LIFE TABLE and SURVIVORSHIP CURVES

1. LIFE TABLES & SURVIVORSHIP CURVES
WELCOME
Student : Prajwal Gowda M.A
Roll No : 12292 Course-incharge :
Ph.D. : 1st year Dr. S Rajna
Course: Insect Ecology & Diversity Department of Entomology
DIVISION OF ENTOMOLOGY
ICAR-INDIAN AGRICULTURAL RESEARCH INSTITUTE
NEW DELHI - 110 012

2.  Lotka (1925) is considered as father of Life tables.
 Life table is a detailed census of a given population. It is sometimes described as biology of a
particular population.
 Life table is summary of a group of population whose number starts life together
 Life table describes the mortality occurring in successive age intervals during the life cycle of insect
species.
 The construction of life table are borrowed from demographical studies.
 The Life tables should be replicated spatially and temporally.
 Richards (1940) opined that there is a successive reduction in the population of insect throughout a
generation.
 Richards (1961) in Annual Review of Entomology coined the word Budget for the Life table.

3.  Life table is basically a study where observations are undertaken frequently
 In Life table studies the word cohort is used very frequently which refers to population born at the
same time with a shared characteristics.
Prerequisites for constructing of a Life table
• Adults : Longevity of adult, Fecundity of adult, pre ovipositional period, mating period, sex ratio,
No. of eggs laid by female, pattern of egg laying
• Eggs : duration, percentage hatchability
• Larva : duration, No. of instars, duration of individual instar
• Pupa : pupal duration, site of pupation.
Requirements of Life table
1) Intensive studies are required for recording age specific survival of species
2) Measurement of variables like weather factors, parasitism, predation should also be recorded.

4. Age Specific Life table Time Specific Life table
Horizontal Life table Vertical Life table
Constructed for those organisms which
have short life span
Constructed for long lived animals
Ex: Annual plants / insects Ex: Herd of elephants because following
cohort of such individuals from birth to
death would take long time
Interval of observation is not regular
but decided by the stage present
Certain fixed intervals are pre decided
and observations are taken during the
fixed intervals
Called Horizontal life table as all the
stages are closely observed moving
horizontally along the life of an
organism
Life history is prerequisite
The flow of life cycle is cut vertically
and observations are noted in each of
the stage
Age determination is pre requisite
The frequency of recording of data should be designed in such as way that no important
event should be lost

5. x is Cohort
lx - is the number surviving at the beginning of age class
dx - is the number dying during the age interval x
Lx - is the average number surviving in age class
Lx = (lx) (lx+1)/ 2
dxf – factors responsible for death of individuals in that age class
Tx - is the no. of units of life at a particular stage. It has no real biological meaning
ex – is the expected life at a particular stage or amount of life left.
ex = Tx/lx
qx = refers to mortality per age interval and is expressed usually as a rate per 1000 alive at the start of that
individual.
qx = 1000 x dx/ lx

6. x lx dx Lx dxf Tx ex = Tx/lx qx
1 1000 300 850
=(1000+700)/2
No hatching
Sterility
Parasitization
or
Unknown
reasons
2180 2.180 300
2 700 200 600 1330 1.90 286
3 500 200 400 730 1.46 400
4 300 200 200 330 1.10 667
5 100 50 75 130 1.30 500
6 50 30 35 55 1.10 600
7 20 10 15 20 1.0 500
8 10 10 5 5 0.5 1000

7. Sx - is the survival rate with in the age mentioned
Sx = x (lx – dx)/ lx
Adult lx – is the no. of moths emerged in the lab from pupa collected from field
N1 = No. of eggs at the beginning of cohort in the column lx
N2 = No. of eggs produced can be worked out by
No. of eggs X average fecundity
N3 = females x 2
Trend Index I = N2/N1 SG = Generation survival = N3/N1
K= log10 N.log10 Ns
K= Difference between successive value for log lx -Hence it is a measure of killing power of a mortality factor
N = No. of individuals before mortality occurs
Ns = No. of individuals surviving the mortality

8.  The "survivorship curves" is the graphical representation of the fall off of numbers with time in a
given age (lx), is plotted against the age "x". The cohort life table data can be used to draw the
survivorship curves.
 The shapes of the curve describes the distribution of mortality with age.
 Survivorship curves shows the no. of individuals which survive per 1000 of population through
each phase of life.
 If the percent of total lifespan of organism is taken along X-axis and no. of survivals on Y-axis
resulting curve is known as Survivorship curve.
 There are two schools of thought regarding survivorship curves.
1) Deevey (1947 & 50) According to him lx of life tables are converted to log values and based on
which a graph should be drawn. He obtained three different types of curves.
2) Slobodkin (1962) used original values to plot curves without log values and obtained four different
types of curves.

9. Curves proposed by Deevey
1) Highly Convex curve
• In this type of mortality rate will be low until the life span is completed, heavy maternal care is
exhibited. Many species of large animals like man and small rotifers exhibit this type of curve.
2) Intermediate or diagnol type curve
This is a theoretical curve.

10.  It indicates a constant rate of mortality occuring at every age, Age specific survival.
 Examples are Hydra, Gulls, Birds exhibit this type of curve.
 This is further divided into
a) Stair step survivorship curve
 In holometabolous insects survival rate differ in successive life history and as a result the curve
becomes stair step type.
 According to it the initial and final steep in the curve represents egg and short lived adult stage
where as two middle segments represents larval and pupal stages which exhibits less mortality.

11. b) Linear
Eg: Birds, Hydra etc.
c) Slightly Concave curve
• This curve is exhibited by those organisms where mortality is high in young stages later stages it
more or less constant.
• Honey bees, Mice, rabbits exhibit display this type of curve.

12. 3) Highly Concave curve
• Animals which do not have mechanisms to protect their young ones exhibit this type of curve. Since
the mortality is very high in the early stages of life of an organism. However such organisms
compensate by laying numerous eggs.
• Eg: oyster, shellfishes, oaktree.

13. Highly Convex curve
• Type 1
• Type 2

14. Highly Concave curve
• Type 1
• Type 2

15. • Identifies “weakest link” in population
• Relates birth/dates rates to each other which provides information about survival strategy
• Useful for modelling population dynamics
• Can be used with fecundity tables to estimate intrinsic rate of increase
Biases/disadvantages
• Sample may not reflect age structure in the population
• Marked animals may die at different rates than unmarked
• Other biases such as differential survival by sex, increased predation at certain age

16. 1. Insect Ecology, Peter Price, 1997, III edn, John Wiley and Sons Newyork.
2. Insect Ecology, P W Price, R F Denno, M D Eubanks, D L Finke and I Kaplan, 2011,
Cambridge University Press.
3. Fundamentals of Ecology, E. P. Odum and G. W. barret, Thomas brooks/Cole Publn.

17. Thank you