Bio. 317 – Sept. 26, 2013 Principles of Ecology Lab
Constructing a Life Table for a Human Population
Objectives: We will use mortality and survivorship data to construct life tables and to compute vital statistics of a human population. Introduction: A summary of mortality, survivorship and expectation of further life, by age, is called a life table. The most straightforward type of life table starts with a cohort of young organisms and follows them through time until the last individual dies. Cohort data can be difficult to obtain, so most life tables are calculated using other information. If we can obtain the current mortality rates by age of a population, we can, after the appropriate assumptions and calculations, construct a life table (called a time-specific rather than an age-specific cohort table). In the rare case in which neither population size nor mortality rates change with time, all we need for a time-specific life table are the sizes of the different age classes. A frequent approach, and the one used here, is to use age at death to estimate mortality rates and calculate the other vital statistics from that. Tables produced in this way are fairly accurate, even though the cohort is a composite, made up of individuals that started life in different years. Methods: We will obtain our data of numbers of deaths at different ages by visiting Hope Cemetery in Galesburg. This will give us a picture of mortality rates in our region during historic times. Next week in lab we will collect data on current mortality rates from the obituary section of the Peoria Journal Star newspaper during the entire month of September 2013. 1. We will divide the cemetery into sections. In each section, a pair of students will use the information on the gravestones to determine the age at death of each person who died before 1900. Record data for males and females separately. Each pair should record as many gravestones as possible to give a total class sample of at least 300 of each sex; 500 would be better. Use common sense when collecting the graveyard data. If you are unable to tell from the name or other evidence whether an individual was male or female, omit that person from the sample. If only initials are used rather than given names, the individual was usually male. "Infants" or children without names can be put into the 0-0.99 year category. Occasionally infants or stillborn children are recorded simply as "baby" or some other designation from which sex cannot be determined. Tally these for the class and we will put half in one sex and half in the other. 2. Pool the class data and record on the data sheet.
3. Analyze the two sets of data (historic for males and females who died prior to 1900; current for males and females who died in September 2013), constructing life tables for each data set.
In the life table, each line represents mortality and survival for an age interval.
ax - the number alive (survivors) at age x
dx - the number dying in each age interval; dx = ax - ax+1
In this analysis we will, in effect, treat the sample as a cohort. First add up all the individuals of a particular sex and then enter the total number of individuals in the sample in the zero age class of the ax column for that sex. This is the number of individuals that started out at age zero (even though they did so at different times). The easiest way to do the table will be to next enter the number dying at each age in the dx column. This will give us mortality in each age interval. Then we will calculate the size of each age class ax this way – subtract the number in dying in the previous age class from those present in that previous age class:
Note that our mortality rate (qx) for the 0-0.99 year age class is an annual mortality rate. However in other age intervals, the mortality rate is calculated for a 5 year period. To make the figures comparable, you must convert the later mortality rates to annual rates by dividing each by 5. Results and Discussion: 1. Plot survivorship curves (ax against x) for the males and females on arithmetic and semi-log plots (semi-log plots have lograthims on the y axis and arithmetic scale on the x axis). Plot the data from the historic cemetery data and the current obituary data on the same graph – so one with current and historic for males and one with current and historic for females. That way you can easily compare the two different times. Put ax on the semi-log scale. How does the form of the semi-log plots differ from the arithmetic plots? Are there are straight-line portions of the semi-log plots? What does this indicate about survivorship rate during that period of life? 2. Can you divide the tables up into a juvenile segment (high but declining mortality), an adult segment (low mortality) and a senile segment (rising mortality)? What ages are included in the three segments? Do the sexes differ in these segments?
3. How do the sexes differ in survivorship and longevity? At what ages do the highest mortality rates occur? Do you have any hypotheses about why the differences exist? Do the different time periods have different mortality rates? If so, suggest reasons why.
Life Table Data Sheet: Deaths pre-1900 – from cemetery
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