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Demography is the statistical sample of people, particularly in terms of size and density, distribution and vital statistics such as births, marriages, deaths, etc. History has its own ups and downs forever, and demography has been helping us since forever to study the effect of different eras on population dynamics. Life tables are used with the information of the life and death rates. Demography is a core related science to evolution and ecology as it is mentioned in the article Demography beyond the population by Alden B. Griffith, connecting demography beyond the population level offers promising avenues of innovation in ecology and evolution Demography is not only linked with a certain criteria but also help with the wide variety of sciences. In demography the statistical method is used to measure the population growth rate, its responsiveness to life-history variables and related figures, such as stable population spread and reproductive quality of exponentially growing populations, in which the history of individual life is defined as a continuous evolution over time.
In order to understand demography, one needs to understand that births and deaths are facts of life. The natural phenomenon for this life cycle quite straightforward; when the birth rate exceeds the death rate, the population increases and when death rate is more than the birth rate the population decrease. In order to understand the conceptuality of this process a lot of life tables and survivorship curves have been introduced, and were quite logical to compute the arithmetic and geometric data. In terms of demography and life histories a lot of researches have been done to enumerate human population growth and transition, fertility and mortality, future population trends, age dependency ratio, population momentum, and many more. This proves that the basic purpose of using demographics tool is to record and predict the events of death and birth rates along with acknowledging the change in population size- thus a feasible technique not only for documenting the statistical studies of humans but also plants, animals, wildlife, forestry etc. Each population varies from the other, and so the survivorship such as humans and Eel life cycle is different so does their death and birth rates. Humans have internal fertilization, and Eels have external fertilization. Humans produce twenty plus youngs in their life (most live) , while Eels have ten thousand plus youngs (most die).
Humans offsprings required parental care that last for sixteen plus years while Eels offspring does not require any parental care. All these features that differentiates one population from the other proved that their survivorship curve would also have variation. The reason for variation is because of the difference in age and fecundity, as fecundity are the factor that got affected the most by the age and environment particularly in humans. In order to discriminate the patterns between variety of populations, biologists use life tables. Life table would permit them to collect information about life spans for each of them individually. Cemetery is the best place to understand the core idea of human demography. All the gravestones have date of death and birth which makes it easier to collect data relatively. Later that data can be used to make a data of a human population, and make a survivorship curve. In demography and life history there is no true if and then statement but it is noteworthy that collecting accurate data would help us document statistical studies.
In terms of collecting data, all the cemeteries are a perfect place to visit. I visited a cemetery near my house Rosehill Cemetery The information was collected from ninety plus gravestones for the individuals born between 1870-1930.The headstones were not chosen in specific order instead were picked randomly from the whole cemetery. Later death rates were used to calculate the age for the deceased individuals. The data I had collected may have some sort of biases based on variation in religious preference, social stratas etc.
As it can be seen through the data from the survivorship graph I found at Rosehill Cemetery that most of the people died in their fifties to the eighties. This data might have some fluctuation because of fact of data can be biased as cemetery has people from different social strata and religions. While we usually have three types of curves, Type 1(human), Type 2 (Birds), Type 3 (Tress). In the survivorship curve I had, the mortality rate increased during the old age, and its pretty common to live through the middle and young age. In type1(humans) they usually have a small number of offspring, and provide excessive parental care which became one of the most common reasons for the longest life span. This data was recorded during 1870-1930, when the world was in the havoc of wars and economic crisis, when technology was not as advanced as it is today but the majority of people were able to compete and survive.
There was one example in the textbook about the female Lacerta Vivipara, survivorship for the curve for Lacerta Vivipara is Type 2. Most bird species have a Type II success curve. In the Type II curve, organisms die somewhat equally at each age bracket. Organizations with this form of survival curve may also have fairly few children and provide substantial parental care. The life span for Lacerta Vivipara could be approximately four to six years. As lizard gets older it has higher survivorship as compared to human beings. When a human being gets older they have less tendency to work as their bones get weaker. Most of the women tend to have osteoporosis while when lizards get older they have a tendency to produce more offspring.
In another example, Dall mountain Sheep (Ovis Dalli) showed the same survivorship curve as humans (Type 1). This a more of K-selected species, which usually live in an environment with intense competition which gradually makes them more stable and stronger. There was a study done in Denali National Park in by Olaus Marie in 1944, in which the age of death was recorded for 608 species. The data shows some variation as in the beginning there would be more death rates as compared to humans but soon it slows down and most death occurs during the old age,which shows similarity with Type 1 curve. In the data I collected from the cemetery the graph shown as humans get older the death rate increased. In this study by Olaus Marie, same pattern has been seen but in the last 12-15 years the death rate drops rapidly.
Based on what I have found, it is probably true that in most of the species parental care can increase the life span, as most of the species with type1 curve have had heavy parental care. In terms of human growth other factors also contributed such as technology, food and agricultural growth, rapid urbanization, female literacy, access to healthcare etc.
It was quite shocking for me to see how different populations have different curves. I was expecting a slight change in the curve, but there are drastic changes. Before I used to think as all the species grow their fertility rate goes down, it was new to me that lizards have more offsprings in the end years of their life. To understand this topic in a better way, I decided to draw survivorship curves for different population. I used findagrave.com website just to learn more about recent death rates and had played around while making different survivorship curves. It was for humans so the results were the same as Type 1. After this, I am feeling more acknowledge about making curves and how I can make a curve for any species if right darta for life table is given.
References
- British Ecological Society Special Feature: Demography Beyond the Population (2016) (Vol. 104) Pg (271-610)
- Johan Ehrlén, Climate change in grasslands demography and population dynamics, Grasslands and Climate Change, (172-187), (2019).
- Citation: Rauschert, E. (2010) Survivorship Curves. Nature Education Knowledge 3(10):18
- Sanderson, Population Ecology Lab, Dall sheap life table, Nature Education Knowledge (2010)
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