Natural Environment

All living and non-living things that occur naturally on earth constitute the natural environment. All living organisms are dependent on the environment for their survival. Their life is regulated by the environment and in turn they influence the environment.


Living organisms draw their nutrition and oxygen for survival from the environment. In the process, plants and animals interact with each other and also their physical environment. An ecosystem is defined as a biological environment consisting of all organisms living in a particular area, as well as the non-living physical components of the environment with which the organisms interact.

An ecosystem can either be natural or human-designed. All ecosystems that exist in nature are Natural ecosystem. They can either be terrestrial or aquatic. Grasslands and deserts constitute the terrestrial ecosystem while rivers, ponds and oceans form the aquatic ecosystem. On the other hand, man-made or human designed ecosystem is an artificial ecosystem e.g. gardens, aquarium, crop fields, etc.

Components of Ecosystem

Both, non-living and living things constitute an ecosystem. They are termed as abiotic and biotic components. All of these components vary over space and time.


Abiotic components are the non-living physical and chemical factors in the environment of an ecosystem. Examples: Sunlight, Temperature, Precipitation, Water or moisture, Soil, Air, etc.


Biotic components are the organisms which include plants, animals and micro-organisms in an ecosystem. Examples: Primary producers, Herbivores, Carnivores, Omnivores, Detritivores, etc.

Biotic Community

Biotic community refers to populations of various kinds of organisms living together and sharing the same habitat. An ecosystem houses several biotic communities which interact with each other. For example, you can observe populations of different kinds of birds, insects and many other animals on a tree, living in the same environment, mutually sustaining and interdependent. This assembly of different organisms constitutes a biotic community. Depending on the mode of nutrition, members of a biotic community are categorised into autotrophs, heterotrophs and saprotrophs.


All plants (except for a few parasitic plants) can manufacture their own food by the process of photosynthesis. Since the plants provide food to all the animals directly or indirectly they are also called producers. Autotrophs form the basis of any biotic system as they trap solar energy to manufacture food for all.

In terrestrial ecosystems, the autotrophs are mainly the rooted plants while in aquatic ecosystem, floating plants called phytoplankton and shallow water rooted plants called macrophytes are the examples of autotrophs.


Heterotrophs are called consumers which feed on plants and animals. Consumers include herbivores (that eat plant material) and carnivores (which eat other animals).


Saprotrophs also called decomposers feed on dead and decaying matter. They break down the complex organic compounds of dead plants and animals into simpler forms and return them back into the environment. Decomposers form an important link between the living and non-living component of the ecosystem. Some bacteria and fungi belong to this category.

Food Chain and Food Web

A simple food chain basically consists of producers, herbivores and carnivores.For example, small fish feeds on phytoplanktons which in turn is being eaten by a bigger fish. This constitutes the food chain.

A simple food chain in a terrestrial ecosystem links the trees and shrubs (producers), the giraffes (herbivores that eat trees and shrubs) and the lions (carnivores that eat the herbivores). Each link in this chain is food for the next level and is said to be at a particular trophic level.

Food chain is defined as succession of organisms in an ecological community that constitutes a passing on of food energy from one organism to another as each consumes a lower member and in turn is preyed upon by a higher member of the food chain.

Different types of food chain can exist in an ecosystem.

Grazing food chain

Grazing food chain is found both in aquatic as well as grassland ecosystem. It is the most common food chain found in the terrestrial ecosystem.

Food chain in a terrestrial ecosystem:

  • Grass → Rabbit → Wolf → Tiger
  • Grass → Grasshopper → Frog → Snake → Hawk

Food chain in an aquatic ecosystem:

  • Phytoplankton → Amoeba → octopus → Seal → Whale
  • Phytoplankton → Zoplankton → Prawn → Mosquito → Small Fish → Big Fish → Crocodile

Detritus food chain

This type of food chain starts from dead organic matter. The dead organic matter is broken down into simple nutrients by micro-organisms like fungi and bacteria. These simple nutrients and decomposers are then consumed by smaller carnivores which in turn become food for larger carnivores.

