Population demography

John Graunt is known as the father of demography (a Greek word, “demo” means “people”, and “graph” means “measurement, description” etc). Population Demography is the statistical study of any population in which we study about size, growth, density, and distribution of the population in a quantifiable manner. Statistics regarding birth, death, disease, and changes over time within the population are also covered in demographic studies. In short, it means all the characteristics of any population that can be expressed in quantity or numbers.

Scientists study the population of any area with the help of mathematical numbers and tools. Big data of any population is collected from that area and based on this collected data, scientists describe the health of this population. Demographic parameters like the average age of individuals in a population, male/female sex ratio, population size, density, population distribution, birth and death rates, etc are considered to describe any population demographics. All the factors that promote and decline in the growth of the population are also studied in population demography.

The following are the population attributes that can be discussed in the demographics study:

Population size
Population density
Population dispersion
Population distribution
Recovery/resilience
Age structure
Fecundity
Natality
Mortality
Sex ratio
Life tables
Survivorship curves

Population size.

Population size is defined as the total number of individuals found in a specific region or area. Counting the exact number of organisms is impossible when the geographic region is large (like forest, river, etc). So in that case, this large area is divided into many small sample regions and ecologists considered the organisms numbers within all these sample areas. This will give an estimated population size of that region.

Maintaining a population size is very important because it maintains and balances the food chain of that region. Small-sized populations face great stress of extinction because any disaster like flood, fire, or disease can remove all the population from that region. The large population has also great competition for food and housing.

Population density.

Population density is the number of individuals per unit area. It is very similar to population size. The only difference between population size and density is that population size is considered for the whole region or area while population density is not for the whole area but for some selected areas.

The number of individuals added or removed from the unit area or changes over time with the population of that area is studied under population density.

The number of individuals can be increased by the birth of new individuals or by the immigration of individuals towards that area.
Death and emmigration caused to decrease in the number of individuals from a specific area or region.

Population dispersion.

Population dispersion is the separation of individuals. The dispersion can be caused by different ways like food competition, predators, etc. Environmental and social factors influence the dispersion.

If the individuals are evenly distributed then it is called uniform dispersion. While dispersion can be random as the dispersion of seeds by plants with the help of wind, birds, animals, etc.

Population distribution.

The population of any area is distributed by both biotic and abiotic factors. Population density depends on biotic factors like predators, parasitic diseases, inter and intra-specific competitions, waste of accumulation, etc. All these biotic factors can distribute the population from any specific area and hence decrease its population density.

It may be possible that these factors reduce the population density from any observable region while increasing the density at another region where suitable conditions are found for that group of organisms.

The abiotic factors that affect the population density are weather, natural disasters, pollution, etc. These factors affect the mortality rate of that population. For example, seeds are not growing due to bad weather and external conditions or some group of trees died due to fire in the forest. So in those conditions, population size is reduced which decreases the population density of that area.

Population recovery/resilience.

Population resilience is the recovery of any population after it is disturbed or damaged. In reality, it is very difficult to understand the effects of all the interactions between biotic and abiotic factors on a population. Sometimes a biotic factor influences the population to grow well (i.e. weather for plants) but any abiotic factor seriously damages that population (forest fire). As the population lives in the form of communities, it is difficult to extract the exact conclusion about any population density.

However, the recovery of any population maintains a balance in its ecosystem. For example, a plant population is affected by drought conditions but rain in that region can recover its size and numbers. In this way, a cycle runs that maintains a balance but this cycle is continuously disturbed by human activities that resist a population to recover.

Some specific features of any population, population demographic properties, geophysical environment and conditions, and other interacting populations (community) mostly help a population to recover.

Age structure.

Ag structure always plays an important role in establishing a population. Although all the individuals in a population did not contribute equally to maintaining a population. The individuals of specific ages and features (i.e. mature individuals) contribute more to establishing a population than the younger ones. For example, a seedling or very young plant contributes less to a population establishment than a tree that can produce seeds so that population size can be increased.

Population fecundity.

It is the measure of the reproductive rates of any population within a period. It varies over time based on the population size, density, and age structure of any population. External conditions like biotic and abiotic factors also affect fecundity.

For a given population of plants factors like season rains, intensity of sunlight, day-night time duration, weather, etc affect the fecundity value. Its value is maximum in ideal conditions that we thought in our mind or on paper but in reality, its value is less than our expectations because many factors are acting on a population.

Natality.

Natality simply means the birth or seed germination in plants. It refers to terms like fusion, birth, seed germination, hatching, etc. In ideal conditions, the natality rate is maximum when there is no competition for space and unlimited resources. But in reality, it is not possible so it is controlled by population density and other environmental factors.

Mortality.

Mortality is totally opposite to natality. It is the measure of number of deaths in a population. Many biotic and abiotic factors contribute to the process of mortality.

Sex ratio.

It is the counting of male and female organisms in a population. For plants, this counting is a little bit different because plants may be monoecious and diecious.

Monoecious plants have both male and female parts on a single individual plant.
Dioecious plants have a male part on one plant and a female part on the other plant within the same species.

Mostly monoecious plants maintains 1:1 of male and female parts while in other type of plants and animals, this ratio can vary. Knowing of sex ratio in a population density is important for researchers because they have to predict the future conditions of a population.

Life tables.

A life table is an age-specific summary of the survival pattern of a population. It is drawn based on the age structure of different individuals in a population. For example, the life table of a plant’s population has the following information:

Number of seeds = 100000

Seedlings = 10000

Over-wintering plants = 1000

Flowering plants = 100

Fruiting plants - 10

Survivorship curves

The representation of data obtained from the life tables of a population in the graphical form (which usually forms curves in graphical representation) is called survivorship curves.

It varies with different types of populations. For example, normally plants produce large amounts of seeds but some of them can germinate and form a new mature plant. So when we represent this data in a graphical form, we will observe that from thousands of seeds, only a few hundred are germinated and this will form a curve on the graph (I.e Type |||).

In the same way, animals or humans produce fewer new ones but take care of their new generation, and because of this caring most of them move towards maturity (i.e. Type |). some of the birds lay their eggs but are not able to protect and care for them (i.e. Type ||).