|Human Population Ecology
Biology 154: Widener University
Note: This handout is modified from the lab activity "Cemetery Demography" by Nancy Flood (U. Toronto) in the Ecological Society of America's Experiments to Teach Ecology, edited by Dr. Jane Beiswenger and available from the ESA Public Affairs Office, Washington, DC., for about $15.
page updated: 9/2/97
Synopsis of Today's Lab.
Today we will travel to a local cemetery and record dates of birth and death etched on the headstones commemorating previous Pennsylvania residents. Back in lab we will pool our class data and examine demographic parameters such as survivorship and mortality of males and females during two time intervals: pre-1950 and 1950 to the present.
Objectives for Today's Lab.
At the conclusion of this lab,
(1) you will understand some of the basic concepts of population demography - survivorship and mortality.
(2) you will understand how factors such as advances in medicine and environmental protection may have affected human demography over the past 150 years.
(3) you will understand how human demography might change in the future, based on current socio-political reality and the prevalence of presently incurable diseases such as AIDS.
Equipment Needed for Fieldwork Today.
§ field clothing (which will include raingear if it is raining),
§ this handout, graph paper, notebook, and pencils,
Exactly What Is Due Next Wednesday at Noon From This Weekís Laboratory.
From this lab due by next Wednesday at noon each of you should submit
(1) your written responses to the hypotheses on page 3 (+5 points),
(2) your original data (Data Sheet #1) that you collected at the cemetery (+5 points),
(3) your analyses of your groupís data (Data Sheet #2) (+5 points),
(4) a summary table (Data Sheet #3) and graph clearly showing the survivorship differences for all of your classís data (+10 points),
(5) answers to questions #1, #2, and #3 and any other two questions for further thought contained in this handout, and
(6) a critical review of the lab activity.
Local cemeteries are an excellent place to study human demography. Etched in the gravestones are the dates of birth and death of the person below, at least in most cases. From these data, we can calculate death rates and draw survivorship curves. A survivorship curve is simply a graphical representation of the chance that an individual will survive from birth to any particular age. By comparing survivorship curves for different periods of time we may look for historical trends in demography over the decades. Also, different cemeteries may represent different socio-economic cross-sections of the population, and comparing data among cemeteries may reveal different patterns of mortality.
Over the last few centuries, advances in health care and large-scale global political conflict have left rather opposing marks on the demographics of our population. Two major time intervals stand out: before 1950 and from 1950 to the present. Firstly, the time interval before 1950 includes the industrial revolution, the ravaging effects of polio, infection and other presently curable diseases, as well as World Wars I and II. Following 1950, numerous vaccines and antibiotics were widely used and, with the exception of the Korean, Vietnam, and Gulf Wars (not to mention a few other incidents...), this has been an era of relative peace in North America. What are your predictions about how the demographics of the Pennsylvania human population have changed during these two time periods?
In order to study the demography of the entire human population that once lived in Pennsylvania, we would have to study all of the local cemeteries and assume that no one emigrated from the area and was buried elsewhere. Neither is likely. We only have a few hours, and many deceased residents of Pennsylvania were buried, or otherwise, elsewhere. Thus for now, we will assume that the cemeteries we will visit are representative of all humans in this area, although we should be aware of these sorts of biases in the data.
Methods for Today's Lab.
Study Sites. Today we will travel to a local cemetery (the exact location to be announced).
Data Collection. We will divide up into four groups and collect data from as many headstones as possible where each group is in charge of collecting data from a separate set of headstones:
Group 1: FEMALES WHO DIED BEFORE 1950
Group 2: MALES WHO DIED BEFORE 1950
Group 3: FEMALES WHO DIED AFTER Jan 1, 1950
Group 4: MALES WHO DIED AFTER Jan 1, 1950
BE CAREFUL TO NOT DUPLICATE DATA WITH ANOTHER MEMBER OF YOUR GROUP. ALSO, NO ONE MAY WANDER OFF ALONE. Also, please exercise restraint when collecting these data. Do not run, shout, stomp on graves, etc. We do not want to attract any attention from any source, above or below ground.
Hypotheses. What Types of Survivorship Curves Might We See?
Q1 - In general, what are your predictions about death rates of people before or after 1950?
Now let's try to predict some of the specifics:
Q2 - For infants of both sexes, would you expect infant mortality to be higher or lower before or after 1950? Why?
Q3 - For females ages 20-50 (reproductive and working ages), would you expect females before or after 1950 to have a higher death rate? Why?
Q4 - For males ages 20-50 (reproductive and working ages), would you expect males before or after 1950 to have a higher death rate? Why?
Q5 - For females ages 50-80, would you expect females before or after 1950 to have a higher death rate? Why?
Q6 - For males ages 50-80, would you expect males before or after 1950 to have a higher death rate? Why?
Q7 - Given what you said above for the causes of mortality for males and females, which sex would you predict has a higher death rate for...
...for the time period before 1950?
...for the time period after 1950?
Now that you have made your predictions, you are ready to go out and collect the data to test them.
Data Analysis Back in the Lab.
To estimate demographic characteristics of the Pennsylvania population, we need to know the ages of people when they died for each sex and time interval. To get this, simply examine your field data sheets (Data Sheet #1), and count the number of people who died in 10 year intervals, 0-9, 10-19, etc.
