The population is defined as a group of individuals of any species living in a habitat. Population ecology is the study of how populations change over time, space and interact with each other. Demography is the branch of population ecology that carries out statistical studies on human populations.

Honey bees in a honeycomb, shoals of fishes in the sea, termites in termite hills are a few examples of populations. Scroll down to learn more about population, its attributes, growth and much more.

Population Definition

In biology, a population is a group of individuals of a species that occupy a specific geographical area and are capable of interacting with the organism of the same or other species. It is the basic unit to study evolution.

Terminologies Associated with Population

1. Species: The basic unit of taxonomic classification comprises organisms having maximum similar morphological and genetic attributes and are capable of interbreeding among themselves.
2. Demography: Demography is the branch of population ecology that carries out statistical studies on human populations.
3. Evolution: It is defined as the change in the heritable characteristics in the organisms of the biological populations over successive generations resulting in the development of a new species.
4. Habitat: The place where an organism lives and has all the necessary environmental factors required for an organism to survive is called habitat.

Population Attributes

1. Population Density
   a. Population density is defined as the number of individuals per unit area at a given time.
   b. It is expressed as the total number of individuals or biomass per unit area or volume.
   c. A population is a dynamic entity and keeps changing over time. Hence density of the population also keeps changing and depends upon intrinsic and extrinsic factors.
   
   i. Intrinsic factors are populations’ response towards it and include intraspecific competition, emigration (going out), immigration (coming in) and physiological and behavioural changes affecting reproduction and survival of individuals.
   
   ii. The extrinsic factors are the interactions of the population with other communities.
2. Age Structure
   a. Age structure is the proportion of the population in different age groups.
   b. Every population has individuals of varied age groups, but their proportions differ, and thus one gets different models.
   c. Age structure models are more popular for human populations and predict population growth.
   d. Populations have three age groups as pre-reproductive, reproductive and post-reproductive.
   e. The proportion of these three age groups decides the shape of a population pyramid. Population pyramids are of three types:
   
   i. Expansive Pyramid or Triangular Shaped Pyramid: The population in younger age groups is large. Populations with high birth rates and low life expectancy show this type of age structure. Here, many organisms are in the Pre-reproductive > Reproductive group > Post-reproductive group.
   
   ii. Declining Pyramid or Urn-shaped Pyramid: The population in the reproductive age group is higher than the pre-reproductive age group. Populations with greater life expectancy, low birth rate, and low death rate generally show these kinds of pyramids. Here, many organisms in Reproductive > Pre-reproductive group > Post-reproductive group.
   
   iii. Stationary Pyramid or Bell-shaped Pyramid: When a population has an almost equal number of organisms in pre-reproductive and reproductive age groups, it is called a stationary pyramid. In such populations, the number of deaths and births roughly balance each other.

3. Sex Ratio
a. It is the ratio of the number of males to the females in a population.
b. In most sexually reproducing species, it tends to \(1:1,\) but for various reasons, it can deviate from this ideal ratio, temporarily or permanently.
c. The human sex ratio is of particular interest to anthropologists.
d. World’s human sex ratio is \(101\) males to \(100\) females. The ratio in local populations varies.
4. Natality– It simply means birth. Population increases by the addition of individuals in two ways- immigration and birth.
a. Natality under ideal conditions is a theoretical maximum figure and is called maximum natality.
b But this is rarely possible and the natality under given ecological conditions is the ecological or realized natality.
c. Maximum natality is constant while realized natality is variable for a given population.
d. Natality is expressed as the rate (number of births) in a given time duration. Natality rate is of two types:
i. Crude or absolute natality rate: It is the number of new births per unit time.
ii. Crude natality rate, for all practical purposes, is expressed per \(1000\) individuals and can be expressed as
\({\rm{Crude}}\,{\rm{natality}}\,{\rm{rate = }}\frac{{{\rm{Total}}\,{\rm{number}}\,{\rm{of}}\,{\rm{births}}\,{\rm{over}}\,{\rm{a}}\,{\rm{period}}}}{{{\rm{Average}}\,{\rm{population}}\,}}{\rm{ \times }}\,{\rm{time}}\,{\rm{period}}\)
iii. Specific natality rate: It is the number of newborns per unit time per unit of population.
\({\rm{Birth}}\,{\rm{rate}}\left( {{\rm{\Delta B}}} \right){\rm{ = }}\frac{{{\rm{\Delta N}}}}{{{\rm{N\Delta t}}}}\)
5. Mortality: Simply means death.
a. Population decreases in size in two ways-death and emigration.
b. The number of deaths of individuals is expressed as rate and referred to as mortality rate. It is expressed as the number of deaths per unit time.
c. The number of deaths under given ecological conditions is referred to as ecological or realised mortality. It varies depending on the size and age composition of the population and environmental conditions.
d. The number of deaths under ideal conditions is constant and referred to as minimum mortality.
e. The survival rate of individuals is of more importance than the death rate in most cases.
6. Population Growth: Natural populations have the ability to grow as individuals mature and reproduce.
a. The size of the population is not static but keeps changing with time.
b. The changing nature of a population is attributed to factors such as natality, mortality, density, age structure, immigration, emigration, environmental conditions etc.
c. But four basic processes are responsible for changes in population. They are:
i. Natality (B): Number of births.
ii. Mortality (D): Number of deaths.
iii. Immigration (I): Incoming of the individuals into the population
iv. Emigration (E): Outgoing of individuals out of the population

