Study of life table and survivorship curve from a hypothetical data set SET-II

SET-I : https://biomassfootprints.blogspot.com/2025/07/study-of-life-table-and-survivorship.html

SET-III: https://biomassfootprints.blogspot.com/2025/07/study-of-life-table-and-survivorship_36.html

🟢SET-II

Life table of a Snail species population in Brazil is given below-

X	0	1	2	3	4	5	6
nx	500	400	250	150	75	25	0

🔷Calculation:

X	nx	lx	dx	qx	Lx	Tx	ex
Age in Year	Observed number alive	nx / n0	nx - n(x+1)	dx / nx	[nx+ n(x+1)]/2	∑ Lx	Tx / nx
0	500	1	500-400=100	100/500 =0.2	[500+400]/2 =450	450+325+200+ 112.5+50+12.5+0 =1,150	2.3
1	400	0.8	150	0.375	325	325+200+ 112.5+50+12.5+0 =700	1.75
2	250	0.5	100	0.4	200	200+ 112.5+50+12.5+0 =375	1.5
3	150	0.3	75	0.5	112.5	112.5+50+12.5+0 =175	1.16
4	75	0.15	50	0.66	50	50+12.5+0 =62.5	0.83
5	25	0.05	25	01	12.5	12.5+0=12.5	0.5
6	0	0	0	0	0	0	0

 

🔷Conclusion:

1. Moderate mortality in first two years followed by decreasing in number.
2. Moderate mortality in initial stages is reflected in the medium expectation for further life of individuals in the age class, drastically reduced number of snail live to their 4th year.
3. Life expectancy also changes with age.
4. If life expectancy gradually declines with ages.

🔷Comment: Expectation of life declines with age. A moderate life table parameters reflects medium-level of environmental uncertainty & their survival strategies in their native environment.

🔷Plotting & study of Survivorship curve from a hypothetical data set:

X	0	1	2	3	4	5	6
nx	500	400	250	150	75	25	0
log nx	2.7	2.6	2.4	2.2	1.9	1.4	0

🔷Survivorship curve on log scale: 

On a graph paper nx (log 10 scale) is plotted against time interval (X) & following graph is obtained.

🔷Conclusion: This convex survivorship curve of a snail population concluded that low mortality in initial stage and showing approximately constant chance of survival up to 4th year then gradually mortality rate is increasing & touches zero.

🔷Survivorship curve on arithmetic scale: On a graph paper nx is plotted against time interval (X) & following graph is obtained. 

🔷Observation: The survivorship curve shows 'concave' type (Type-III). 

🔷Conclusion: Here, Population mortality is moderate during initial stage, then  increasing throughout the life span.