A project is aiming to protect the hundreds of unique and threatened invertebrate species that are only found on St Helena – the tiny British Overseas Territory in the South Atlantic Ocean

Lying around 1,200 miles (1,900 km) from Africa and 1,800 miles (2,900 km) from South America, St Helena is among the world’s most isolated islands.

It’s really hard to get to a lot of places on the island and new things are discovered pretty regularly

Its volcanic origins on the Mid-Atlantic Ridge are still evident in its rugged and highly varied landscape and the island’s original wildlife thrived, alone in the middle of the ocean for millions of years.

St Helena was discovered by the Portuguese in 1502, and colonised by the British in 1659, yet its distinctive evolutionary conditions are still said to have impressed Charles Darwin on his visit in 1836.

Last year a survey established for the first time that the island has 502 species that are not found anywhere else on the planet. The majority of these unique species are invertebrates (416), a higher number than those found in the UK and its 13 other overseas territories put together.

Examples include the spiky yellow woodlouse (Pseudolaureola atlantica), a brightly coloured isopod with a population of around 90 living in highly restricted areas, making it one of the rarest animals in the world.

One of the most biodiverse square kilometres in the world

But new research has shown the “spiky”, as it's commonly known, is not alone in facing an uncertain future. The majority of St Helena’s endemic invertebrates are believed to be on the brink of extinction, thanks mainly to the fragmentation of their habitats since humans arrived on the island 500 years ago.

Some species like the Saint Helena giant earwig (Labidura herculeana) – once the world’s largest earwig – have been declared extinct, while others such as the giant ground beetle (Aplothorax burchelli) and St Helena darter (Sympetrum dilatatum) are thought to have been wiped out.

“We have 502 unique species that we know about at the moment on St Helena. The UK and all of the other overseas territories between them have 1500, so on this tiny little isolated rock of 123 square kilometres [47 square miles] we have a third of the unique species, which is pretty dramatic,” Jeremy Harris, director of St Helena National Trust, tells BBC Earth.

“There’s a section of habitat at the very top of the island that’s particularly rich in these species so we have about 119 unique species in half a square kilometre, the size of a football field. That’s got to be one of the most biodiverse square kilometres in the world.

“The mountainous nature of the island means that it’s really hard to get to a lot of places on the island and new things are discovered pretty regularly.”

Buglife, the St Helena National Trust, the Centre for Ecology and Hydrology and the St Helena Government are now working together to protect what is left of the unique and threatened wildlife of the island.

Raising the international profile of the place is really important

As part of their Bugs on the Brink project, the team began submitting species assessments to the IUCN Red List. Project manager David Pryce has been responsible for documenting all of the invertebrate life on St Helena and suggests that around 83% of the unique species are likely to be categorised as threatened.

He believes that of the 416 endemic species so far described, 42 can be labelled vulnerable, 146 as endangered, 156 as critically endangered and 23 as already extinct. Mr Pryce’s assessment also suggests that up to 44 species in the critically endangered group could also be extinct as living specimens have not been found since the 1960s.

As well as the loss of the natural environment, the native fauna’s survival has also been threatened by other pressures such as the introduction of rats, mice and other invertebrates.

The island is also preparing for the opening of its first airport, which will make St Helena more accessible than ever before and paves the way for large-scale tourism.

Mr Harris says those concerned with the conservation of the island’s biodiversity hope having the threatened species recognised by the IUCN will help them to secure more external support.

“Raising the international profile of the place is really important, we’re a long way stuck out in the middle of the Atlantic,” he says.

"The island needs to be a tourist destination so it can begin generating money from a tourism economy, and there’s a real recognition that the environment and the quirkiness of these species plays a real role in that, the natural world is going to be one of the draws."

Along with St Helena, research was also conducted into the unique species found on the UK's other overseas territories. It was found that together these islands contain 94% of the known endemic British species. Mr Harris has called on all of the islands to assess the conservation status of their native species in order to gain official IUCN listing for them all.

You can follow Zoe Gough and BBC Earth on Twitter

Like BBC Earth on Facebook and follow us on Instagram.