  • Micro-organism → Earthworm → Frog → Snake → Peacock

Food Web

Most animals form a part of more than one food chain and eat more than one kind of food in order to meet their food and energy requirements. These interconnected food chains form a food web.

Significance of food chain and food web

  • They help to maintain ecological balance.
  • They help in understanding the feeding relations among organisms.
  • Energy flow and nutrient cycle takes place through them.

Energy Flow in Ecosystem

Food provides energy and thus in a food chain, energy is passed from one link to another. This energy flow is unidirectional - the energy which is transferred from one trophic level to the next does not come back. When a herbivore eats, only a fraction of the energy (that it gets from the plant food) becomes new body mass; the rest of the energy is lost as heat or is used up by the herbivore to carry out its life processes. When the herbivore is eaten by a carnivore, only a small amount of total energy is received by the carnivore. Because of the large amount of energy that is lost at each link, the amount of energy that is transferred gets lesser and lesser going up the food chain.

Biogeochemical or Nutrient Cycles

There is a constant need of nutrients by the biotic community for their survival and they take these from the environment. Nutrients in the form of oxygen, carbon dioxide, nitrogen, phosphorus, sulphur or water exists in a definite amount in the environment. The amount of these nutrients however varies in different parts of an ecosystem at a given time. But these elements are never lost and nature has its own method of replenishing them in a cyclic manner.

The movement of these nutrients in a cyclic manner in the environment constitutes the biogeochemical cycles. A biogeochemical cycle is the cycle in which nitrogen, carbon, and other inorganic elements of the soil, atmosphere, etc. of a region are converted into the organic substances of animals or plants and released back into the environment.

It is a cyclic pathway by which a chemical element or molecule moves through the environment unlike energy flow which is unidirectional.

Carbon Cycle

The carbon cycle is the biogeochemical cycle by which carbon is exchanged between soil, water and atmosphere (air) of the earth. It is the most important cycle of the earth and allows for carbon to be recycled by all of its organisms.

Nitrogen Cycle

Nitrogen cycle is the biogeochemical cycle that describes the transformation of nitrogen and nitrogen-containing compounds in nature. Atmospheric nitrogen is the biggest source of nitrogen. Green plants absorb nitrogen in the form of nitrates and nitrites from the soil and water. Animals get nitrogen when they feed upon plants. Nitrogen is an essential component of proteins and nucleic acids in living organisms.


The nitrogen cycle is studied in five steps:

1. Nitrogen Fixation

Nitrogen can be fixed in two ways:

a. Lightening during cloud formation: Nitrogen and oxygen combine with each other to form oxides of nitrogen in the atmosphere by lightening. These nitrogen oxides then dissolve in rain water and on reaching the earth’s surface becomes a part of the soil and water.

b. Free living micro-organisms present in the soil and by the symbiotic bacteria in the root nodules of certain leguminous plants: Microbes like the blue green algae and bacteria fix the atmospheric nitrogen into nitrites and nitrates. These nitrogenous compounds are then released into the soil.

2. Nitrogen Assimilation

Plants absorb nitrogen in the form of nitrates to prepare amino acids. This nitrogen is then taken up by the animals in the form of proteins through the food chain.

3. Ammonification

The proteins in the body of the animals are broken down into simpler form like urea and ammonia. These are then removed from the body along with urine and excreta. Dead plants and animals also return nitrogen to the soil as ammonium compounds. These ammonium compounds are then converted to ammonia by ammonifying bacteria.

4. Nitrification

Conversion of ammonia into nitrates is called nitrification. Nitrifying bacteria like Nitrosobacter and Nitrosomonas found in the soil convert ammonia into nitrate. Some other bacteria present in the soil convert ammonia into nitrites. Some of this nitrates and nitrites are again taken up by the plants for their nutrition.

5. Denitrification

Denitrifying bacteria like Pseudomonas and Clostridium living in the soil reduce the soil nitrites and nitrates into nitrogen which is returned back into the atmosphere.

Water Cycle

Water is very essential for all living organisms but the earth has a limited amount of water. The water keeps going from one component of an ecosystem to another component in a cyclic manner which is called the water cycle.

Population Interaction

What is population?

