In case of external fertilization, gametes are at the mercy of surrounding. Gametes can get blown off by wind, washed away by water or eaten by predators. So, most of the gametes perish before fertilization. To ensure the survival of the species, it is necessary to produce as many gametes as possible so that at least some of them would be able to carry out fertilization. Hence, organisms which exhibit fertilization produce a large number of gametes.