Biotic Community

Organisms And Their Environment of Class 12


Biotic community is a grouping different but interacting populations of different species which live harmoniously in a given locality, e.g., pond community, lawn community, forest community.

Each such community may further be split into

  • animal community
  • plant community
  • microbe community

Various members of community do not get assembled at a place by mere chance but after a long drawn struggle for adaptations to the environment, establishment of interactions and interdependence amongst themselves and modifying the environment to suit them. Each member of biotic community is called species. It is represented by a population.


Population is a nearly permanent aggregation of individuals of the same kind which inhabit a particular space or geographical area at a particular time. There may be several, rather numerous, such geographically localised groups belonging to one type of organism or species, e.g., spotted deer in various forests, grass in various localities, bull frogs in different ponds.The different populations of the same organism are often called local populations or deme. Members of a local population freely interbreed. A local population may be adapted genetically to its particular environment. It is then called ecotype. Metapopulation is set of local populations connected by dispersing individuals. Populations can also be considered at the scale of regions, islands, continents or oceans. Even the entire species can be viewed as a population.


Various characteristics of a population are : population density, natality (birth rate), mortality (death rate), age distribution (ratio of one age group to the other), biotic potential, dispersion and growth forms.

Different populations can be compared by measuring these characteristics.

  • Population Density
  • Birth or Natality rate
  • Death or Mortality rate
  • Dispersal
  • Age distribution
  • Population growth
  • Biotic or Reproduction potential
  • Environmental resistance
  • Carrying capacity
  • Sex ratio
  • Age ratio
  • Population fluctuations and Population cycles.

Population density

Population density of a species is the number of individuals of a species per unit area or volume e.g., number of animals per square kilometre; number of trees per hectare; number of phytoplanktons per cubic litre of water.

Population density (P.D.) can be calculated as :

P.D =Biotic Communityor D = Biotic Community

Where N = Number of individuals in a region.

S = Number of unit areas in a region or space

Population of an area is described on the basis of three parameters :

  • Number and kind of individuals of a species.
  • A given space or an area.
  • Time.

Population density reflects the success of a species in any given area. It can be determined by random counting (by counting the individuals in definite and small sized sampling units) or by total counting (e.g. of human population and is done after every ten years, called census).

Population density is end result of interaction of the biotic potential and environmental resistance. It depends upon a number of factors like availability of natural resources, natality, mortality, age structure climatic conditions, etc . It varies from time to time and from one area to another e.g. population density of common grass and other plants is more during favourable rainy season than unfavourable dry season.

Natality or Birth rate

The increase in number of individuals in a population under given environmental condition is called Natality. It is generally expressed as number of births per 1,000 individuals of a population per year. The actual birth rate being achieved under existing conditions is called realized natality. It increases the population size (total number of individuals of a population) and population density.

Mortality or Death rate

It is opposite to natality rate. The loss of individuals due to death in a population under given environmental conditions is termed as mortality. It is commonly expressed as the number of deaths per 10,000 individuals of a population per year. Lowest death rate for a given species in most favourable conditions is called potential mortality, while the actual death rate being observed in existing conditions is called realized mortality. It decreases the population size and population density both.

Dispersal or Population dispersal

The population size and population density of a given area changes from time to time due a movement of majority of individuals at one time or other time during their life cycle. Entry of certain individuals of same species into an area from outside is called immigration, while departure of individuals of same species from an area to outside is called emigration. Immigration increases the population size and density while emigration decreases the population size and density but the species spreads to new areas.

Age Distribution (Age composition)

The relative abundance of a organisms of various age groups in the population is called age distribution of population.

In a population, three age groups are classified :
  • Pre-reproductive
  • reproductive
  • post-reproductive age groups

Age distribution depends upon the natality and mortality, and determines the population growth. With regard to age distribution, there are three kinds of populations.

Rapidly growing or Expanding population.

It has high birth rate and low death rate, so there are more number of young individuals in the population. According to a recent survey, more than 42% of the Indian population consists of children below the age of 14 years, so Indian population is called young population.

Stationary or Stable population It has equal birth and death rates, so population shows zero population growth.

Declining population It has higher death rate than birth rate, so the population of young members is lower than that of old members, e.g. Japan (Ageing population).

Biotic Community

Fig. : Age structure of hypothetical populations which are expected to increase,reuwin stable or decline with the passage of time

Population growth

It is determined by number of individuals added to the population by births (natality) and immigration; and the number of individuals lost from the population by death (mortality) and emigration. If more individuals are added than lost, then the population shows negative growth. But if two rates are equal, then the population will become stationary and is called zero growth.