Population is a group of similar individuals living in a particular geographical area. Populations of different species of organisms live in the same ecosystem. When organisms encounter one another in their habitats, they can influence each other in a number of ways. Some interactions are harmful to one or both of the organisms.  


Mutualism in an interaction between individuals belonging to two different species, that benefit both members. Lichen is a complete entity formed by the association of an alga and a fungus. The main body of the lichen is formed by fungus. The alga manufactures food for itself as well as for the fungus, while the fungus provides water, minerals and shelter to the alga.


Commensalism is an interspecific interaction where one species benefits and the other is unaffected (neither harmed nor benefitted). Commensal relationships may involve one organism using another for transportation or for housing. For example, hermit crab lives in gastropod shell to protect their body. Sucker fish attaches itself to the under surface of shark and gets a free ride. It is thus protected from its predators and is also widely dispersed in this way.


It includes one organism living in or on the body of another living organism from which it derives nourishment and in the process harms its host. For example: tapeworm living in the intestine of man.


A close interaction between two or more different organisms of different species living in close physical association. For example, pollination of flowers where the flowering plants are cross pollinated by the bees which benefit by getting nectar from the plants. Plants pollinated in this manner produce less pollen than do plants that rely on the wind to transfer pollen.

Population Growth

Population is an aggregate of individuals belonging to the same species. Population of any species does not remain static.

Population growth is the change in the number of individuals of any species in a population at a given time. The size of the population depends upon the density, natality (birth rate), mortality (death rate), population dispersal, age distribution, and environmental resistance that the population has to face.

Population Dispersal

It is the movement of individuals or groups of living organisms by which they expand the space or range within which they live. Dispersal operates when organisms leave the space that they have previously occupied, or in which they were born and settle in new areas. It affects the size of the population. Population dispersal can be of two types:

  1. Emigration: It is the permanent outward movement of the organisms from a given population. It decreases the size of the local population.
  2. Immigration: It is the permanent inward movement of the organisms from outside into a given population. It increases the size of the local population.

Environmental Resistance

It is the resistance presented by the environmental conditions to prevent the species from reproducing at maximum rate and thus limiting a species from growing out of control. Environmental resistance includes both abiotic factors like temperature, space, etc. and biotic factors like natural enemies. Environment keeps a check on the rise in the population size.

The physical and biological factors that together prevent a species from reproducing at its maximum rate is called environmental resistance.

Carrying Capacity

It is the maximum population that the environment can sustain indefinitely.

Growth Curves

The growth of a population can be expressed in the form of a mathematical expression called the growth curve. If the number of organisms is plotted against time, you get a curve which is called the population growth curve. Population growth curve has a characteristic shape. There are two forms of growth curves - J-shaped growth curve and S-shaped or sigmoidal growth curve.

S-shaped Growth Curve

When a small number of organisms first enter a previously unoccupied area, the growth is slow at first as it adapts to new conditions and establishes itself. Reproduction in these organisms takes place after a certain period of time. This is called the lag phase. During this phase, both natality and mortality remain small and relatively constant.

Gradually, the growth becomes rapid and the population increases rapidly. Now, the natality rate increases while the mortality remains low. This is called growth phase. The rapid rise in population is because of the availability of plenty of food and also because there is no competition between the biotic potential and the natural resources.

But the number of organisms cannot continue to increase at a faster and faster rate because eventually something in the environment will become limiting and cause an increase in the number of deaths. For animals, food, water or resting sites may be in short supply, or predators or disease may kill many individuals. Plants may lack water, soil nutrients or sunlight.

Eventually, the number of individuals entering the population will become equal to the number of individuals leaving it by death or migration and the population size becomes stable. This part of the population growth curve is called stable phase where the natality rate and mortality rate are approximately equal. The graph so obtained is S-shaped and is called the sigmoid curve.

J-shaped Growth Curve

The J-shaped growth curve describes a situation in which the population growth continues in an exponential form until the environmental resistance becomes effective. As the environmental resistance becomes effective, there is a stiff competition for survival and the growth rate stops abruptly. There is a sudden increase in mortality (population crash).