GENERAL ECOLOGY Cemetery
BIO 340
chapter 9: Human Demography
Introduction
As the United States has progressed through the industrial revolution over the last 150 years, changes in the life-styles of citizens have been reflected in their age at death. Factors such as diseases and accidents have changed in their relative impacts. One way to study these changes in human demographic patterns is to visit a local cemetery and collect dada recorded on tombstones.
By collecting information on the year of death for all individuals who died in the same time period you can produce a graphical representation of their survivorship called a survivorship curve. For this exercise three time periods will include all individuals born before 1901, all individuals born between 1901 and 1944, and all individuals born from 1945 to the present.
For the numerous species studied, the curves usually fit one of three general shapes (Figure 1). Human survivorship typically fits a type 1 curve. However, slight, but distinct, differences can be seen when survivorship curves from separate human communities are compared, and when cohorts from different time periods for a single community are compared, as you will see during this lab.
Objectives
In today's lab you will
· Compare and explain the differences in the survivorship curves for at three cohorts in a community with respect to local or national history.
· Speculate on future changes in demography, based on current community changes.
· Collect age data and calculate survivorship for at three cohorts in a community. Graph this data to show a survivorship curve for each time period you are studying.
KEY WORDS
Survivorship Curves
/ Type 1 / Type 2Type 3 / Cohort Life Table / Static Life Table
Fecundity
BACKGROUND
There are several ways to estimate the survival patterns of a population. The most reliable way to follow a cohort (all individuals within the population that were born in one time period) and follow it from the time they are born until the time that the last individual dies. During this study information on the number of individuals that die in each time period and the number of young that an individual has (fecundity) during each time period. The definition of a time period is variable and depends on the characteristics of the species being studied. This data can then be used to construct a cohort life table. This table can then be used to determine the net reproductive rate (R), the generation time (T) and per capita rate of increase (r). However, while this method gives the researcher a large amount if information about the population it is obtain time consuming and difficult to do.
The second estimator that can be used is done by collecting the death date of the individuals within a population independent of when they were born. This information can then be used to construct a static life table. Unlike the cohort life table the static life table only gives you a snap shot of the survivorship within the population. While R, T, and r can not be estimated in this type of life table due to a lack of fecundity information a survivorship curve for the population can be constructed.
A survivorship curve is a graphic version of the survivorship within a population (Figure 1). These curves usually follow one of three different shapes. A type I survivorship curve occurs in population in which there is high survivorship in early and middle age with low survivorship in old age such as seen in humans. Other populations, such as seen in many birds, have a constant survival rate throughout every age group in the population. This is called a type II survivorship curve. A type III survivorship curve is seen in populations that have a low rate of survival in early and middle age with a high rate of survival in later ages. This curve type can be seen in such species as insects or fish.
directions
Field:
1. Divide into groups of 3.
2. Each group responsible for one region of cemetery (90-150 graves). Roads surround each section of the cemetery.
3. Each person in a group is responsible for collecting data on one specified time period only: Died before 1900, died 1900-1944, died 1945 – present.
4. Record birth date, death date and sex of all individuals within your cohort.
5. Keep males and females separate for easy collation (males in one column and females in the other).
6. For each individual calculate the age at death (Table 1).
Laboratory
1. Form new groups with all of the students that collected data for the same death cohort and summarize the data using Table 2.
2. Summarize the data on Table 3 or in the Excel sheet provided. Note data are clustered into age classes (first column) of ten year intervals. For the 0-9 age class, count all of the individuals which died at an age of 9 or less. Continue to record the number of deaths for each of the age classes. The number of deaths for an age class is commonly abbreviated as “dx”. When you reach the bottom of the column, determine the total number of deaths and record this number in the indicated space. This number should equal the total number of tombstones counted for that cohort.
3. The third column of for survivorship data (nx) or the number of people surveyed that were alive at the beginning of a given time period. To calculate this, begin by placing the total number of deaths in the first column in the top cell since everyone counted was alive at the beginning of the first time period. To determine the number of for the next box down subtract the number of deaths (dx) that occurred in the first time period from the previous survival rate. This will tell you the number of people counted that were alive at the beginning of the second time period. Continue this process of subtracting the number to the left and one row up to determine the data for each row of the survivorship column. When you reach the bottom you should have the total number of 0. (See Table 4 for an example of data in this manner.)
4. In order to calculate the proportion surviving at the beginning of each time period (lx) take the number surviving at the beginning of each time period (nx) and divide by the total from the first column. The values in this column should range from 0 to 1 with 1 for the top row and 0 in the bottom row.
5. Standardize the survivorship data to per 1000 (S1000) to allow you to compare data form the three decades. Use these equations to make the necessary calculations.
S1000 = (lx) * (1000)
To check your calculations for correctness, the top line should be 1000 and the bottom line should be zero.
6. In order to make graphing easier calculate the logarithm to the base ten (10) of each number in the “survivorship per 100” column. Once again to check your calculations, the number in the top row should be 3 and the bottom line will produce an error (log 0 is undefined) on your calculator. For this number just put “0”.
7. Using excel, plot your data with the x-axis representing time and the y-axis representing the log of the proportion of individuals surviving to compare males and females within your cohort.
