Human Demography: The Study of Age-Structured Population

Abstract

Demography is the study of the characteristics of human populations, such as size, growth, density, distribution, and vital statistics. Human demography can be influenced by human caused or natural caused events. Demography is important in predicting the future growth or decline of a population. A census was done in a local graveyard to determine whether or not there were any differences in the survivorship of individuals of two centuries: 19th and 20th. The survivorship of the 20th century was hypothesized to be higher than in the 19th century. Also, the role of sex in the survivorship of the individuals of the two centuries would be determined, as it was hypothesized that females would have better survival abilities than the males. The survivorship curve of the 20th century was higher than the one for the 19th century, as the curve for females was slightly higher than the males. However, the differences of average age of death between the two genders were not significant, with a p-value of 0.121. Therefore, the differences in survivorship between the two centuries were valid, considering the p-value of less than 0.001. This could be due to the advancement of technology and the discoveries of antibiotics, such as penicillin, which improved the life expectancy of the individuals in the 20th century. The improved educational system of the 20th century also played a role in improving the survivorship of the individuals.

Introduction

Demography is the scientific study of characteristics and dynamics relating to the human population. Demography covers many characteristics: size, density, distribution, growth rate and 'vital statistics'. The vital statistic is a set of qualitative data concerning human life or the conditions and aspects affecting it. Some examples of vital statistics are birth rates, death rates, and migration patterns. These particular characteristics of demography play bigger roles than the others: they predict whether a population will be growing or declining, its stable age distribution and how selection might affect individuals of each cohort. The vital statistics are some of the qualitative data that will compose a higher level component of demography, called a life table.

Human demography and population dynamics were highly influenced by human historical events or trends. These events or trends could be either human-caused or natural-caused, such as wars, famine, poverty, illness and natural catastrophes. For example, the flu pandemic of 1918 took the life of 50-100 million individuals in only 18 months. This event was shortly followed by the Great Depression, which further added to the plunge in population size. Some of these events could also cause an 'after-effect', such as the 'Baby Boomer Generation' that happened after the WWII period: boosted birthrate, total fertility rate (TFR), child-woman ratio, etc.

Demography of a specific population can be gathered by a population census or vital statistical records. From there, the data can be used for various purposes that differ by subjects: political, scientific and marketing. Demography is utilized by governments politically to determine the allocation of federal funding for education, law enforcement, federal projects, aids, etc. It is also used by scientists and mathematicians to predict potential problems of the human population. For example, Robert Malthus and Albert Bartlett are two of many famous scientists that studied demography and its relationship to limited resources of human population. Demography can also be used in commercial applications, such as business advertising and by the real estate industry.

There are two main components of demography that are often used to analyze population patterns: life table and survivorship curve. Life table is a record of survival and reproductive rates in a population, as it is broken out by size, age or developmental stage (juvenile or adult). Life table is crucial in predicting the future growth or decline of an organism, as the growth of the population depends upon the age-structure of its cohorts. Cohort is a group of individuals in a population, all born at the same time, that are followed until death. A cohort analysis allows us to directly measure the mortality rates of different ages and to construct the survivorship schedule [l(x)] for the population (Gotelli 2008). Life table is also useful for the purpose of species conservation, such as in the case of Loggerhead Sea turtle of the southeastern United States (Crouse et al. 1987). The Loggerhead is an endangered species and its population is declining. The mortality rate of Loggerhead hatchlings is relatively high, which led conservation biologists to promote the protection of nesting beaches. When these measures proved ineffective in preventing the population decline, compiling and analyzing a life table for loggerheads indicated that reducing mortality of older turtles would have a greater probability of reversing the population decline, which proved to be a correct hypothesis.

Life table is composed of a variety of quantities: cohort survival [S(x)], fecundity survival [b(x)], survivorship schedule [l(x)], survival probability [g(x)], net reproductive rate (Ro), generation time (G), and intrinsic rate of increase (r). The survivorship curve can be obtained by plotting the logarithm of survivorship schedule [l(x)] on the y axis and age (x) on the x axis. The curve represents the survival chance of an individual from birth to a particular age. The purpose of this experiment is to examine local demographics and population dynamics by collecting information from tombstones of a local cemetery. The collected information includes birth year, death year, age of death and sex of individuals from both 19th and 20th centuries. The life table variables will be calculated and the survivorship curves will be plotted from the data. Based on the historical events of human population, it is hypothesized that there will be significant differences in survivorship of individuals between the two centuries. The survivorship curve of the 20th century will be higher as the individuals will have higher survival abilities, due to higher life expectancy, better education, advancement in technology, etc. Sex will also play a role in determining survivorship of individuals between the two centuries. It is hypothesized that survivorship curve of female will be higher than the one of male, because the male gender will have a higher mortality rate primarily due to wars.

Method

The main concentration of this lab was to examine the demography of a cemetery, which is located in Burlington, VT. The individuals' tombstones were divided into two categories: 19th century individuals (1801-1900) and 20th century individuals