Usually, geneticists postulate a minimum number of 600 pairs for a population to survive without incest problems. However, the data provided in this books show that the minimum viable population size depends solely on the species in question. By means of existing small bird, mammal, fish, insect and other animal populations, the minimum number of individuals necessary for survival is analyzed. The effects of random event on small populations, like volcanic eruptions on islands are also investigated. An important scientific study, of great interest to a wide range of conservationists.