Four of the six great apes, including the Eastern Gorilla – the largest living primate and one of our closest cousins – are now Critically Endangered, just one step away from extinction, according to a new report.

The latest updated Red List of Threatened Species also put the Western Gorilla, Bornean Orangutan and Sumatran Orangutan in the Critically Endangered category, while the Chimpanzee and Bonobo were listed as Endangered.

The list also reports the decline of the Plains Zebra due to illegal hunting, and the growing extinction threat to Hawaiian plants posed by invasive species.

Thirty eight of the 415 endemic Hawaiian plant species assessed for this update are listed as Extinct and four other species have been listed as Extinct in the Wild, meaning they only occur in cultivation.

Released by the IUCN (International Union for Conservation of Nature and Natural Resources) World Conservation Congress, the Red List now includes 82,954 species of which 23,928 are threatened with extinction.

The Eastern Gorilla (Gorilla beringei), which is made up of two subspecies, has moved from Endangered to Critically Endangered – one step away from Extinction – due to a devastating population decline of more than 70 per cent in 20 years. Its population is now estimated to be fewer than 5,000.

Grauer’s Gorilla (G b graueri), one subspecies of Eastern Gorilla – has lost 77 per cent of its population since 1994, declining from 16,900 individuals to just 3,800 in 2015.

Killing or capture of great apes is illegal; yet hunting represents the greatest threat to Grauer’s Gorillas.

The second subspecies of Eastern Gorilla – the Mountain Gorilla (G b beringei) -is faring better and has increased in number to around 880 individuals.

"To see the Eastern gorilla – one of our closest cousins – slide towards extinction is truly distressing," said Inger Andersen, IUCN Director General.

"We live in a time of tremendous change and each IUCN Red List update makes us realise just how quickly the global extinction crisis is escalating," said Andersen.

The once widespread and abundant Plains Zebra (Equus quagga) has moved from Least Concern to Near Threatened.

The population has reduced by 24 per cent in the past 14 years from around 660,000 to a current estimate of just over 500,000 animals.

The Plains Zebra is threatened by hunting for bushmeat and skins, especially when they move out of protected areas.Three species of antelope found in Africa – Bay Duiker (Cephalophus dorsalis), White-bellied Duiker (Cephalophus leucogaster) and Yellow-backed Duiker (Cephalophus silvicultor) – moved from Least Concern to Near Threatened.

Whilst the populations of these species within protected areas are relatively stable, those found in other areas are decreasing due to continued illegal hunting and habitat loss, the report said.