(a) Migration refers to movement of animal populations from one place to another and their subsequent return to avoid unfavourable climatic conditions .
(b) It is the layer of atmospheric that extends between altitudes of about 16-50 kilometers . It has a ozone layer (ozonosphere) between altitude 25-50 kilometers.
(c) Groups of organisms belonging to several different species that live together in the same area of habitat and interact through trophic and spatial relationship to form a self-sustained unit is called biotic community .
(d) All the ecosystems of the world together form a biosphere.