Use Data Sheet #2 to guide you through the calculations to estimate the survivorships. Plot the data on graph paper provided by your instructor.
Exact steps for data analysis.
1. On Data Sheet #2, write your Group Number (either 1, 2, 3, or 4), and write whether you collected data on MALES or FEMALES and BEFORE or AFTER 1950.
2. In column A, write down the number of people who died for each 10-year age interval listed (0-9, 10-19, etc.) from your groupís data set from Data Sheet #1.
3. At the bottom of column A, write down the total number of people who died in this data set (i.e., add all of the numbers in the column).
4. Copy the total from the bottom of column A to the first row of column B (age interval 0-9). This is the total number of people in your groupís data set upon which death took its toll as they grew older.
5. Then, subtract the number who died in each age interval (from column A) from the number who are were "alive" in your sample from the beginning of that age interval (from the same row in column B), and write this number in the next row in column B. Repeat this for all ages in B.
6. Calculate the SURVIVORSHIP. For each row in column C, divide the number in column B by the total that you found at the bottom of column A. This gives you the fraction of the people that survived to each age interval. By definition, the SURVIVORSHIP of the first age interval equals 1.000, regardless.
7. Use the graph paper provided to plot the SURVIVORSHIP in column C as a function of age from your data.
Questions to Answer After You Have Collected and Plotted Your Data.
Q #1. What is your interpretation of juvenile mortality pre- and post-1950 for males and for females? List all factors that might account for any differences you see.
Q #2. What is your interpretation of mortality for reproductive age adults ages 20-40 for pre- and post-1950 for males and for females? List all factors that might account for any differences you see.
Q #3. What is your interpretation of mortality for adults ages 60-80 for pre- and post-1950 for males and for females? List all factors that might account for any differences you see.
Q #4. What shifts in the survivorship and mortality curves would you expect if AIDS continues to increase in prevalence without cure?
Q #5. What shifts in the survivorship and mortality curves would you expect if environmental problems worsen and pollution-related diseases increase?
Q #6. What shifts in the survivorship and mortality curves would you expect if cutbacks to social services such as prenatal and infant care are enacted?
Q #7. Why might data that you have collected be useful to an insurance company?
Q #8. Many people carry recessive and hidden genetic defects that sometimes pre-dispose the carrier to a curve of higher disease incidence and mortality. Even though the person may have no physical symptoms, what do you think would happen to his or her health insurance premium if his or her insurance company found out about the hidden genetic defect? Do you believe that this is fair?
Comments to Instructors.
Use Data Sheet #3 to summarize the class totals. One suggestion is to copy this table to the blackboard and have students fill it in as they complete their analyses. Or, xerox this sheet onto a transparency and fill it in on an overhead projector.
SOME QUESTIONS FOR DISCUSSION WITH YOUR CLASS :
Q - What are the differences among your 4 survivorship curves?
Ans - you should see three main ones.
A1 - You should see a huge reduction in mortality (compare the slopes of the survivorship curves) for the first age interval (age 0-9) after 1950 relative to before 1950. This is mostly due to reduced infant mortality and improved pre-natal care, BUT point out that there is ample room for further reduction in the present rate of infant mortality in the US.
A2 - You should see a huge reduction in mortality (compare the slopes of the survivorship curves) for working and reproductive ages of both males and females. For both sexes this is mainly due to breakthroughs in disease control (penicillin, polio, tetanus, etc.). Additonally, for both sexes, although mostly for males, improved worker conditions and a shift of jobs from the manufacturing to service sectors have combined to reduced job-related exposure to toxins, carcinogens, and injury. Lastly and only for females, improved medical care has greatly reduced the risk of mortality during childbirth. (Point out that the World Wars have left relatively little signature in THESE data since those killed in these wars were for the most part not returned and buried here in Chester.)
A3 - The third pattern is less perceptible, but still important. For older people (ages 50-80), you should see very little difference in the rates of mortality (slopes of the curves) before and after 1950. The 1950 curves will be shifted to the right because of lower mortality at earlier ages, BUT the actual rate of mortality once one survives to older ages is not that different. Basically, humans age now at the same rate as we did before 1950. Our bodies have fundamentally not changed in this short of a time, and modern medicine still has a very poor understanding of the aging process.
Directions to Chester Rural Cemetery from Widener University: proceed north on Providence Rd., Walnut, Chestnut, or Melrose and turn left on 20th St. heading southwest. Proceed about 4 blocks past Providence to Edgemont Ave. at the intersection of 20th and Edgemont you'll see Chester Rural Cemetery gate and main entrance. Turn briefly onto Edgemont and enter the cemetery. Follow the road along to "Soldiers' Circle". The oldest graves are nearest this circle.
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Disclaimer: Adult supervision is recommended when performing this lab activity. We also recommend that common sense and proper safety precautions be followed by all participants. No responsibility is implied or taken by the contributing author, Dr. Bruce W. Grant, nor by his academic employer, Widener University, for anyone who sustains injuries as a result of using the materials or ideas, or performing the procedures put forth at this web site or in any printed materials that derive directly therefrom.
Copyright - Bruce W. Grant, 1997.