Module 3.3 — Age Structure & Life Tables

Age Structure

Distribution of individuals among age classes

Expanding Population

Many young individuals relative to older individuals

Stable Population

Similar numbers across age classes

Declining Population

Fewer young individuals than older individuals

Age Structure Pyramid

Graph showing age distribution in a population

Survivorship Curve

Graph showing survival through life

Type I Survivorship Curve

Low juvenile mortality and high mortality late in life

Type I Examples

Humans, elephants, whales

Type II Survivorship Curve

Constant mortality throughout life

Type II Examples

Birds, squirrels

Type III Survivorship Curve

High mortality early in life and high survival later

Type III Examples

Fish, insects, plants

Life Table

Table summarizing age-specific survival and reproduction

Nx

Number alive at age x

sx

Survival from age x to age x+1

lx

Survivorship from birth to age x

bx

Fecundity at age x

Fecundity

Number of offspring produced

Net Reproductive Rate (R₀)

Average female offspring produced per female during her lifetime

Net Reproductive Rate Equation

R₀ = Σ(lxbx)

R₀ > 1

Population grows

R₀ = 1

Population remains stable

R₀ < 1

Population declines

Cohort Life Table

Tracks the same individuals throughout their lives

Static Life Table

Examines many age classes at a single point in time

Advantage of Static Life Table

Faster and works for long-lived species

Advantage of Cohort Life Table

Directly measures survival and reproduction through life