Organisms and Population


It is the branch of science that studies the reciprocal relationship between organisms and their physical and biological environment.


It is a specific physical place or locality occupied by an organism, population and community which has a particular combination of abiotic or environmental factors.

Ecological Niche:

Elton defines the Ecological niche as "An Ecological refers to an animal place in the biotic environment and its relation to its food and enemies or its functional role in the ecosystem."

Biological Organisation:

Molecular level of Organisation:

Individual level of organisation:

Higher level of organisation:

Ecology is basically concerned with 4 levels of Biological Organisation.

  1. Organism
  2. Population
  3. Community
  4. Biomass


Every Individual of a species is called an organism. It forms the basic unit of study of Ecology. It is a quantitative unit and can be counted or measured.
Organisms with similar features and potential to interbreed among themselves and produce fertile offspring constitute a population.


It is defined as the total number of individuals of a species in a specific geographical area.
Intraspecific competition exists in a population for basic needs.


It is constituted by an assemblage of populations of all different species that live in an area and interest among themselves.

Biomass (Biome):

A biomass is a large unit consisting of major vegetation type and associated fauna in a specific climate zone.


  • It is the total sum of all biotic and abiotic factors that surround and potentially influence an organism not only in survival but also in reproduction.
  • The rotation of earth around the Sun and its even tilted axis causes annual variation in temperature and thereby seasons which leads to the formation of major biomass.
  • Regional and local variation within each biome leads to the formation of a wide variety of habitats.

Ecological factors:

They are those components of the environment which determine the variations in the physical and chemical nature of different habitats.
  • Abiotic/ non-living /physical factors
  • Biotic /living /biological factors

  • It is the difference of hotness and coldness of a substance. The average temperature on land varies seasonally.
  • It decreases progressively from the equator towards the pole and from plane to mountain top.
  • The physicochemical functions and geographical distribution of organisms is governed by the temperature of the organism.
On the basis of thermal tolerance, animals are divided into the following two categories:
Eurythermal organisms: These can tolerate a wide range of temperature fluctuations.
For example- toad, man etc.
Stenothermal organisms: These can tolerate a small range of temperature.
For example- polar bear, fishes etc.

Thermotaxis: Temperature changes also induce certain behavioural patterns. For example- thermotaxis, that is orientation towards heat. Ticks locate their warm blooded host, Rattlesnake, and vipers can locate/ detect their prey by body heat.

  • It covers more than 70% of the Earth's surface. The chief source of soil water is atmospheric precipitation. About 45% of water flows into river, 20% percolate into soil and 35% loss by evaporation.
  • The productivity and distribution of plant is dependent on water.
  • For aquatic organisms the chemical composition, pH, salinity and temperature of water are important.
The aquatic animals are divided into two categories on the basis of natural water in which they can grow:
Stenohaline: These can withstand only freshwater or saline water.
Euryhaline: These can live in water with a wide range of salinity.

Light is an important factor for life to exist on the earth as plants prepare food, release oxygen through photosynthesis only in presence of light.
Light also affects the colour vision, eye size, skin pigmentation, reproduction and biological productivity of the organisms.

Parameter of light affecting the living organisms:
The activities of both plants and animals depend upon three parameter of light-
  • Light intensity
  • Light quality
  • Light duration
Small herbaceous plants and some shrubs that live under the canopy of forest trees are adapted to photosynthesise under very low light intensity they are called Sciophytes.
The distribution of red, brown and green algae at different depths in the ocean depends on light.

Soil is the surface layer of your land. The nature and properties of soil in the given place at dependent upon:
  • Climate
  • Weathering process
  • Whether soil is segmented Or transported

Properties of soil:

  • Soil composition, grain size and aggregation determine the water holding capacity of soil.
  • The physical and chemical properties of soil such as grain size, pore size, pH, mineral composition determine the type of plant that can grow in a particular habitat and the type of animal that depends on these plants.

