Life tables

Life tables concept was first formulated by Raymond Pearl (1924)
Life tables is the systematic tabulation of births and deaths of an organism. It is summary statement on the life of a typical individual of population or a cohort of individuals. . 
It is an especially useful approach in entomology where developmental stages are discrete and mortality rates may vary widely from one life stage to another. 
From a pest management standpoint, it is very useful to know when (and why) a pest population suffers high mortality. 

1. Life tables and their application to insect biology Presented By: Hemlata M.Sc.(Ag.)Previous Year Dept. of Entomology

2. Life tables Life tables concept was first formulated by Raymond Pearl (1924) Life tables is the systematic tabulation of births and deaths of an organism . It is summary statement on the life of a typical individual of population or a cohort of individuals . .  It is an especially useful approach in entomology where developmental stages are discrete and mortality rates may vary widely from one life stage to another.  From a pest management standpoint, it is very useful to know when (and why) a pest population suffers high mortality.  

3. ROLE OF LIFE TABLES • Study of factors to determine the need to the need to modify the life system of pest with a view to reducing its number below ETL. • Study of applied ecology which involves application of biological knowledge for achieving desired populations. • Determining the key mortality factor and predicting the appearance in future. • Dividing pest control procedures that suits the available technology compatible with the economic and environment quality requirement . • ETL gives for timing application of need based application of insecticides. Estimation of intensity of pest population.

4. IMPORTANCE OF LIFE TABLES • Life tables describes for the successive age intervals the deaths, remaining survivor, rate of mortality and expectation of further life. • Life table provides an important tool in understanding their life cycles. • By this technique we can determine the mechanical relationship of various environmental factors and find out the key factor that accounts for large part of the change in population size. •Especially useful approach in entomology where developmental stage are discrete and mortality rates may vary widely from one life stage to other.

5. Life tables are two types Age-dependent life tables: Age dependent life tables show organisms mortality and reproduction rate as a function of age. In nature , mortality and reproduction rate may depend on numerous factors: temperature , population density , etc. when building a life table, the effect of these factors is averaged. Only age is considered as a factor that determines mortality and reproduction . Only females are considered in this life-tables .

6. Stage –dependent life tables: These are used mainly for insects and other terrestrial invertebrates. Stage-dependent life tables are built in the cases when: The life cycle is partitioned into distinct stages(e.g ., eggs, larvae, pupae and adult in insects) Survival and reproduction depend more on organism stage . Age distribution at particular time does not matter (e.g ., there is only one generation per year).

7. For example,In a hypothetical insect population, an average female will lay 200 eggs before she dies.  Half of these eggs (on average) will be consumed by predators, 90% of the larvae will die from parasitization , and three-fifths of the pupae will freeze to death in the winter.  (These numbers are averages, but they are based on a large database of observations.)

8. A life table can be created from the above data.  Start with a cohort of 200 eggs (the progeny of Mrs. Average Female).  This number represents the maximum biotic potential of the species ( i.e. the greatest number of offspring that could be produced in one generation under ideal conditions).  The first line of the life table lists the main cause(s) of death, the number dying, and the percent mortality during the egg stage .  In this example, an average of only 100 individuals survive the egg stage and become larvae.  The second line of the table lists the mortality experience of these 100 larvae:  only 10 of them survive to become pupae (90% mortality of the larvae).  The third line of the table lists the mortality experience of the 10 pupae - three-fifths die of freezing.  This leaves only 4 individuals alive in the adult stage to reproduce.  If we assume a 1:1 sex ratio, then there are 2 males and 2 females to start the next generation

9. If there is no mortality of these females, they will each lay an average of 200 eggs to start the next generation.  Thus there are two females in the cohort to replace the one original female -- this population is DOUBLING in size each generation!! In ecology, the symbol "R" (capital R) is known as the replacement rate .  It is a way to measure the change in reproductive capacity from generation to generation. The value of "R" is simply the number of reproductive daughters that each female produces over her lifetime.

10. If the value of "R" is less than 1, the population is decreasing -- if this situation persists for any length of time the population becomes extinct. If the value of "R" is greater than 1, the population is increasing -- if this situation persists for any length of time the population will grow beyond the environment's carrying capacity.  (Uncontrolled population growth is usually a sign of a disturbed habitat, an introduced species, or some other type of human intervention.) If the value of "R" is equal to 1, the population is stable -- most natural populations are very close to this value.

11. Applications of life tables The weakest link in the life cycle can be determined and made use of to control pests. The effect of most effective biotic factors causing death of the pest can found out and used effectively. Field life table studies on natural enemies can help in determining the cause of their failure in field information can help to find out the best release techniques . Based on survivorship curves one can operate control measures when the mortality factors operating slow and thus can obtain economic results It provides useful information on their mortality factors, information thus obtained can be incorporated in mass production techniques and make it more efficient. Studies on pest and natural enemies may provide exact time of release of predator and parasitoid and utilize them to their best potential.

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