Natural Extinction

This happens when a species declines in numbers gradually but steadily at the end of its evolutionary period on earth. The length of the period depends largely on the success of the species as a whole and its ability to adjust to changes in climate and vegetation and the appearance of predators or (in the case of predators) the disappearance of prey. It is thought that 90% of all organisms that ever lived on earth are now extinct.  It is worth remembering that a species dying a natural evolutionary death is nearly always replaced by new forms or groups.