The rates of species extinction are currently 1,000 times greater than natural background levels, but this may soon increase to 10,000 times. Over 99% of modern extinctions, all of which are leading to the rapid loss of ecosystem services, can be attributed to human activity. The IUCN has developed quantitative criteria that assign species to nine conservation categories based on their risk of extinction. Species that are Extinct in the Wild (EW), Critically Endangered (CR), Endangered (EN), and Vulnerable (VU) are officially considered “threatened with extinction”. The five other categories are Extinct (EX), Near Threatened (NT), Least Concern (LC), Data Deficient (DD), and Not Evaluated (NE). Furthermore, species with the following characteristics are particularly vulnerable to extinction: species with small populations, species with declining populations, species with restricted distribution ranges, species with one or only a few small populations, species that are exploited by people, and species with critical symbiotic relationships. Small populations are at high risk of extinction because they are vulnerable to several deleterious genetic factors, as well as demographic and environmental stochasticity. Such populations often require intensive management to prevent them from becoming a victim of an extinction vortex. De-extinction as a scientific field aims to revive extinct species using methods such as selective breeding and cloning. But this practice is controversial, and not practical with current technology.