Survivorship curve, Life table

 

Practical 1: Study of Life Tables and Plotting of Survivorship Curves

Q:  Construct a life table from the given data and calculate:

1.       Proportion surviving at each age

2.       Death rate at each age interval

3.       Plot a survivorship curve (logarithmic scale) using the proportion surviving values against age.

Age (x)	Number Alive (Nx)	Number Dying (dx) = Nx - Nx+1
0	1000	200
1	800	200
2	600	200
3	400	300
4	100	100
5	0	-

 

Objective:

To construct life tables and plot survivorship curves from the given population data.

Materials Required:

1. Data on population survival
2. Graph paper or spreadsheet software (e.g., MS Excel or Google Sheets)
3. Calculator

Procedure:

Step 1: Obtain age-specific survival data from the given table

Step 2: Construct the life table and calculate proportions and make the life table

Age (x)	Nx (No. alive)	dx (No. dying)	lx = Nx/N0 (Proportion surviving)
0	1000	200	1.0
1	800	200	0.8
2	600	200	0.6
3	400	300	0.4
4	100	100	0.1
5	0	-	0.0

 

Step 3: Plot the Survivorship Curve On graph paper or Excel:

X-axis: Age (x)

Y-axis: log(lx) (logarithm of proportion surviving)

Age (x)	lx	log(lx)
0	1.0	0
1	0.8	-0.09691
2	0.6	-0.22185
3	0.4	-0.39794
4	0.1	-1.0

Plot these values and connect the points to form the survivorship curve.

Observation & Result:

 Survivorship curve

Interpretation:

1.       Proportion surviving (lx) decreases steadily with age.

2.       Death rate (qx) increases with age, showing higher mortality in later stages.

3.       The curve reflects a Type I pattern, with moderate to late-age mortality.

In the given data it remains high for early age and declines later stages, indicating Type I survivorship curve, common in humans and large mammals.

Types of Survivorship Curves:

Type	Pattern	Example Organisms
Type I	Convex	Humans, elephants (low juvenile death)
Type II	Straight	Birds, rodents (constant death rate)
Type III	Concave	Fishes, insects (high early mortality)

 

Conclusion:

Survivorship curves provide insight into the life history strategy of a species. They help ecologists understand mortality patterns and the survival probability of organisms at various life stages. The shape of the curve reflects the organism’s strategy for survival, reproduction, and care for offspring.