How do organisms live in their environment:

Many organisms have evolved a relatively constant environment that allows all the biochemical reactions to proceed with minimum efficiency. Their constancy could be in terms of optimal temperature, osmotic concentration of body fluids.
Other organisms have following four possibilities
  • Regulation
  • Conformation
  • Migration
  • Suspension

The organisms maintain homeostasis and/or behaviour means and ensure a constant body temperature (thermoregulation), osmotic concentration (osmoregulation).


  • Most of the animals or nearby all plants can't maintain a constant internal environment. Their body temperature changes within the ambient temperature.
  • The osmotic concentration of body fluid of aquatic animals changes with those of ambient water or osmotic concentration, such animals are called osmoconformers.
  • Thermoregulation is an energy expensive process, heat loss or gain is a function of surface area.
  • Since small animals have a large surface area relative to their volume, they lose body heat very fast in cool conditions; then they have to spend much energy to generate body heat through metabolism.
  • Hence, small animals are not found in polar regions. For example- Hummingbird


  • By this mechanism, the organisms can move temporarily from the stressful conditions in the habitat to other habitats with hospitable conditions.
  • Birds undertake long distance migration during winters. For example- Bharatpur National park hosts more than 1000 of migratory birds from Siberia and extremely cold Northern regions.

Those organisms which can't migrate suspend their metabolic function during the stressful period and resume their function at the return of favourable conditions.
  • Hibernation: In frogs, certain reptiles and Polar bears.
  • Aestivation: Some snails, fishes and some zooplanktons.


  • These adaptations are the useful morphological, physiological, behavioral changes developed in a living organism over a long period of time to adjust according to a particular environment.
  • Adaptation makes organisms more fit to their environment and increases their chances of survival and reproduction or the continuation of Race.

Adaptation for thermoregulation:
Thermoregulation in animals:
  • The birds and mammals (warm -blooded animals) those of colder regions are large sized than those of warmer regions. For example- polar bear. It is so as a large sized body will have less surface area per unit weight so loses less heat.
  • The tail, snout, ear and legs of mammals are relatively shorter in the colder apart than in the warmer part. (Allen's rule)
  • The fishes found in the water of low temperature tend to have more vertebrae than those of warmer water.
  • The mammals, birds and insects of tropical warmer regions are darker in colour and heavily pigmented than those of Arctic regions.

Thermoregulation in plants:
Plants protect themselves from extreme temperature by the development of thick cuticle, thorn, thick cortical layer, dense hair port, mucilage, tannins etc.

Adaptation for or Xerophytic conditions:
Desert animals and plants face the problem of scarcity of water. These organisms develop different methods for either moisture getting or moisture conservation.

Osmoregulation in desert plants:
  • They have thick cuticles on the leaf surface.
  • They have sunken stomata.
  • They also have a special photosynthetic pathway called CAM pathway (Crassulacean Acid Metabolism).
  • In plants like opuntia leaves are modified into spines to reduce transpiration and photosynthesis is carried out by flattened stem.

Osmoregulation in desert animals:
  • Desert rabbit, kangaroos, pocket Mouse (Peronathus) feed on dry seeds and vegetation.
  • Camels have water cells in the wall of the stomach to store metabolic water formed during the biological breakdown of fat stored in its hump.
  • In desert mammals the number of sweat glands is greatly reduced or absent.
  • Desert insects have waxy waterproof layers on their body surface.

Adaptation in aquatic plants:
  • Aquatic plants have evolved aerenchyma for buoyancy and floating.
  • They have a waxy covering to avoid damage by water.

Aquatic adaptation in organisms:

  • Body is generally streamlined/ boat shaped which offers minimum resistance of water in swimming.
  • Absence of neck and loss of heads, mobility in fishes.
  • Lightness in the endoskeleton to increase the buoyancy.
  • Aquatic animals develop different types of locomotory structures.
  • Swim bladders act as hydrostatic organs.
  • The aquatic animals are adapted to derive oxygen from water either through body surface or gills.


