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Grave Yard Demography Activity


Demography Background:

 A population is composed of all of the living individuals of a given species living in a given place.  The measurement and description of populations is called demography.  Demography was first developed by social scientists studying human populations, but the principles of demography can be applied to populations of any species, and demography is an important component of the sciences of ecology and conservation biology. Demographers study population statistics in order to make predictions about the future size and age structure of a population.  Demographic data can also be used to test hypotheses about the effect of the environment on populations.  A hypotheses is a provisional explanation for an observation.  To be useful, the hypothesis must be testable.

Demographers summarize their measurements of population statistics in a life table.  The statistical data contained in life tables allow demographers to calculate the mortality and survivorship rates of a population.  The mortality rate is calculated by dividing the number of individuals that die during a given time period by the number alive at the beginning of the period, while the survivorship rate is calculated by dividing the number of individuals that survive during a given time period by the number alive at the beginning of the period.** Note that survivorship and mortality rates are the exact opposites of one another. Knowing the mortality and survivorship rates of a population allows demographers to predict changes in population over time and to test hypotheses about the population.

There are two types of life tables, which differ in the method of data collection used in their creation.  Cohort (or generational) life tables are constructed by following the fate of a cohort (a sample of individuals born at approximately the same time) throughout their lives.  Though highly accurate, this method is usually impractical.  Static (or time-specific) life tables are constructed based on a sample of individuals drawn from a population at one specific time.  One such method for gathering demographic data to construct a static life table is to sample individuals of known age at death, by collecting data from gravestones in a cemetery, for example.  This is referred to as the age at death observed method.

Survivorship Background:

In this exercise, you will focus on  a single statistic, the survivorship rate, that can be calculated from the data contained in a life table, although you will not be required to construct an entire life table for a population.  The survivorship rate of a population can be graphically represented as a survivorship curve.   Ecologists generally recognize three types of survivorship curves.  A Type I survivorship curve is characterized by high mortality late in life.  That is, if an individual survives infancy, it is likely to live a long life.  The Type I curve is typical of many large mammals, including humans in developed nations.  A Type II survivorship curve is characterized by the constant mortality throughout life.  An individual has a fairly constant risk of death throughout its life.  The Type II curve is typical of many many birds, small mammals, and humans in developed nations. A Type III survivorship curve is characterized by high mortality early in life.  That is, many individuals will be killed in their infancy, with only a few surviving to old age.  The type III curve is typical of most fishes, invertebrates, and plants.  These three types of survivorship curves are highly generalized; no population will correspond exactly to these ideal types.  For example, the survivorship curves of most populations of wild animals probably fall somewhere between the Type I and Type II models.

 

 


Contact Information:    Jeana Forsyth
Prairie Central High School - Fairbury, Illinois
jforsyth@prairiecentral.org

Last Updated
09/19/2012