Biology 3A
Age Structure and Survivorship
Objective: To analyze the age structure and survivorship of the human population in southern
California and the east coast of the United States.
Introduction
The distribution of ages for any given population (animals, plants, fungi, etc.) can be described in
various time units from days, to weeks, months and even years. If the actual age is difficult to
determine, individuals can be assigned into qualitative age groups such as hatchling, nestling,
juvenile, subadult and adult. The collective proportion of individuals belonging to various age groups
in a population is referred to as the population’s age structure or age distribution.
To obtain the age structure of the population, one may: 1) track a particular cohort (individuals born
within the same time interval) group’s survivorship (birth till death); 2) track all members of all age
groups in a population at the same time (assumes constant birth and death rate and a stable age
structure); and 3) track the age of death in a population.
By knowing the age structure of a population, one can assess various population dynamics and can
construct life tables indicating age-specific mortality, survivorship and life expectancy. One can also
estimate population growth rates based upon the number of females and births in the population.
The age structure of a population is dependent on numerous factors such as longevity, mortality,
intrinsic rate of increase as well as environmental factors. In general, a decreasing population will
have a great proportion of members in older age classes with fewer members in younger age
classes.
In populations
such as Mexico, there are a
greater
proportion
of
individuals in the younger
age classes as opposed to
the older age classes. In
this case, this population is
increasing.
When one
compares the age classes in
countries such as Sweden
where there is not much
difference between age
classes,
therefore
the
population is rather stable.
If
the
proportion
of
individuals in an age class is
plotted in a horizontal
histogram, with age classes on the y-axis
Figure 1: Age structure for three different countries.
and the proportion of individuals on the yaxis, a “pyramid” forms (Figure 1). The age
pyramids have males on one side and females on the other side. These are useful in comparing
populations of different localities or the same locality from year to year.
Bio 3A Lab
Age Structure and Survivorship
Page 1 of 4
Life Table
There are numerous ways to generate life tables. By compiling various statistics for each cohort, we
can compare different populations, the numbers dying or surviving and can generate survivorship
curves. We will use the following information to generate a cohort life table in this exercise: cohort or
age interval (x), number in each cohort (no), number living at the start for each cohort (nx), number
dying during x, (dx), probability of dying (age specific death rate) during x, (qx), probability of surviving
(age specific survival rate) to interval x, (lx).
Procedure:
1. Develop a hypothesis before you begin to collect your data.
For example: Hypothesis: The age structure and survivorship of males and females in the Lake
Forest/Mission Viejo area population are similar.
Collecting data from cemetery headstones:
1.
2.
3.
4.
5.
6.
Work in pairs.
Record the birth and death year of individuals from grave markers.
Record the age at death of individuals from grave markers.
Determine the sex of each individual.
Continue until at least 50 males and 50 females have been recorded.
Combine all data from all groups.
Cohort Life Table Directions:
1. Log onto the computer using your user name (first initial, last name and maybe a number from
your saddleback email) and your student ID (password).
2. Download/open the demography worksheet from:
http://www.saddleback.cc.ca.us/faculty/steh/bio3afolder/bio3aindex.html
3. Tally and group all data from Tally sheet from all groups.
4. Open demographywkst.xls Æ excel file
5. Transfer class data to demographywkst.xls in the dx (# dying during age x) column
6. Calculate no (initial number of individuals in each cohort for males & females).
Sum of all of the dx values for each female and male group
7. Calculate nx (initial # of individuals of age x in a population)
nx for 0 – 4 cohort => no
nx + 1 = nx – dx Æ n2 = n1 – d1 (values are from the row above)
8. Fill down after you have calculated n2 for the remaining columns.
9. Calculate lx (proportion of individuals surviving from start of life table to age x) for each cohort.
lx = nx/no
(do not fill down to fill out this column)
10. Calculate qx (age-specific death rate)
(both values are from the same row)
a. qx = dx/nx
11. Fill down
Should range from 1 – 0.
May get a #DIV/0! message if any nx = 0.
12. Calculate 1000*lx (used to plot survivorship curves)
a. = 1000*lx (same row)
13. Fill down (values will range from 1000 Æ 0 or near zero)
Bio 3A Lab
Age Structure and Survivorship
Page 2 of 4
Survivorship curves
On type of graph that can be generated from life table data include survivorship curves,
expectancy curves and mortality curves. The ones most often seen in basic biology textbooks
Campbell and Reece include survivorship curves. These curves are generated by using
proportion of individuals surviving to age x (lx) plotted on semi-logarithmically against age.
comparative purposes, we will use lx data bases on 1000.
life
like
the
For
There are three generalized survivorship curves that are recognized: type I, type II and type III (Figure
2). In a type I survivorship curve, there is high survivorship by young individuals up to a certain older
age where survivorship decreases. Young individuals in a type III survivorship curves have low
survivorship or high mortality until a certain age and then mortality rates are relatively lowered. A
type II curve is an intermediate between type I and III. Individuals in a type II curve have a constant
mortality rate throughout their lifespan.
Figure 2: Three generalized survivorship
curves.
Analysis:
1. Construct life tables for males, females, and both sexes combined. Use 5-yr age intervals
(e.g., 0 - 4, 5 - 9, 10 – 14, etc.).
2.
3. Using Excel generate graphs of:
a) log female survivors vs. age – for each cemetery/locality
b) log male survivors vs. age – for each cemetery/locality
c) log children survivors vs. age (do for the first 20 years only)
Use 1000*qx values when doing the log transform.
Can put a & b on the same graph or
Generate 2 graphs, one for each cemetery/locality with males & females
d) generate an age pyramid (age interval vs. percent of population)
4. Construct a survivorship curve from each set of life table data. Remember to use log
survivorship as the vertical axis for your plots.
5. Construct a single age pyramid from the life table data, with females on one side and males on
the other.
Bio 3A Lab
Age Structure and Survivorship
Page 3 of 4
Tally
Sheet
Cemetery/Locality:
Born:
Age at Death
F
0-4
5-9
10-14
15-19
20-24
25-29
30-34
35-39
40-44
45-49
50-54
55-59
60-64
65-69
70-74
75-79
80-84
85-89
90-94
95-99
100+
Cemetery/Locality:
Born:
Age at Death
F
0-4
5-9
10-14
15-19
20-24
25-29
30-34
35-39
40-44
45-49
50-54
55-59
60-64
65-69
70-74
75-79
80-84
85-89
90-94
95-99
100+
Bio 3A Lab
Name:
M
Born:
F
M
M
Born:
F
M
Age Structure and Survivorship
Hypothesis:
Page 4 of 4