Biological diversity is being depleted worldwide through habitat loss, introduced species, overexploitation, and pollution. These conditions reduce population sizes and increase susceptibility to chance fluctuations (environmental, catastrophic, demographic, and genetic), and so increase the risk of extinction. Human intervention is required to ensure the survival of many species: 11.7% of mammals and 10.6% of birds are categorized as threatened species by the World Conservation Union (IUCN). A recent assessment from the Conservation Breeding Specialist Group of the IUCN classified 38% of vertebrates as threatened.