Life tables

Life tables are a very useful tool to study population growth, as they summarize birth and death rates for organisms at different stages of their lives.

There are different types of life tables: dynamic (or cohort, or age-specific), static (or time-specific) and composite.

A cohort is a group of individuals born during the same time period. Cohort tables are built using data for the same individuals across time throughout their lifetime or an extended period.

For static life tables, age-distribution data are collected from a cross-section of the population at one particular time or during a short segment of time.

Composite life tables use data that are gathered over a number of years and generations, using either cohort or time-specific techniques.

For this, mark-recapture methods are used to sample individuals. This is exactly what we have been doing here on Astakos IV. We capture juvenile and adult goslins and mark them. Then, the following year, we recapture marked individuals and capture and mark new ones, and those will then be recaptured the following year and so on.

Here is an example of a mark-recapture life table for the first two years at the wild lake:

Figure 1 - Life table for goslins at the wild lake for years 1 and 2

Numbers, sex and age distribution data can be used to calculate a number of population growth indicators, such as mortality, survivorship, and fecundity.

The mortality rate in year x is proportion of individuals of age x dying by age x+1, which can be translated by (qx = 1-dx/nx).

The probability of survival for a certain age group can be calculated by multiplying the mortality rates for the previous years.