The International Union for Conservation of Nature (IUCN) reports that tens of thousands of species are threatened with extinction. But what does it mean for a species to be ‘threatened with extinction’? How do researchers evaluate extinction risk?

The IUCN Red List of Threatened Species is regarded as the definitive source of extinction risk. Every year, the IUCN publishes its latest assessment of the status of each evaluated species. This article from Our World in Data looks at how species are categorized and assessed for their extinction risk.

There are 9 categories of extinction risk and these are listed below and in the graphic:

  • Not Evaluated (NE)
  • Data Deficient (DD)
  • Least Concern (LC)
  • Near Threatened (NT)
  • Vulnerable (VU)
  • Endangered (EN)
  • Critically Endangered (CR)
  • Extinct in the Wild (EW)
  • Extinct (EX)

Species are evaluated across four key metrics. However, it’s important to note that this doesn’t include every possible way to assess the future health of a species: aspects such as food supplies or future hunting threats aren’t defined as measure risks. Any of the following would indicate that a species is at risk:

  1. If the population size is very small.
  2. If the population size is small and it’s declining.
  3. If there has been a large decline in population size (regardless of its total size).
  4. The geographic range of the species – the area or region it lives in – is small and declining.

Within each of these measurements, researchers set thresholds that detail whether a species is vulnerable, endangered, or critically endangered. 

The number of described species in the world is now over 2 million (with more than 1 million being insects, which the IUCN does not assess).3 The IUCN has assessed more than 150,000 species for their extinction risk. A large number, but a small fraction of the total.

They aim to re-evaluate every species in a peer-reviewed process every five years – every ten years at most. Many at-risk species are monitored and evaluated more frequently. That’s an incredible achievement. But it’s still just a fraction of the species that exist. We don’t know the extinction risk for most of the world’s species.

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