female. Under the stated assumptions, this predicts long-term growth, not necessarily immediate growth.
  4. Type II: constant risk · A constant death probability in equal age intervals defines Type II. Survivors decrease by a constant proportion, not a constant number.
  5. 0.4 · Divide survivors by the starting number: 400/1000 = 0.4. This is the proportion surviving from birth to age two.
  6. More offspring versus more care · Limited energy can be allocated to producing many offspring or to providing more care per offspring. The examples illustrate different allocations.
  7. Age alone cannot tell us which grows · Both populations have 500 animals, but young animals may not survive to breed. Births, deaths and migration are needed to compare their future growth.
  8. 0.7 daughters per newborn female · Weight births by survival at each age: R0 = 0.5 times 1 + 0.1 times 2 = 0.5 + 0.2 = 0.7 daughters per newborn female. Adding births alone ignores survival.
Worksheet · LightMySky