Fig: Factors Affecting Population Density

d. Out of these Natality rate \(\left( {\rm{b}} \right)\) and immigration cause an increase in population while mortality rate \(\left( {\rm{d}} \right)\) and emigration cause a decrease in population.
e. So, if N₀ is the density of at time t, then population density at time \({\rm{t}} + 1\) is :
\({{\rm{N}}_{{\rm{t + 1}}}}{\rm{ =  }}{{\rm{N}}_{\rm{0}}}{\rm{\; + }}\left\{ {\left( {{\rm{B + I}}} \right) – \left( {{\rm{D + E}}} \right)} \right\}\)
f. Since \({\rm{B}} > > {\rm{I}}\) and \({\rm{D}} > > {\rm{E}},\,{\rm{B}} + {\rm{I}}\) is approximately \({\rm{B}}\) and \({\rm{D}} + {\rm{E}}\) is approximately \({\rm{D}}.\) Hence,
\({{\rm{N}}_{{\rm{t + 1}}}}{\rm{ =  }}{{\rm{N}}_{\rm{0}}}{\rm{\; + }}\left( {{\rm{B  –  D}}} \right)\)
g. Thus, if \({\rm{B > D,}}\) population increases.
h. If \({\rm{B = D,}}\) the population is stable.
i. If \({\rm{B < D,}}\) population decreases.
j. The birth rate and death rate are the most important factors influencing population growth
k. Another parameter called vital index, i.e. ratio of births to deaths in a population at a given time becomes important.
\({\rm{Vital}}\,{\rm{index}}\,{\rm{ = }}\,\frac{{\rm{B}}}{{\rm{D}}} \times 100\)

Population Growth Models

The growth of population is different under different environmental conditions and one can mathematically model it. The population growth models are of two types, i.e. exponential growth model and logistic growth model.

1. Exponential Growth Model 
   a. When a small population is living in a large habitat, the unlimited availability of food and no competition makes the population grow exponentially or in a geometric fashion.
   b. In such conditions, the increase or decrease in the density \(\left( {\rm{N}} \right)\) of the population at a unit time period \(\left( {\rm{t}} \right)\) can be calculated as:
   \(\frac{{{\rm{dN}}}}{{{\rm{dt}}}}{\rm{ = }}\left( {{\rm{b – d}}} \right){\rm{ \times N}}\)

a. Where \({\rm{dN}}/{\rm{dt}}\) is the rate of change in population density
i. \({\rm{b}}\) is the natality rate
ii. \({\rm{d}}\) is the mort ality rate and
iii. \({\rm{N}}\) is population size initially.
b. If we replace \(\left( {{\rm{b}} – {\rm{d}}} \right)\) with \({\rm{r}},\) then
\(\frac{{{\rm{dN}}}}{{{\rm{dt}}}}{\rm{ = rN}}\)

a. Where \({\rm{r}}\) is called the intrinsic rate of natural increase.
b. The exponential model can also be represented in another form of equation as
i. \({\rm{N = }}{{\rm{N}}_0} \cdot {{\rm{e}}^{{\rm{rt}}}}\)
ii. Where, \({\rm{N }} = \) population size at time \({\rm{t}}\)
\({{\rm{N}}_0}\; = \) original population size
\({\rm{r }} = \) intrinsic rate of natural increase
\({\rm{t }} = \) time
c. In simpler terms, the intrinsic rate of natural increase or biotic potential is the capacity of a population to reach its maximum size under ideal environmental conditions.
d. \({\rm{r}}\) is an important parameter to assess the effects of biotic and abiotic factors on population growth.
e. The value of r is different for different species. For example, for Norway rat, \({\rm{r}}\,{\rm{ = }}\,{\rm{0}}{\rm{.015}}\) while for flour beetle \({\rm{r}}\,{\rm{ = }}\,{\rm{0}}{\rm{.12}}\)
f. If we plot population density \(\left( {\rm{N}} \right)\) against time, in exponential growth we get a \({\rm{J}}\) shaped curve.

