What it Means to be a Threatened Species

Tigers are currently grouped into six extant subspecies on the basis of distinctive molecular markers: Amur Tiger, Northern Indochinese Tiger, Malayan Tiger, Sumatran Tiger, Bengal Tiger, and the South China Tiger (possibly extinct). Three subspecies previously recognized on the basis of morphology are extinct: the Bali Tiger, Javan Tiger, and the Caspian Tiger. (Photograph by IUCN Photo Library/Steve Winter)

Most people know what it means to be a threatened species—it’s something that’s rare and may become extinct. What isn’t often explained is how we know something is threatened and who decides whether a species is threatened or not.

Read almost any article about species in peril and there will be some reference to the level of threat faced by that species. Words like Vulnerable, Endangered, and Critically Endangered are usually mentioned without any real explanation of where these categories come from or how they are determined. Part of the reason for this is that it’s just easier to take the author’s word for it and move on. However, in science, the author’s word is not good enough—statements must be supported by data and methods and these must be presented for open (and often scathing) review by other scientists. This process of peer-review is the foundation of all credible scientific work and species science is no exception.

When determining the threat status of a species, the International Union for Conservation of Nature (IUCN) has developed a set of peer-reviewed categories and criteria based on the best available science. When a species is assessed it is assigned one of nine categories: Not Evaluated, Data Deficient, Least Concern, Near Threatened, Vulnerable, Endangered, Critically Endangered, Extinct in the Wild, or Extinct. The category to which a species is assigned is based on a rigorous evaluation of a set of four broad criteria. These include an estimate of current population size, geographic range, reductions in population size, and the probability of extinction in the wild.

Scientists throughout the world collect data on the population size of species in as many places as possible to get an idea of how many individuals there are in nature and how far they roam. Often, despite their herculean attempts, researchers are unable to count every single living individual. In reality, species are tricky—most of them are quite small and make a habit of not being conspicuous.

What scientists must then rely on are proven statistical methods of counting a sample of individuals to come up with estimates of total population size. These estimates are then combined with where a species lives—its geographic extent or range. The likelihood of extinction in the wild is what the estimated population size and ranges are compared against. Scientists must base their assessments of extinction probability on life histories, habitat requirements, and threats facing a species, each of which must be appropriate, documented, and defendable. Following this process, an assessed species will be assigned to one of the IUCN Red List categories.

Bunaea alcinoe_NE_Craig Beatty
This Common Emperor Moth (Bunaea alcinoe) has not yet been evaluated by the IUCN Red List. It’s worth noting that the Common Skate (Dipturus batis) is a Critically Endangered Species, despite its name. (Photograph by Craig Beatty)

Based on these criteria more than 21,000 species of the 71,576 species on The IUCN Red List are threatened with extinction. For instance, it is estimated that fewer than 400 Ethiopian Wolves (Canis simensis) remain in the wild. They are restricted to seven isolated mountain enclaves in the highlands of Ethiopia at altitudes above 3500 meters. Their habitat is being encroached upon by agriculture and canine distemper disease, and climate change may continue to increase the threats to this species.

Ethiopian Wolf
Photograph by Brent Huffman/UltimateUngulate

The IUCN Red List currently records 799 Extinct species and without appropriate conservation action this number will increase. Sixty-one species are listed as Extinct in the Wild (existing only in captive populations) and 4,286 are listed as Critically Endangered. Each one of these species is the tip of an evolutionary branch, millions of years in the making. Unfortunately, an unknown number of species have been pushed to extinction before they were described by science or assessed by the IUCN Red List.

People and organizations all over the world make it their life’s work to conserve species, biodiversity and habitats. We feel a ground-swell of support as more people around the world are realizing just how dependent we are on the biodiversity that surrounds us.  The interactions among species and between nature and people cascade through our lives in ways that we are sincerely starting to recognize on a global level. The IUCN Red List is an indispensable asset for building awareness and informing communities, conservationists, and governments in making the best decisions about the future of life on this planet, including our own.


The International Union for Conservation of Nature is the world’s oldest and largest global environmental organization made up of more than 1,000 organizations, as well as 10,000 individual scientists and experts working on conservation around the globe. Perhaps we are best known for the IUCN Red List of Threatened Species, which is the global standard for species science and conservation information and the connection to human livelihoods and is celebrating 50 years of conservation action in 2014.