Population

In the ecological system, the organisms are categorized into four levels of the organization, they are as follows, the organism level is also known as the individual level, the second is the population level, third being the community level and finally the fourth which is the largest level of organization which is a biome. To understand population biology, it is important to understand the basics of ecology. Ecology can be defined as the interdisciplinary science that studies the relation between the organism’s growth, behavior and environment. The population can be defined as the group of individuals of the same species inhabiting the same area. In this article, we will learn about the basics of population biology, which includes the understanding of population, various attributes of a population, change in population, factors affecting such change in population, and finally, the type of population growth models.

Attributes of Population

There are certain attributes of the population that only this level of organization has. According to population biology, these attributes decide whether the group to be qualified as a population or not, these attributes are also used to study population biology, the pattern of growth in the given population. According to the concepts of population biology a population can be described as the group of individuals of the same species inhabiting the same area. It is important to note that the definition of population differs in genetics, in genetics the population can be described as the group of interbreeding individuals of the same species, which is reproductively isolated from the members of the other species. 

Population attributes are the features of the population that defines its characteristic like growth pattern, population density and, population regulation, these attributes are as follows,

1. Sex ratio

2. Mortality rate

3. Natality rate

4. Dispersion

5. Population density

Sex Ratio

At the individual level, the individual can identify as male or female, whereas in population biology or at the population level the quantitative measurements are called sex ratio. The sex ratio can be defined as the number of females per male, it is the ratio between the number of males the female. It is generally considered for sexually reproducing dioecious populations.

An example of a sex ratio can be a population where 60% of the population is female and 40% is male. The ideal sex ratio is 1:1 where there are 50% females and % males, this diversion is the result of various causes like low survivorship, societal norms, and misconception in the population. In a study of human population, it shows that we have the highest difference in sex ratio where there is a comparatively low number of females than male. It is majorly because of female foeticide.

Mortality Rate

Mortality can be defined as the death of an individual in the population. The mortality rate is also commonly known as the death rate. The mortality rate can be defined as the number of individuals who died during a given period of time. The mortality rate is often calculated as the number of deaths per 1000 individuals per year. It is important to note that the mortality rate is time specific.

mortality rate can be mathematically expressed as the following,

\[\frac{\text{Total Number of New Death in a Defined Period of Time}}{\text{Total Number of the Individual at the Given Population in Start}}\]

The mortality rate can be age-specific, ethnicity-specific, or other parameters. An important type is the crude death rate; it is defined as the total number of deaths which is inclusive of all the reasons for the death. 

Natality Rate

The natality rate is also known as the birth rate. The term natality is more commonly used in population biology when describing a study of the human population. Natality is defined as the birth of an individual in a population, whereas the mentality rate refers to the number of individuals produced per female per unit of time.  They are an important factor used in the calculations of the dynamics of the population. In population biology, natality can be divided into two groups: absolute natality or realized natality.

Absolute natality, it is also known as the maximum or the physiological natality, it can be defined as the maximum number of individuals that can be produced by the female in an ideal condition.  Ideal condition refers to the condition where there is no limitation of resources and no competition.

Realized natality is also known as ecological or actual natality. It can be defined as the number of births per individual per unit of time in their normal ecological habitat. 

There are two terms that are widely used in the study of population biology namely, fecundity and fertility. It is important to understand the difference between these terms to understand the population dynamics in the scientific study of the human population. 

Fecundity can be described as the maximum reproductive output potential of an individual under ideal environmental conditions

Fertility can be described as the actual reproductive performance of an individual under prevailing environmental conditions.

Dispersion 

Dispersion can be defined as the spatial and temporal distribution pattern of the individual of the population. There are two groups or classifications of the method of dispersion are known as random dispersion and clumped dispersion. 

Random dispersion can be defined as dispersion where the position of one individual is unrelated to the other individual.

Clumped dispersion is also known as the aggregated dispersion, it is the natural condition in which populations are found in their ecological habitat. In this type of dispersion, the position of one individual affects the position of the other, they are interrelated. 

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Age Class and the Age Structure

It is important to understand that at any given point in time the population can be studied according to age class. The age class is the parameter that is used to categorize different individuals of the population according to characteristics of interest. Age class is also sometimes more commonly referred to as the age group. The class is generally categorized on the basis of the reproductive phase of an organism. The population is generally categorized into the following

1. Pre reproductive age class

2. Reproductive age group

3. Post reproductive age group

The age structure can be defined as the proportion of individuals in each age group or class. They are used to plot the age pyramid.

The age pyramid is the graphical illustration showing the number of individuals in different age classes in a population. Age pyramids are typically used for the scientific study of the human population, specifically the demography of the population. There are three types of age pyramid they are as follows,

1. Triangle Pyramid

2. Bell-Shaped Pyramid

3. Clover Leaf Pyramid 

Triangle Pyramid - triangle pyramid is formed when the population is still growing. Triangle pyramids are the indicators of the expanding population

Bell Pyramids - Bell-shaped pyramids are generally found in the population where there is no substantial growth in population, but neither the death rates are very high. These pyramids are the indicators for a stable population. The number of individuals in the pre-reproductive phase and post-reproductive phase is the same.

Clover Leaf Pyramids - clover leaf pyramids are the indicators of the population that is diminishing. In this type of pyramid, the number of individuals in the pre-reproductive phase is lower than the reproductive phase.

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Population Growth

Population growth can be described as the change in the number of individuals in a population with time. The change can be positive or negative. Positive change is associated with an increase in the number of individuals. The negative growth can be associated with the decline in the population. Change in the population can occur due to the following reasons,

1. Birth of individual

2. Death of individual

3. Immigration

4. Emigration

The change in closed population is mainly attributed to the birth and death rate of the population. Population gain is due to the increase in birth and decrease in death rate.

The change in population in an open population is due to all the above mentioned factors. An open population can be defined as the population where immigration ( incoming of the individual) and emigration ( migration of individuals outside the population). When there is a net increase in the sum of the birth and immigration in the population there is a net positive change in the population.  

Types of Population Growth

There are two types of population growth pattern they are namely, exponential growth and logistic growth

Exponential Growth- It is the growth pattern where the number of individuals increases in a geometric pattern. The mathematical equation is

\[\frac{dN}{dT}\]= rN

N= number of individual

rN= rate of intrinsic increase

Where the rate of change of population size is equal to the intrinsic rate of increase multiplied by the number of individuals.

Logistic growth is the growth pattern where the resources are limited, it is defined by carrying capacity which can be defined as the population of a particular species that a particular system can support indefinitely. Mathematical equation is

\[\frac{dN}{dT}\]= rN \[\frac{K-N}{N}\]

Where K is is the carrying capacity. 

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