Life Tables

How can life tables (and an understanding of life histories) be useful?

Population demography is the characteristics of a population as it relates to its range and distribution of age. For humans, a census may be used to gather this type of information, but for organisms that cannot be counted quite as easily, sampling is often used instead. Life tables are a way of presenting the information that has been gathered through this process. They record the mortality, survivorship and fecundity of the population under examination. Mortality refers to the number of individuals in an age class that die before reaching the next one. Survivorship is often seen as the opposite, as it counts how many organisms reach the next age class. Fecundity is the number of offspring produced in a period of time.

Life tables can be useful in determining the strategies of different organisms. Each species attempts to improve survivorship, decrease mortality, and alter fecundity to maximize population growth. Some species will produce huge numbers of offspring, and a high juvenile mortality rate will occur, but the individuals that survive the first age class will be the ones with greatest potential fitness and therefore the survivorship in remaining age classes will be higher. Other species have a low fecundity, but the parental care of each offspring will be greater and the juvenile mortality rate will be lower. Competition ensues later in life, however, so the survivorship of higher age classes is lower in these strategists. Still other species opt for a combination or moderate approach to these two strategies.

The analysis of life history comes to determine how many offspring an individual creates, how often it reproduces, when it starts and stops reproduction, how much parental care it give, and essentially how long it lives.


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