Life Tables Protocol
By Jim Cronin and Barry Aronhime
THEORETICAL BACKGROUND Populations of most organisms have age structure -- that is, more than 1 age class exists at any point in time. For example, the population of snow geese migrating south for the winter consists of 1-year olds, 2-year olds, and adults of various ages. Counter-examples include annual plants (e.g., dandelions) and many temperate-zone insects (e.g., mayflies) that have only l age class alive at any time. Age structure can have a significant effect on the growth and dynamics of a population and the study of its effects on these 2 parameters is known as demography. From a knowledge of age structure, a life table can be constructed which contains a summary of age- specific survivorship and mortality. These tables were developed by demographers to predict the lifespan of humans and are the basis for determining costs of life insurance. Life tables were first employed by the Egyptians and later used extensively by the Romans. During this century, the growth of many plant and animal populations have been analyzed using these demographic techniques.
There are a number of ways to generate a life table. The best, and most direct, method is to mark a group (cohort) of individuals of known age and follow them until their deaths. A table constructed from this method is known as a cohort life table. You can well imagine that this method would require a great deal of time and patience for someone interested in human demography. A more practical method, but indirect method, for creating a life table is to use data on the age-at-death to determine mortality and survivorship. This is known as a static life table because it results from data obtained from one fixed point in time. For example, a table could be constructed for whitetail deer by collecting skulls in the woods over a several year period. The age of death of these animals can be easily determined by the wear on their teeth. Many other animal or plant remains can be used to determine the age-at-death: horns, teeth, scales, fish otoliths, annual growth rings on trees, etc. A critical assumption made in using static life tables is that there must be no significant change in survivorship during the period over which those organisms were alive.
From life table information, you can build survivorship curves (Figure 5.1). A type I survivorship curve depicts a population where the highest mortality is in old age (dall sheep are
life (many birds). Finally, a population with a type III survivorship has highest mortality in the early stages of life (most aquatic invertebrates).
1
1,000
Type I Type II Type III
Number Of Survivors
Young
PIO
Age
Figure 1 shows the three types of survivorship curves for a given population.
LAB OBJECTIVES: 1) Create a life table for human populations in Baton Rouge, 2) determine age-specific survivorship and mortality rates, 3) develop a hypothesis regarding a factor (or two) that might affect mean life