Population is defined as the "Total number of individuals of a species in a specific geographical area that can interbreed under natural conditions to produce fertile offspring as a unit of biotic community."

Characteristics of population:
Population size and density
Birth on Natality rate
Death or Mortality rate
Age distribution
Population growth
Age ratio
Population fluctuation and cycles

Population Size and Density:
Population size indicates the total number of organisms of a species present in a specific geographical area at a specific time.
Population density of a species is a number of individuals per unit area per unit volume.

Population density can be calculated as population density = N/S
Where N= Number oof individual s in a region
S= Number of unit areas in a region

Birth rate (Natality rate):
It is generally expressed that number of birth per thousand individuals of a population per year.

Death rate (Mortality rate):
It is expressed as number of deaths per thousand individual per year.

Age distribution:
The relative abundance of organisms of various age groups in the population is called age distribution of population.
That three ecological age groups are:
  • Pre- reproductive
  • Reproductive
  • Post reproductive

Population growth:

It is indicated by the change in population density is in terms of time.

The size of any population keeps changing with time depending upon the factors like-
  • Food availability
  • Weather
  • Competition
  • Predation pressure
The density of a population in a given habitat during a given population, density changes due to four basic processes namely-
  • Mortality
  • Natality
  • Immigration
  • Emigration

Equation for Population Growth:
N (t+1) = Nt + [(B+I) - (D+E)]
B = Birth rate
I = Immigration
D = Death rate
E = Emigration
Nt = Population density at time t

Population Growth Models:
Growth of a population can be expressed by a mathematical expression called Growth curve in which logarithm of total number of individuals is plotted against time factor.
Growth model shows the specific and predictable pattern of growth of a population with time.

There are two models of population growth
Exponential growth model
Logistic growth model

Exponential growth model (J-Shaped growth curve, Geometric growth model):
When the resources availability is unlimited in a habitat, the population growth in an exponential for geometric fashion.
The equation is:
dN/dt = (b - d) × N
Let b - d = r
The equation is:
dN/dt = rN
'r' is the constant which is called intrinsic rate of natural increase.
When a population shows exponential growth, the curve plotted with 'N' in relation to time assumes J - shaped.
Derivation of Population Density:
Population density of a population showing exponential growth is
Nt = N0ert
Where, Nt = population density after time t
N0 = population density at time 0
r = Intrinsic rate of natural increase
e = The base of natural logarithm

Logistic growth curve (Sigmoid/S-shaped growth curve or Verhaulst growth curve):
No population can continue to grow exponentially, as the resource availability becomes limiting at a certain point of time.
A population showing Logistic growth shows a sigmoid curve when we plot 'N' in relation to time. Such a growth is called Verhulst pearl logistic growth.

$\frac {dN}{dt} = rN [\frac {K - N}{K}]$
Where, N = population density at time t
r = Intrinsic rate of natural increase
K = carrying capacity
This group model is more realistic in nature because no population can sustain exponential growth exponentially as they will be competition for basic needs.

Population Interaction:

Living organisms can't live in isolation and they do interact in different ways to form biological communities.
Interspecific Interactions:
The interactions of populations of two different species are called interspecific interactions.
Such interactions are following types:
  • Beneficial to both in case of mutualism.
  • Beneficial to one and harmful /detrimental to the others as in Parasitism and Predation.
  • Beneficial to one and neutral to other as in Commensalism.
  • Detrimental to both as in competition.
  • Detrimental to one and neutral to other as in Amensalism.

It is defined as the interspecific interaction in which both the interacting species are benefitted.
For example-
  • Lichens show an intimate mutual relationship between a fungus and a blue-green algae (cyanobacteria) or an algae. Hence, the fungus helps in absorption of nutrients and provides shelter while algae prepares the food.

  • Mycorrhizae are a mutualistic association between fungi and roots of higher plants.

