B.Sc. Zoology -Study of Life tables (SET-I)

 🔷Definition of a Life Table:

A life table is a convenient format for describing the mortality schedule of a population in a particular period of time. It is an age specific account of mortality operating in a population.

This concept was introduced by Raymond Pearl in 1921.

🔷Type:

A. Cohort or Dynamic life table: Here fate of a group of individuals, born at a given time, is followed from birth to death.

dynamic life table is further modified to composite life table. Here, a cohort is constructed from individuals born over several time periods instead of one. 

B. Time specific life table: It is constructed by sampling the population in some manner to obtain a distribution of age classes during a single time period.

🔷Use: Life table is useful for-

1. Showing age composition of a species population.
2. Project the future age structure, size & growth rate of a population.
3. Indicating critical stages in the life cycle at which mortality is high.
4. Showing success of same species in different biotypes.
5. Computing average longevity of a species population.
6. Many socio-economic & medical applications rely on human life table data.

🔷Construction of Life Table:

It begins with a cohort (A group of individuals born in the same period of time).

Notation	Explanation
X	Age Class/Class interval
nx	Number of survivors at the start of age class x
lx	Proportion of individuals alive at the start of age class x; or lx= nx/no
dx	Number of organisms dying between two classes ; or dx = nx – n(x+1)
qx	Mortality during successive interval ; or qx= dx/nx
Lx	Average number of individuals surviving in each interval; or  Lx=[nx+n(x+1)]/2
Tx	Cumulative value of Lx; Tx= L0+L1+L2+L3+…..
ex	Average life expectancy ; ex = Tx / nx

🟢SET-I

A cohort of 530,Grey Squirrels (Sciurus sp) from a population in Pennsylvania were observed for 5 years continuously . Construct a life table from the survivorship data given below.[X- in year]

X	0	1	2	3	4	5
nx	530	159	80	45	20	05

🔷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	530	530/530 =1.00	530-159 =371	371/530 =0.70	(530+159)/2 =344.5	344.5+119.5 +62.5+32.5 +12.5+2.5 =574	574/530 =1.09
1	159	159/530 =0.30	159-80 =79	79/159 =0.496 =0.50	(159+80)/2 =119.5	119.5 +62.5+32.5 +12.5+2.5 =229.5	229.5/159 =1.44
2	80	80/530 =0.150	80-45 =35	35/80 =0.44	(80+45)/2 =62.5	62.5+32.5 +12.5+2.5 =110	110/80 =1.375
3	45	45/530 =0.084	45-20 =25	25/45 =0.55	(45+20)/2 =32.5	32.5 +12.5+2.5 =47.5	47.5/45 =1.055
4	20	20/530 =0.037	20-5 =15	15/20 =0.75	(20+5)/2 =12.5	12.5+2.5 =15	15/20 =0.75
5	5	0.01	5	1.0	2.5	2.5	2.5/5 =0.50

 🔷Conclusion:

1. High rate of mortality in first two years followed by generally decreasing in number.
2. High mortality in initial stages is reflected in the low expectation for further life of individuals in the age class, drastically reduced number of squirrels live to their 3rd year.
3. Life expectancy also changes with age.
4. If life expectancy remains high for initial  age classes, it gradually declines with ages.

🔷Comment: Expectation of life declines with age. Highly variable life table parameters reflects high degree of environmental uncertainty & their survival strategies in their native environment.

This life table guide us to propose strategy for conserving any species population. For this purpose we need to choose those age group whose life expectancy is more.

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

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

🟢SET-IV

🟢SET-V

🟢SET-VI

🧿Study of Survivorship curve & Plotting of Survivorship curve from a hypothetical data set: https://biomassfootprints.blogspot.com/2025/07/bsc-zoology-plotting-study-of.html