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

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

💹SET-II :https://biomassfootprints.blogspot.com/2025/07/study-of-life-table-and-survivorship_18.html

💹SET-III

Life table of a Darwin finch, Galapagos cactus finch (Geospiza sp) is given below-

X	0	1	2	3	4	5	6
nx	1000	980	920	800	750	620	550

🔷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	1000	1	1000-980 =20	20/1000 =0.02	[1000+980]/2 =990	990+950+860+ 775+685+585+275 =5,120	5.12
1	980	0.98	980-920 =60	0.061	950	950+860+ 775+685+585+275 =4130	4.21
2	920	0.92	120	0.130	860	860+ 775+685+585+275 =3180	3.45
3	800	0.8	50	0.06	775	775+685+585+275 =2320	2.9
4	750	0.75	130	0.17	685	685+585+275 =1545	2.06
5	620	0.62	70	0.112	585	585+275=860	1.38
6	550	0.55	550	1.0	275	275	0.5

 

🔷Conclusion:

1. More or less constant death throughout life. Individuals have approximately constant chance of surviving throughout the life span. 
2. Life expectancy also changes with age.

🔷Comment: Expectation of life declines with age. This life table parameters reflects low-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	1000	980	920	800	750	620	550
log nx	3	3	2.96	2.90	2.87	2.79	2.74

🔷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 diagonal survivorship curve of a Geospiza sp population concluded that individuals have an approximately constant chance of survival throughout the life span, because the rate of death is more or less constant.

🔷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 'convex' type (Type-I). 

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