Life Table and survivorship curves and their role.

Life Table and survivorship curves and their role.

1. DEMOGRAPHIC
TECHNIQUES

2. A technique to summarize how mortality occurs in a population
Is mortality high among juveniles?
Do older organisms have a higher mortality rate than younger
organisms?

3. Life tables
•One way to determine the survival and mortality of
individuals in a population is to examine a cohort of
individuals from birth to death.
•A cohort is a group of same-aged young which grow and
survive at similar rates.
•A population is often made up of organisms from a
variety of different cohorts, each of different ages, so we
say the population consists of different age classes.
•Cohort-may include the entire population, or only males
or only individuals born in a given year

4. •A life table provides data on the number of
individuals alive in different age classes and the age-
specifi c survival or mortality rates in these age
classes.
•We can also present information from life tables
graphically in the form of survivorship curves.
•A survivorship curve is a graphical representation of
the numbers of individuals alive in a population at
various ages.

5. Types of life tables
1. Cohort life tables
•generation or horizontal life tables
•following the cohort throughout life –eg., annual seeds or lambs born.
2. Static life tables
•stationary, time specific, current, vertical life tables
•records of age at death –individuals in sample are born at different
times on basis of cross section of a population at a specific time.
For some long-lived organisms such as tortoises, elephants, or trees,
following an entire cohort from birth to death is impractical, so a
snapshot approach is used. This is the approach that Adolf Murie
adopted in his studies of the Dall mountain sheep.

6. e.g of cohort life table for the song sparrow
Age in
years (x)
Observed
no of birds
alive (nx)
Proportion
surviving at
start of age
interval x
(lx)
No dying
within age
interval x to
x+1 (dx)
Rate of
mortality
(qx)
0 115 1.0 90 0.78
1 25 0.2176 0.24
2 19 0.1657 0.37
3 12 0.10410 0.83
4 2 0.0171 0.50
5 1 0.0091 1.0
6 0 0.0 - -

7. Types of survivorship curves
Type 1-low type of mortality for most of the life span
and then high losses of older organisms
Humans and large mammals
Type 2-constant per capita rate of mortality independent of age
eg birds, squirrels
Type 3-High per capita mortality early in life, followed by a period of much lower
and relatively constant loss
Fishes, invertebrates, parasites