Living organisms are all around us, even deep under the oceans and in the snow covered Arctic and Antarctica. There are organisms which are single celled and microscopic, as well as animals as large as the elephant, the rhinoceros, the hippopotamus and the whale.

The enormous variety of organisms is termed biodiversity (bios means life and diversity means variety). There is not only diversity in size among organisms but also in complexity. For example, bacteria are simple single celled organisms and humans are made of a trillion cells and are highly complex.

All organisms have come to exist on earth because of evolution and are related through ancestry. The humans are at the top of the evolutionary ladder. It is sad that lots of different kinds of organisms have been lost due to the impact of human activities. Therefore, we have to be conscious and aware so that damage to the earth on which we live along with other organisms, is avoided.

Organisms live up to 8 km in air and up to 5 km below sea level. This part of the earth which supports life is the biosphere. Biosphere has diverse ecosystems such as the pond, river, oceans, and mountains, deserts. Various kinds of organisms or different species live in these ecosystems. They interact with each other and also interact with the
physical components of the ecosystem such as light, temperature, etc.

Biosphere: The physical part of earth on which organisms can survive.

Ecosystem: Definite geographical region in which various species of organisms live and interact with each other and the physical environment.

Species: Group of very similar organisms which can interbreed to produce fertile offspring.