Excel Instructions:
a. Highlight the data found in male/female comparison including the labels and click on the graphing wizard.
b. Choose line graph and the first graph in the second row of choices then click next 2 times.
c. Label the axis and graph and click next.
d. Choose “as object” and click finish.
8. In order to compare the three different death cohorts studied collect the log (S1000) data for males and females combined and place it in the chart on sheet 2 of the excel workbook.
9. Create a graph comparing the survivorship curves for the three different death cohorts.
DISCUSSION QUESTIONS
1. Do the curves for the different decades differ from one to another? If so, what might have caused the differences?
2. How do your graphs compare Figure 2, survivorship curves for Newberry, South Carolina. Can you explain any differences?
3. How would your data have been altered if your cemetery had closed in 1965 rather than being opened to the present day?
4. One problem with studying survivorship curves when using birth cohorts is that all individuals must be followed until they have died. What differences would you expect to see between the survivorship curves from an 1870’s cohort and a 1940’s cohort?
5. What would future survivorship curves look like if
a. AIDS continues to increase and no cure is found?
b. Medical advances continue and most diseases and infant mortality are eliminated?
c. Environmental problems deteriorate and pollution-related disease increases?
6. What assumptions did you need to make in order to develop the survivorship curves for Mt. Pleasant using the data from this cemetery?
REFERENCES
Condran, G. and E. Crimmins. 1980. Mortality differentials between rural and urban areas of states of the northeastern United States 1890-1900. Journal of Historical Geography 6 (2): 179-202.
Dethlefsen, E.S. and K. Jensen. 1977. Social commentary for the cemetery. Natural History 86(6) 32-29.
Gwatkin, D.R. and S.K. Brandel. 1982. Life expectancy and population growth in the third world. Scientific American 246(5): 57-65.
Kuntz, S. 1984. Mortality change in America, 1620-1920. Human Biology 56: 559-582.
Mahler, H. 1980. People. Scientific American 243 (3):67-77.
Molles, M.C. 2002. Ecology: Concepts and Applications Third edition. McGraw-Hill, Boston.
Table 1. Data collection sheet - MALES
Time Period ______
# of indiv. / Birth Yr. / Death Yr. / Age at Death / Sex / # of indiv. / Birth Yr. / Death Yr. / Age at Death / Sex1 / 26
2 / 27
3 / 28
4 / 29
5 / 30
6 / 31
7 / 32
8 / 33
9 / 34
10 / 35
11 / 36
12 / 37
13 / 38
14 / 39
15 / 40
16 / 41
17 / 42
18 / 43
19 / 44
20 / 45
21 / 46
22 / 47
23 / 48
24 / 49
25 / 50
Table 2. Data collection sheet - FEMALES
Time Period ______
# of indiv. / Birth Yr. / Death Yr. / Age at Death / Sex / # of indiv. / Birth Yr. / Death Yr. / Age at Death / Sex51 / 76
52 / 77
53 / 78
54 / 79
55 / 80
56 / 81
57 / 82
58 / 83
59 / 84
60 / 85
61 / 86
62 / 87
63 / 88
64 / 89
65 / 90
66 / 91
67 / 92
68 / 93
69 / 94
70 / 95
71 / 96
72 / 97
73 / 98
74 / 99
75 / 100
TABLE 3. Data compilation for all of the groups of one time period.
AGE / GRP 1 / GRP 2 / GRP 3 / GRP 4 / GRP 5 / GRP 6 / GRP 7 / GRP 8 / Total
0-9
10_19
20-29
30-39
40-49
50-59
60-69
70-79
80-89
90-99
100-109
110-119
Female
AGE / GRP 1 / GRP 2 / GRP 3 / GRP 4 / GRP 5 / GRP 6 / GRP 7 / GRP 8 / Total
0-9
10_19
20-29
30-39
40-49
50-59
60-69
70-79
80-89
90-99
100-109
110-119
TABLE 4: Data calculations for survivorship curve.
TIME PERIOD
Age Class
(Years)
/ # Deaths(dx) / # Survivors
(nx) / Proportion Surviving (lx) / Survivorship per 1000 (S1000) / Log (S1000)
0-9
10-19
20-29
30-39
40-49
50-59
60-69
70-79
80-89
90-99
100-109
+110
TABLE 5: Survivorship table summarizing data collected for the 1830’s from a cemetery in Newberry, South Carolina.
Age Class
(Years)
/ # Deaths(dx) / # Survivors
(nx) / Proportion Surviving (lx) / Survivorship per 1000 (S1000) / Log (S1000)
0-9 / 8 / 160 / 1.000 / 1000 / 3.00
10-19 / 7 / 152 / 0.950 / 950 / 2.98
20-29 / 13 / 145 / 0.906 / 906 / 2.96
30-39 / 9 / 132 / 0.825 / 825 / 2.92
40-49 / 13 / 123 / 0.769 / 769 / 2.89
50-59 / 21 / 110 / 0.688 / 688 / 2.84
60-69 / 23 / 89 / 0.556 / 556 / 2.75
70-79 / 39 / 66 / 0.413 / 413 / 2.62
80-89 / 22 / 27 / 0.169 / 169 / 2.23
90-99 / 4 / 5 / 0.031 / 31 / 1.49
100-109 / 1 / 1 / 0.006 / 6 / 0.78
+110 / 0 / 0 / 0.000 / 0 / *0
* Log (0) is undefined. Consider this value zero.
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