Population growth = (Births + immigration) – (Deaths + emigration)

Growth of a population can be expressed by following formula :

Nt = No + B + I – D – E

Where Nt = Final population size

No = Initial population size

B = Natality rate

D = Mortality rate

I = Immigration rate

E = Emigration rate

Carrying capacity

The maximum number of individuals in a population that can be sustained indefinitely in a given habitat represents it carrying capacity. It is defined as “Feeding capacity of an environmental of an ecosystem for a population of a species under provided set of conditions”. It is also defined as the “Level beyond which no major increase can occur.” This limit as a constant and represented by K. When a population reaches the carrying capacity of its environment, the population has zero growth rate. So the population generally stabilizes around the carrying capacity.

Biotic or Reproductive Potential

It is defined as the inherent maximum capacity of an organism to reproduce or increase in population number is termed as biotic potential. It is represented by the symbol “r”. It is calculated as the multiple of number of young produced at each reproduction and number of reproductions in a given period of time. If biotic potential is unchecked, the number of any species will soon be overcrowded. However, nature keeps a check on the expression of biotic potential. e.g., if a pair of flies is allowed to reproduce unchecked the fly population may outweigh the earth in a few years.

Environmental resistance

The environmental check on population size, or its biotic potential is called environmental resistance. In nature full biotic potential of an organism or population is never realized, since conditions are rarely ideal. Various harmful environmental (abiotic) factors like non-availability of food and shelter; natural calamities like drought, cloud bursts, floods, fires, temperature fluctuations, accidents, etc. and certain biotic factors like pathogens, parasites, predators etc. check the biotic potential from being realized. The sum of all these inhibitory factors is called environmental resistance. Thus, actual increase is the balance between biotic potential and environmental resistance. Thus environmental resistance does not allow population growth to soar towards infinity.

Population growth forms

Populations have characteristic patterns of growth with time. These patterns are known as population growth forms.

There are two basic population growth forms, designated as

  • J-shaped
  • S-shaped

J-Shaped growth forms: In the case of J-shaped growth form, the population grows exponentially, and after attaining the peak value, the population may abruptly crash. The exponential growth cannot be sustained infinitely because not only environment is ever changing, food and space are limited. For example, many insect populations show explosive increase in numbers during the rainy season, followed by their disappearance at the end of the season. The J-shaped growth form is represented by the following exponential equation : dN/dt = rN where, dN/dt is the rate of change in population size,r is the biotic potential, and N is the population size.

S-Shaped growth forms: It shows an initial gradual increase in population size, followed by an exponential increase and then a gradual decline to a near-constant level. This slow-down following the exponential phase, occurs due to increasing environmental resistance. In such cases, plotting of the rate of increase of population over time gives an S-shaped or sigmoid curve.

Biotic Community

Fig. : Population growth forms : (a) J-shaped, (b) S-shaped. K represents the carrying capacity

The S-shaped sigmoid growth form is represented by the following equation, which includes an expression for environmental resistance :

Biotic Community = (K-N)/K = rN

where dN/dt = rate of change in population size

r = biotic potential

N = population size

Sex ratio

The number of females in a population per 10,000 males is called sex ratio. Average female to male ratio in world population is 1 : 1

Age ratio

On the basis of reproductive period, individuals of a population can be divided into three groups :

  • Pre-reproductive group includes individuals below 18 years of age.
  • Reproductive group includes individuals capable of reproduction (from 20 to 40 years)
  • Post-reproductive group includes individuals between 40 to 70 years.

Population fluctuations and Population cycles

The populations are not stable and do change to a number of extrinsic as well as intrinsic factors.

These variations in the population size are of two types :

Population fluctuations: In these changes, population density tends to fluctuate irregularly above characterized by sudden increase in population in short time which is followed by equally quick decrease in population size. These are caused by random seasonal or annual changes in availability of resources (food or energy) or extrinsic factor (e.g. temperature, rainfall etc.). e.g. more birds during early summer due to their hatching period; more insects during summer months and more weeds in rainy season. Later, many of these birds and insects are destroyed by environmental hazards.

Population cycles:These are regular changes in the population size. In these, population size is nearly constant over long period of time. These are caused by seasonal changes in environment e.g. population cycles (of 3 to 4 years) of lemmings (Lemmus lemmus) of Tundras (Elton, 1942).Lemmings (small mouse-like rodents found in arctic regions of Canada and Norway) increase in their number for a period of about 3 years when it reaches a peak beyond the carrying capacity of that area. They eat up all the available food. In the winter months, the lemmings migrate in large numbers in the sea and swim till they are drowned due to exhaustion. The surviving lemmings multiply and repeat the process.

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