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### Theory:

Biosphere
The biosphere is the fourth sphere of Earth that encompasses all other three spheres, i.e. lithosphere, hydrosphere and atmosphere. The biosphere is a place where living things thrive and lives. It consists of flora and fauna, i.e. plants, animals and microorganisms. It extends for about $$20$$ km from the ocean floor to the troposphere. However, most plants and animals live within a very short range of $$1$$ km above and below the Mean Sea Level (MSL). The biosphere is made up of different ecosystems and biomes. All living things (large or small) are classified as species. The place where these species live are called their habitat.

A wide range of plants and animals living in a particular habitat is known as Biodiversity.
Ecosystem
An ecosystem is a community where all living organisms live and interact with one another and also with their non-living environment such as land, soil, air, water etc.
The size of an ecosystem may range from a bark of a tree to the global ecosystem or ecosphere.
Example:
Agricultural land, Pond ecosystem, Lake ecosystem, Forest ecosystem, Desert ecosystem.
The biosphere holds all ecosystems on the Earth and sustains life forms, including mankind.
Ecology: The branch of science that deals with the ecosystem.
Ecologist: A person who studies ecology.