Organism and Environment

Each and every living organism has its specific surrounding medium of environment with which it continuously interacts and remains fully adapted. The environment is the sum total of physical and biotic conditions influencing the responses of the organisms. The life supporting environment of planet earth - the biosphere is composed of following 3 chief media - air, water and soil, which are the components of 3 major sub-divisions of the biosphere - atmosphere, hydrosphere and lithosphere respectively.

The Three Components of the Biosphere

These media are not completely isolated from each other, some of the atmospheric gases are dissolved in all natural waters and some moisture is present almost everywhere in the atmosphere.

The material contributions of each of the three subdivisionsof the biosphere towards the maintenance of living matter

Physical hierarchy

Every organism consists of parts that can be arranged in increasing levels of complexity. Each higher level is more complex than the one below it. The smallest structural unit of matter, both non-living and living are protons, neutrons and electrons. These particles combine in a specific manner to form the atom. The atoms in turn combine to form the next larger unit called as molecules or chemical compounds. The complexes of compounds combine to form the organelles, which are sub-microscopic bodies. Organelles by themselves do not qualify as living units. A specific combination of organelles forms a living unit called the cell.

Hierarchy of levels in the organisation of living organisms and nonliving matter

Biological hierarchy

A cell is regarded as the structural and functional unit of life. Thus a living organism must consist of at least one cell. Cells combine to form higher units, the tissues, which in turn come together and constitute the next higher unit, known as the organs. The organs occur in groups, to form the organ systems. All the organ systems together form an organism.

The units at each level are specialised entities. They have their own structure and function, but no unit can exist in strict isolation in nature. They are mutually dependent on one another. Moreover, they derive the energy and materials needed for their survival from their environment. At any level of organisation there exists an intimate relationship between a unit and its environment.

Ecological hierarchy

Ecological hierarchy begins at the level of the organism and proceeds to levels of greater complexity.

Ecological hierarchy

Organism

An organism, may be small unicellular or multicellular. Organisms have a fixed life span and organised life cycle comprising of birth, growth, maturity, old age and death. They are always derived from pre-existing ones through the mechanism of reproduction. The offspring resembles the parents very closely.

Population

A group of the same species inhabiting a specific geographical area at a particular time form a population.

Biotic community

The organisms of all the populations that live in a defined area or habitat and interact in various ways with one another to form a self-sustained unit are collectively called a biotic community.

Examples: Animal community, plant community or microbial community.

Ecosystem

Living organisms cannot live isolated from their non-living environment. Living and the non-living environment interact with each other to form a stable system. A natural self-sufficient unit of the world comprising a biotic community (living organisms) and its abiotic physico-chemical environment (non-living environment) is known as an ecosystem. The geographic area providing uniform conditions for life is called biotope. (Greek - bios-life; topos - place).

An ecosystem has two components, the physical part or biotope and living part or biotic community. Ecosystems may be natural or artificial. A pond, a lake and a forest are a few examples of natural ecosystems. An aquarium, a garden are instances of artificial ecosystems.

A landscape is a unit of land with a natural boundary having patches of land each representing different ecosystems.

Biosphere

All the ecosystems of the world together form the biosphere. It consists of all the living organisms and all the regions of the earth and its atmosphere where they live.

A Biome is a large unit characterised by flora and fauna found in a specific climatic zone. For e.g., a desert or a tropical rain forest.

In ecology, levels of biological organisation range from organism to biosphere Species

All the populations of the same kind of organisms form a species. As a unit, a population is sub-ordinate to species.

A species is defined as a group of organisms similar in structure, function, behaviour and which can inter breed among themselves. But there are examples where the individual members of the same species have considerable differences in structure and function depending on their stages of development, sex or specialisation of the members. For e.g., the larvae and adults of butterfly, moth, frog and mosquito are different in structure as well as mode of life.

Young and adult stages of cabbage butterly

Many animals and plants have races or varieties. In Africa, members of the Zulu tribe are short (1.2 m) and those of Watutsi tribe are tall (2.2 m). Mongoloids, negroid, caucasoid and australoid have considerable physical difference between them regarding skin colour, hair, nose, lip etc.

There are several varieties of dogs such as greyhound, bulldog, alsatian, irish terrier. All these races or varieties of the same kind of organism inter breed freely among themselves. Hence, they belong to the same species.

Some breeds of dogs showing variation within a species

So a species is defined as a group of individuals which

a) are similar in essential structural, functional and behavioural characteristics in the normal adult forms.

b) visibly differ from all other kinds of organisms.

c) inter-breed in nature with one another forming fertile offsprings.

d) are reproductively isolated from other such groups.

e) have a common ancestry.