  • The Mediterranean orchids, Ophrys employs sexual deceit to get it's flower pollinated. In this Orchid one petal of flower resembles the female of the bee species in size, colour etc. The male bee perceives it as a female and pseudo copulates with it. During this process the pollen grains are dusted on his body. When the Bee is attracted by another flower of the same species and repeats the process, pollen grain falls on stigma that is pollination achieved.

It is an interspecific interaction when one animal called Predator kills and consumes the other weaker animal called prey.
Predation is a nature's way of transferring the energy fixed by plants to higher trophic level.

Predators play following important role in predation:
  • They act as conduits for energy transfer to higher trophic levels.
  • They keep the Prey population under control and balance the ecosystem.
  • They help in maintaining species diversity in a community by reducing the intensity of competition among the competing Prey species.

Prey species evolved various defence mechanisms to reduce the impact of predation.
For example-
  • Certain insect species and frogs have camouflage (cryptic coloration) to avoid detection by their predators.
  • Some animals for example Monarch butterflies are highly distasteful to their Predators.
  • This butterfly species accumulates a Chemical by feeding on a poisonous weed used during its Caterpillar stage.
  • Some prey are poisonous and hence, avoided by predators. For example- Dart frogs like phyllobates and dendrobates.
Plants have certain morphological features against herbivores.
  • Thorns in Bougainvillea
  • Spines in Cactus, Acacia, etc.
  • Calotropis produces a highly poisonous glycoside that is a cardiac poison.
  • Nicotine, opium, Quinine etc are the Chemicals produced by plants.

It is an interspecific interaction where one species called parasite depends on other called host for food and shelter.
In this association, parasites are benefited and the host is harmed or killed.
Parasites have evolved one or more of the following adaptation-
  • Loss of unnecessary sense organs.
  • Presence of hooks /adhesive organs/ suckers.
  • High Reproductive capacity.
  • Parasites maybe of following types- Ectoparasites
  • Endoparasites
Ectoparasites- They are those parasites which depend upon the external surface of hosts.
For example- Marine Copepods on fish, Cuscuta Hedge plants.
Endoparasites- They are those parasites which take shelter within the body of the host in the organs like intestine, liver, lungs etc.
Brood parasitism:
Brood parasitism refers to the phenomenon in which one parasite bird species lay its egg in the nest of another Bird species. For example- Koel and cuckroo female lay their eggs in crow's nest.

It is defined as the interspecific interaction where one species is benefited while the other species (host) is neither benefited nor harmed i.e., neutral.
For eg:
  • Orchids grow as epiphytes on mango or other fruit trees for getting shelter.
  • The clown fish living among sea anemones to get protection from their predators which stay away from stinging tentacles of the sea anemones.
  • Cattle Egret and grazing cattle association.

It is the interaction between two species in which one species is harmed and other is neither benefited nor harmed i.e., neutral.
For example-
  • Phenomenon of antibiosis in which a species for example Penicillium notatum secrete antibiotic and inhibit the growth of harmful bacteria.
  • Roots of black walnut secrete Juglone chemical which inhibit the growth of seedling of apple, tomato etc.

It is a type of interaction either among individuals of the same species (intraspecific) or between individuals/populations of different species (interspecific).
It is believed that competition occurs among closely related species when they compete for the same resources that are limited.
But it is not true always because of the following reasons:
  • Completely unrelated species can also compete for the same resources. For example- visiting flamingos and resident fishes competing for zooplanktons in the lake of South America.
  • Gause's Competitive Exclusion Principle: It states that two closely related species competing for the same resources cannot exist together as the competitively inferior one will be eliminated. But this is true only when the resources are limiting and not otherwise.
  • They have also pointed out that species facing competition might evolve mechanisms that promote coexistence rather than exclusion. Mac Arthur had shown that five closely related species of Warblers living on the same tree were able to avoid competition and coexist due to behavioral differences in their foraging activities (eating).