Principle of Ecology

  • It is the scientific study of the processes influencing the distribution and abundance of organisms, the interactions among organisms, and the interactions between organisms and the transformation and flux of energy and matter.


  • The term ecology was first coined in 1869 by the German biologist Ernst Haeckel. It has been derived from two Greek words, ‘oikos’, meaning home or estate and ‘logos’ meaning study. The emphasis is on relationships between organisms and the components of the environment namely abiotic (non-living) and biotic (living).


  • Ecology not only deals with the study of the relationship of individual organisms with their environment, but also with the study of populations, communities, ecosystems, biomes, and biosphere as a whole.
  • A large community unit characterized by a major vegetation type and associated fauna, found in a specific climatic region is a biome.
  • Biosphere-
  • Biomes refer basically to terrestrial areas. The aquatic systems like the seas; rivers etc. are also divided into distinct life zones on basis of salinity.
  • Ecosystem- A communities of organisms and their physical environment, interacting as an ecological unit.
  • Community- A group of organisms consisting of a number of different species that live in an area and interact with each other.
  • Population- A group of organisms consisting of a number of different populations that live in defined area and interact with each other.
  • Organism- It is the smallest unit of study in ecology and refers to an individual member of a species.


  • Habitat is the physical environment in which an organism lives. Each organism has particular requirements for its survival and lives where the environment provides for those needs.
  • A habitat may support many different species having similar requirements. For example, a single ocean habitat may support a whale, a sea-horse, seal, phytoplankton and many other kinds of organisms. The various species sharing a habitat thus have the same ‘addresses. Forest, ocean, river etc. are examples of habitat.
  • The features of the habitat can be represented by its structural components namely- space, food, water and cover or shelter.
  • Earth has four major habitats-Terrestrial, Freshwater, Estuarine (Where rivers meet the ocean) and Ocean.


  • In nature, many species occupy the same habitat but they perform different functions. The functional characteristic of a species in its habitat is referred to as “niche” in that common habitat.
  • Habitat of a species is like its ‘address’ (i.e. where it lives) whereas niche can be thought of as its “profession” (i.e. activities and responses specific to the species).
  • The term niche means the sum of all the activities and relationships of a species by which it uses the resources in its habitat for its survival and reproduction.


  • Every organism is suited to live in its particular habitat. You know that the coconuts cannot grow in a desert while a camel cannot survive in an ocean. Each organism is adapted to its particular environment.
  • An adaptation is thus, “the appearance or behaviour or structure or mode of life of an organism that allows it to survive in a particular environment”.


  • A valid theory of evolution was propounded by Charles Darwin and Alfred Wallace in 1859. This theory has been extended in the light of progress in genetics and is known as Neo-Darwinism. It has the following features:
  • Organisms tend to produce more off springs that can be supported by the environment.
  • Mutation (a change in genetic material that results from an error in replication of DNA) causes new genes to arise in a population. Further, in a sexually reproducing population, meiosis and fertilization produce new combination of genes every generation, which is termed recombination. Thus members of the same species show ‘variation’ and are not exactly identical. Variations are heritable.
  • An evolutionary force which Darwin termed natural selection, selects among variations i.e. genes that help the organism to adopt to its environment. Such genes are reproduced more in a population due to natural selection.
  • Those offspring which are suited to their immediate environment have a better chance of surviving, reaching reproductive age and passing on the suitable adaptations to their progeny.
  • Evolution thus results in adaptation and diversity of the species.


  • The number of species surviving in the world today is the outcome of two processes speciation and extinction.
  • Speciation is the process by which new species are formed and evolution is the mechanism by which speciation is brought about.
  • A species comprises of many populations. Often different populations of a species remain isolated due to some geographic barrier such as mountain, ocean, river, etc.
  • The most common way a population undergoes speciation is by geographic isolation.
  • The members of a population of a species live in a particular environment and are capable of breeding with the member of another population of the same species.
  • The population then becomes separated into two completely isolated populations by a barrier which prevents their interbreeding and gene exchange. The isolating mechanism may be a physical barrier like water, mountain, and ocean represent geographical isolation.
  • Ecological isolation caused by differences in temperature, humidity, pH etc. in the environment of the two populations.
  • Reproductive isolation caused by interference in interbreeding between members of different populations of species i.e. species. When two populations of a species are unable to interbreed due to reproductive barrier.
  • Mutations occur randomly in isolated populations giving rise to new variation within each sub-population of these mutations those that help to adapt to the environment are reproduced in greater numbers in the next generation due to natural selection.
  • In other words since no two environments are identical, natural selection pressures that occur on each separate sub-population are different, depending on local conditions such as climate, disease, predators etc. Natural selection affects each sub-population differently and so different ‘variations’ caused by nutrition or recombination in different subpopulations get established. With the passage of time, the sub populations become more and more different from each other.


  • Biotic communities are dynamic in nature and change over a period of time. The process by which communities of plant and animal species in an area are replaced or changed into another over a period of time is known as ecological succession.
  • There are two types of successions (i) Primary succession and (ii) Secondary succession.

Primary succession

  • It takes place an over a bare or unoccupied areas such as rocks outcrop, newly formed deltas and sand dunes, emerging volcano islands and lava flows as well as glacial moraines (muddy area exposed by a retreating glacier).
  • It is much more difficult to observe than secondary succession because there are relatively very few places on earth that do not already have communities of organisms.
  • Furthermore, primary succession takes a very long time as compared to secondary succession as the soil is to be formed during primary succession while secondary succession starts in an area where soil is already present.

Secondary succession

  • It is the development of a community which forms after the existing natural vegetation that constitutes a community is removed, disturbed or destroyed by a natural event like hurricane or forest fire or by human related events like tilling or harvesting land.
  • It is relatively fast as; the soil has the necessary nutrients as well as a large pool of seeds and other dormant stages of organisms.


The biological community of an area or ecosystem is a complex network of interactions.

The interaction that occurs among different individuals of the same species is called intraspecific interaction while the interaction among individuals of different species in a community is termed as interspecific interaction.


Type of interaction Effects of interaction
Amensalism one species is inhibited while theother species is unaffected
Predation Predator–prey relationship: one species (predator) benefits while the second species (prey) is harmed and inhibited.
Parasitism Beneficial to one species (parasite) and harmful to the other species (host).
Competition Adversely affects both species
Commensalism One species (the commensal) benefits, while the other species (the host) is neither harmed nor inhibited
Mutualism Interaction is favourable to bothspecies
Neutralism Neither species affects the other

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