Population is a group of organisms of the same species which occupies the same habitat and forms a single functional unit of biotic community. This ecological definition doesn't fit well into genetics. For genetic studies, a population refers to a group of interbreeding individuals which belong to a single species. The smallest subgroup of individuals which can interbreed in a population is called a deme. A population generally consists of one or more such demes.

Population can be characterized and analyzed by mainly three different factors.
a. Population density
b. Natality
c. Mortality

a. Population density

It is the size of population in a definite unit of space. Density is the total number of individuals per unit area in a given time.
Crude density refers to the number/ biomass of the organisms per unit of total space.
Ecological (specific) density refers to number /biomass of the organisms per unit of habitable area, i.e. the area which can actually be colonized by the population.
The population density is never static for most of the populations and hence indices are employed to compare the densities at different time intervals. These indices of relative abundance represent the occurrence or sighting of the organisms at a given interval of time.
The frequency of occurrence is also useful to estimate the percentage of sample area which is occupied by the species.

The Lincoln index represents the population density by the popular mark-recapture method. This is given by the following

Population estimate (N) / no. of animals captured and marked in a sample s1 at time t1 (n1)
= number of animals captured in second sample at time t2 (n2) / number of marked animals in sample s2 at time t2 (m2)
Then N/n1= n2/m2

Hence N = n1 *n2 / m2

Relative abundance is suitable for calculating population density of larger animals and terrestrial plants. However, in vegetation studies the factors such as density, dominance and frequency are combined to give the importance value specific for each species which can provide a more precise descriptive study of the fauna.

b. Natality

Natality rate refers to the ability of the population to give rise to new offspring. This rate is equivalent to birth rate in human demographic studies
Natality rates can be either crude or realized one.

Crude/absolute natality rate is the rate at which new individuals are formed in units of time in a population. This is usually maximum natality, the maximum theoretical limit of production of new organisms under ideal conditions when the resources are assumed to be not limiting the population growth. The reproductive rate is assumed to be dependent only on the physiological factors and not on ecological factors.

Specific natality rate or birth rate refers to the number of new individuals in unit time per unit of population.

Ecological or realized natality rate is the increase in population under a given environmental condition. This is not constant and varies with the age structure and other environmental factors.

c. Mortality

This is the death rate of the individuals in the population. Like natality this is equivalent to death rate in human populations.

Ecological or realized mortality rate is the number of individuals lost in a given environmental condition.
Minimum mortality refers to the minimum loss under ideal conditions of environment. This value is constant for a population and is often determined by physiological longevity. This value is often larger than ecological longevity.

The survival rate is the number of surviving individuals in a population expressed as 1-M if M stands for death rate.

A life table is a systematic representation of the specific mortalities of the population at different stages of growth.
There are three main types of survivorship curves which reflects the population density.
Type I- The population death rate is rather low till the end of the life span. The curve is highly convex. E.g.: humans, deer, sheep etc,
Type II - It is a diagonal straight line exhibiting constant rate of mortality over all age groups. E.g.: birds, mice, rabbits, etc. This is more often depicting the amount parental care in these species.
Type III - This curve is highly concave with the death rates at its peak during younger stages of growth. E.g. oysters, shell fish, oak trees.
The curve may be stair-step type if the survival rates differ greatly in successive stages of life. E.g.: butterflies

Apart from the three common characteristics there are many other factors which are unique to a population. Spatial structure of populations are determined by the three factors such as

• Density
• Dispersion
• Distribution

Factors which influence the population density have been described in the previous article.

I. Dispersion is the spatial and temporal distribution pattern of the individuals in the population. They describe the position of individuals in a population in relation to each other. There are mainly three forms - random, regular and clumped (aggregate).

In random distribution, the individuals are having equal probability of being found in anywhere within a given area. The position is unrelated to the presence or vicinity of the individuals of the population. The population is relatively uniform and there are no aggregates found. This is however very rare in actual environmental conditions. The intra and interspecific interactions and ecological interactions are more or less neutral.

In regular dispersion, the individuals are equidistantly spaced from each other. This is usually found in cultivated lands where equidistance is maintained between the crops. The interactions between the individuals are antagonistic in nature and resources get depleted locally.

In clumped dispersion, the individuals are found in aggregates. Most populations in nature fall into this category. There are well marked areas of abundance separated by areas of low abundance of the species. The interactions are more agonistic with clumping of organisms at local abundance of resources.

II. Distribution is mostly limited by the physical environmental factors such as moisture, geographical structure, temperature etc.
This distribution has been well depicted in terms of age of individuals as in age pyramids. Age structure shows the number of individuals in each population classified according to their age groups.

Age distribution affects the natality as well as the mortality rate. The ratio between the different age groups determines the reproductive status of the population. This can be classified into pre-reproductive, reproductive, and post- reproductive distributions. Humans have equally distributed distributions for the three age groups.
Age pyramids are a convenient way to represent the age distribution.
Based on the nature of age pyramids, populations can be classified into '
a. stable population
Pre reproductive as well as reproductive age groups have almost the same number of individuals with the smallest number of individuals in the post reproductive age group. The graphic representation is a bell shaped curve.
b. expanding population and
The successive generations are numerous in number than the original parent population. There will be more individuals in the pre reproductive age group than the other age groups. The graph is usually pyramidal in shape.
c. diminishing population
When the pre reproductive and reproductive age groups are almost similar in size, and there is drastic reduction in the post reproductive group size, it results in a diminishing population. The graph is urn shaped.

Dispersal is the process by which the individuals get added up or removed from a population. The usual causes of dispersal of individuals are migration, immigration and emigration.

Growth forms are characteristic patterns depicting increase in population. There are basically two patterns- J shaped and S- shaped.

J-shaped growth form has the populations increasing in an exponential pattern. The growth ceases when there is environmental resistance or in the presence of any other limiting factors.
The upper limit is a sharp and abrupt and is represented by intrinsic rate of natural increase, r. The maximum value of r is called biotic potential or reproductive potential.

The biotic potential represents the maximum ability of the individuals in a population to reproduce. This can be increased by having a higher growth rate, longer reproductively active life span, and by having the maximum number of offspring at relatively younger age groups when the energy can be efficiently utilized more for such functions.

In the sigmoid or S shaped growth pattern, the individuals of the population increases slowly and slows down after reaching a certain limit of equilibrium. This is a logistic model of population growth.

The upper limit of the sigmoid curve at which the population is considered to attain equilibrium is called carrying capacity. This is usually represented by K. There are various factors which limit this value including predation, competition, environmental conditions etc. The value is not a constant since it includes all the factors into consideration and these factors can vary widely over time.

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