An endangered species is any animal or plant that’s at risk of extinction due to a sharp decline in population or the depletion of its critical habitat.

The United States Endangered Species Act of 1973 (ESA) established criteria to designate a species as either endangered or threatened, and ruled that animals on the endangered species list would be protected under federal law. The ESA also required a recovery plan for species suffering from critical habitat destruction. In the decades since the act was passed, the United States government has significantly weakened the ESA by making it easier to remove endangered animals from the list, decreasing protections for threatened species, and considering economic factors when deciding whether a species deserves protection.

In 1964, the International Union for Conservation of Nature established the Red List of Threatened Species (or the IUCN Red List), a comprehensive global endangered species list. This list uses scientific criteria to evaluate a species’ risk of extinction and develop conservation efforts to protect the thousands of endangered species and subspecies around the world.

There are five primary reasons that a wild animal species might become endangered.

1. Habitat loss: In some cases habitat loss occurs naturally, but the primary cause of habitat destruction is human activity—including land development for agriculture, housing, and industry.

2. Climate change and pollution: Climate change and pollution have dramatically altered the ecosystems where many now-endangered species previously thrived.

3. Overhunting, overfishing, and poaching: Populations of animals like elephants, rhinos, and tigers have dramatically shrunk as a result of hunting and poaching.

4. Invasive species: A non-native species that’s been accidentally or purposely brought to a new ecosystem can wipe out a population of animals that previously thrived in that area.

5. Disease: Genetic variation lets a species adapt to environmental changes, but low-population species have less genetic variation because of inbreeding and thus also have less disease resistance.

The IUCN Red List of Threatened Species classifies species into nine groups.

1. Extinct (EX): The species no longer exists.

2. Extinct in the wild (EW): The species exists only in captivity, cultivation, or outside its native range.

3. Critically endangered (CR): The population of the species is critically low.

4. Endangered (EN): There is a high risk of the species’ extinction in the wild.

5. Vulnerable (VU): The species is at high risk of human-caused extinction.

6. Near threatened (NT): The species is close to reaching a high risk of extinction in the near future.

7. Least concern (LC): The species is unlikely to face risk of extinction in the near future.

8. Data deficient (DD): There is insufficient information to properly assess the species’ conservation status.

9. Not evaluated (NE): The IUCN has not yet studied a species’ conservation status.

In total, 32,000 species are at risk of extinction. All of the species below are notable examples, each categorized as either critically endangered, endangered, or vulnerable on the IUCN Red List of Threatened Species.

1. African wild dog: 1,409 remaining

2. Amur leopard: 84 to 100 remaining

3. Asian elephant: Fewer than 50,000 remaining

4. Black-footed ferret: 370 remaining

5. Black rhino: About 5,600 remaining

6. Blue whale: 10,000 to 25,000 remaining

7. Bluefin tuna: Remaining population unknown

8. Bonobo: 10,000 to 50,000 remaining

9. Chimpanzee: 172,700 to 299,700 remaining

10. Eastern lowland gorilla: Remaining population unknown

11. Galápagos penguin: Fewer than 2,000 remaining

12. Giant otter: Fewer than 5,000 remaining

13. Giant panda: 1,864 remaining

14. Hawksbill turtle: Fewer than 25,000 nesting females remaining

15. Hippopotamus: 115,000 to 130,000 remaining

16. Javan rhino: 72 remaining

17. Kakapo: 209 remaining

18. Leatherback sea turtle: Between 34,000 and 36,000 nesting females remaining

19. Mountain gorilla: More than 1,000 remaining

20. North Atlantic right whale: 300 to 350 remaining

21. Orangutan: 119,346 remaining

22. Polar bear: 22,000 to 31,000 remaining

23. Red panda: Fewer than 10,000 remaining

24. Snow leopard: 4,080 to 6,590 remaining

25. Sumatran elephant: 2,400 to 2,800 remaining

26. Sumatran rhino: Fewer than 80 remaining

27. Tiger: Around 3,900 remaining (including both subspecies of tiger, Panthera tigris tigris and Panthera tigris sondaica)

28. Western lowland gorilla: Remaining population unknown

29. Western Steller sea lion: About 46,000 remaining