Life-History Parameters

Life History and Strategies

Life History

  • Sequence, timing, and nature of events in an organism’s life, from birth to reproduction to death

    • An individual’s life history is determined by a set of life-history traits

    • A population’s life history is often represented as average age-specific rates (e.g. fecundity, survivorship)

Life History Strategies

  • A pattern of life-history traits (heritable trait that determines some aspect of life history for an organism/species). This could be age and size at maturity (first reproduction), number and size of offspring produced, or longevity

    • Evolved through natural selection over time in a population as a response to ecological/environmental conditions

    • Different life histories can succeed within the same environmental conditions

Successful and Unsuccessful Life History Strategies

  • A successful strategy in relation to estimated growth rate values (r)

    • Population estimated growth rate (per capita): Growing or stable r-value

    • Unsuccessful: contracting/shrinking population

A Case Study of Differing Life-History Strategies

  • Different life histories can succeed within the same environmental conditions

    • Dolphins: Very few high-investment offspring (low birth rate) and offspring frequently survives to live a fairly long life (low death rate)

    • Barnacles: Produce thousands of offspring at a time (high birth rate), but invest very little in any one of them (offspring is vulnerable to predation/starvation; high mortality)

    Despite the polar differences in life-history strategies between these two organisms/populations, with the same t, their estimated growth per capita is essentially the same (

Demographic Parameters Explained and Equations

Demographic Parameters

  • Qualitative values that characterize the life history of a species (all per capita/per individual)

    • Birth rate (b): # of individuals born per individual in pop. per unit of time

    • Death rate (d): same as birth, but switch birth with death

    • (Estimated) Growth rate (r): net change in pop. size per unit of time

      • Growing population size: r-value ≥ 0

      • Stable population: r-value ~ 0

    • Contracting/shrinking population: r-value ≤ 0

      This formula can be used to estimate the number of births, per capita, within a population. Divide the total births that occurred during the time period by the length of the time period multiplied by the average number of dolphins during that said time period. This equation can also be used to estimate deaths per capita within a population.

      The estimated per capita growth rate can determine whether a population/species' life history is successful or not. The growth rate depends on the relative values of birth and death rates over a period of time. N(avg) represents the average population number within that time period.

Fecundity

  • Represents the average number of offspring per reproductive female per unit of time

    • Estimated birth accounts for all females, fecundity only considers females within the reproductive age demographic

    • High fecundity: Large number of offspring in a short period of time

    • Calculation: Number of babies born per year divided by the number of females within the reproductive age demographic

Population Age Structures and Population Pyramids

Population Age Structure

  • Distribution of individuals across ages, from birth to the species’ maximum life span

    • Distribution of ages within a population

Age/Population Pyramids

  • Graphical way to illustrate a population’s age structure/distribution

    • Its shape can determine whether the population size is growing shrinking/contracting/stable

A shrinking/contracting population will have more of an inverted pyramid shape. Meaning, lots of older individuals and fewer children than reproductive adults. Characteristic of a population with a low mortality and low fecundity rate. A larger base and a smaller top (pyramid shape) is indicative of a large number of pre-reproductive individuals and that the population will likely increase in size. With a stable/shrinking population, most individuals are post-reproductive and there are fewer children than reproductive adults (pyramid with a narrower base)