suppose in a pond ecosystem or a river ecosystem any new exotic species is introduced,like a new fish species for fulfilling food demand,or a plant species for beauty,but these species if started growing largely in that ecosystem,then other species that were already in that ecosystem will be affected.take an example,the nile perch was introduced into lake victoria in east africa led eventually to the extinction of 200 species of cichlid fish in the lake.Parthenium,Lantana,Eicchornia these are also invasive plant species.now if we exterminate all the deers in any jungle ecosystem,there will be loss of its main predator Tiger also,so any species introduction/extermination can easily harm an ecosystem,by these process also a society economically & ecologically.economically it may be useful,sometimes can be harmful also.