It was announced on Tuesday that Sudan, the world's last male white northern white rhinoceros, has passed away due to age-related complications. The CEO at the Ol Pejeta Conservancy, where Sudan lived, said that the famous rhino would be remembered as a symbol.
"He was a great ambassador for his species and will be remembered for the work he did to raise awareness globally of the plight facing not only rhinos, but also the many thousands of other species facing extinction as a result of unsustainable human activity," Richard Vigne said, according to AP.
The International Union for Conversation of Nature's Red List of Threatened Species aims to keep track of those species, with a catalog 75,000 assessments of species, subspecies, varieties and subpopulations. The IUCN also keeps track of factors that could contribute to the greater decline of each species such as pollution, human intrusions and climate change.
With so many factors at play, it can be difficult to determine which are the "most" endangered of becoming extinct. IUCN, though, does classify some species as "critically endangered."
The Center for Biological Diversity -- a nonprofit group that advocates for endangered species -- has taken action to try to save a variety of different species. Here's a look at five of the critically endangered species the group has advocated for.
The Center for Biological Diversity calls the pangolin "the world's most illegally trafficked mammals." The scale-covered, termite-eating creature curls up into a ball when threatened. Two types, the Chinese Pangolin and the Sunda Pangolin, are listed as critically endangered by IUCN.
Dusky gopher frog
This frog's name comes from the fact that it burrows in the ground. As its habitat is threatened, its population has now been reduced to three small ponds in Mississippi, according to the The Center for Biological Diversity.
Medium tree finch
This bird, one of the original 14 finches identified by Charles Darwin on the Galapagos Islands, is threatened by Philornis downsi, "an introduced parasitic fly whose larvae feed on the finch's nestlings," according to the The Center for Biological Diversity. It is also threatened by habitat destruction.
Southern Bluefin Tuna
Declines in bluefin tuna populations have occurred due to overfishing, according to The Center for Biological Diversity. One type, the Southern Bluefin Tuna, has become critically endangered, but the nonprofit has been advocating against the consumption of all bluefin tuna.
Red wolves were once extinct in the wild, according to The Center for Biological Diversity. After four pairs were released in the wild in North Carolina more than 30 years ago, the population climbed to 130 by 2006. Recently, though, the population has begun to decline again.
To learn more about the thousands of other endangered species and how you can help, visit any of these websites:
IUCN Red List
The Center for Biological Diversity
World Wildlife Fund
These five species are critically endangered