Environment keeps changing with time and these changes may be unfavourable for an organism e.g., extreme heat or extreme cold. Some organisms, migrate from the unfavourable place to the place with favourable conditions e.g., birds. Some organisms, unable to migrate, might avoid the stress by escaping in time. They do so by reducing their metabolic activity and going into a state of ‘dormancy’. Under unfavourable conditions many zooplankton species in lakes and ponds are known to enter diapause, a stage of suspended development.