Environment
All organisms are intimately dependent on the environment from which they derive their sustenance. Organisms from virus to man are obligatorily dependent on the environment for food, energy, water, oxygen, shelter and for other needs. The environment is defined as 'the sum total of living, non-living components; influences and events, surrounding an organism'. The relationship and interaction between organism and environment are highly complex. For the convenience of study, environment is broadly classified into two components: abiotic (or non-living) and biotic (or living).
You should realise that the environment is not static. The biotic and abiotic factors are in a flux and keep changing continuously. The organisms can tolerate changes in environment within a certain range called 'range of tolerance'.