Fig: a- Exponential Growth Model Under Non-limiting Growth Conditions
b- Logistic Growth Model Under Limiting Growth Conditions

2. Logistic Growth Model
a. No population will grow in ideal non-limiting conditions and hence exponential growth is not a practical observation.
b. In realistic conditions, a population having a smaller size initially has slower growth rates.
c. Such a pattern of population growth shows three phases as lag phase, log phase and stationary phase.
d. If we plot the population density \(\left( {\rm{N}} \right)\) against time, the logistic growth shows an \({\rm{S}}\)-shaped or sigmoid curve.
e. This type of growth curve is also called as Verhulst-Pearl logistic growth curve and is defined by the following equation
\(\frac{{{\rm{dN}}}}{{{\rm{dt}}}}\,{\rm{ = }}\,{\rm{rN}}\left( {\frac{{{\rm{K – N}}}}{{\rm{N}}}} \right)\)
i. where \({\rm{N}}\, = \) density of population
ii. \({\rm{r}}\, = \) intrinsic rate of natural increase and
iii. \({\rm{K}}\) is carrying capacity

a. Logistic growth can be also represented using the following equation
i. \({\rm{N}}\,{\rm{ = }}\,{{\rm{N}}_{\rm{0}}}{\rm{ \times }}{{\rm{e}}^{{\rm{r}}\left( {\frac{{{\rm{K – N}}}}{{\rm{K}}}} \right){\rm{t}}}}\)
ii. Where, \({\rm{N}}\,{\rm{ = }}\) population size at time \({\rm{t}}\)
\({{\rm{N}}_0}\; = \) original population size
\({\rm{r}}\; = \) natural rate of intrinsic growth (birth rate- death rate)
\({\rm{K}}\; = \) carrying capacity
b. Carrying capacity is the maximum number of individuals in a population that the given ecosystem can sustain under given environmental conditions.
c. Since ideal environmental conditions are rarely possible, the logistic growth model is considered the most realistic one.

Life History Variations

1. Populations evolve to maximize their reproductive fitness in the habitat in which they live.
2. This fitness towards maximizing reproduction is called Darwinian fitness (high r-value).
3. According to ecologists, life-history traits of organisms have evolved in relation to the constraints imposed by the abiotic and biotic components, in relation to the pressure imposed by both biotic and abiotic factors which exist in the surrounding environment or habit of that particular organism.
4. Thus, organisms evolve towards the most efficient reproductive strategy under a particular set of selection pressures. Some of these are as follows:
   i. Breeding Habit: Some organisms breed only once in their lifetime (pacific salmon fish) while others breed many times during their lifetime (most bird and mammals)
   ii. Number and size of offspring: Some produce a large number of small-sized offspring (oysters, pelagic fishes) while others produce a small number of large-sized offspring (birds and animals).

Summary

The population is a set of individuals capable of interbreeding and found in specific geographical locations. Population ecology is the most researched area as it links ecology, genetics and evolution of a population. The population shows certain attributes that individual organisms do not show. Age structure, population density, sex ratio, natality, and mortality attributes.

The growth of populations can be mathematically modelled, and two models, the exponential growth model and the logistic growth model, exist. The exponential growth model shows the growth of populations under non-limiting environmental conditions, while the logistic growth model shows population growth under realistic limiting environmental conditions.

FAQs

Q.1. What will be the world population in \(2050\)?
Ans: Human populations of different continents differ but the estimated figure of the world’s human population by \(2050\) is \(9.74\) billion people.

Q.2. What are the \(10\) largest countries in the world by population?
Ans: The following table shows the \(10\) most populous countries of the world and their population at \(2020\)

Sr. No	Country	Population in millions
\(1\)	China	\(1398.03\)
\(2\)	India	\(1391.89\)
\(3\)	United States	\(329.15\)
\(4\)	Indonesia	\(329.15\)
\(5\)	Pakistan	\(216.57\)
\(6\)	Brazil	\(209.33\)
\(7\)	Nigeria	\(200.96\)
\(8\)	Bangladesh	\(163.67\)
\(9\)	Russia	\(146.73\)
\(10\)	Mexico	\(126.58\)

Q.3. What is the meaning of population?
Ans: A group of individuals of the same species, living in the same geographical area and capable of interbreeding is called a population.

Q.4. How many types of populations are there?
Ans: Statistically there are \(4\) types of the population as finite, infinite, existent and hypothetical.

Q.5. What is the best definition of Population?
Ans: There are various definitions for the population. One of the best definitions for the population is, a group of individuals of the same species, living in the same geographical area and capable of interbreeding is called a population.

Q.6. What are the characteristics of a population?
Ans: The characteristics that describe a population are called population attributes. Age structure, sex ratio, population density, natality, and mortality are the population attributes.

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