Life tables used to study demography

1
Population ecology

• Life tables used to study demography

• Cohort life table life table based on
  individuals of same age (i.e. a cohort) followed
  from birth to death

• 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

• Life tables

• Mortality (Mx) Proportion of individuals that
  die between ages x and x1

• Mx (Nx-Nx1)/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

• Survivorship curves

• Three general patterns

• Type I high survivorship early, low survivorship
  late in life

• 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-history trait traits associated with
  organisms 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-history traits

• Life histories involve trade-offs

• Resources limited if resources used for one
  life-history trait then fewer resources available
  for another life-history trait

• Increased fecundity in birds correlates with
  higher mortality

Where have all the sea otters gone?
7
Population ecology

• Life-history traits

• Many other life history trade-offs observed

• Early vs. late fecundity

• 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

• Decreasesdeaths 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 ecology

• Population growth

• Growth rates measured in two ways

• Population growth rate change in population
  size per unit time

• 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 ecology

• Population growth

• Exponential growth

• Population growth rate increases as population
  size increases

Population size (N)
Time (t)
Where have all the sea otters gone?
11
Population ecology

• Population growth

• Example of exponential population growth

Where have all the sea otters gone?
12
Population ecology

• Population growth

• Does population growth continue without limits?

• Number of resources usually prevent populations
  from growing exponentially

• Carrying capacity (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
Population ecology

• Population growth

• Logistic growth model

• Population growth rates decreases as population
  approaches its carrying capacity

Where have all the sea otters gone?
14
Population ecology

• Population growth

• Logistic growth model

• Logistic growth produces S-shaped curve
  population growth rate decreases as N approaches K

Population size (N)
K
Time (t)
Where have all the sea otters gone?
15
Population ecology

• Population growth

• Examples of logistic growth

Where have all the sea otters gone?