They are some of the smallest animals on our planet, measuring from 1cm to 2cm in length. But the recent return of thousands of tiny tropical tree snails to French Polynesia represents one of the biggest reintroduction programmes ever attempted by conservationists.

More than 15,000 partula snails – bred by a total of 16 key international conservation organisations, including the Zoological Society of London (ZSL), and Edinburgh, Chester and Amsterdam zoos – have been shipped out to Polynesia over the past five years. A few weeks ago, the most recent arrivals – more than 4,000 Partula rosea and Partula varia snails – were released on the islands of Huahine and Moorea in the Society Islands. (The archipelago, which includes Tahiti and Bora Bora and covers an area of 1,600 sq km in the South Pacific, is noted for its coral-fringed lagoons and rugged mountains.)

For efforts of this magnitude to be expended on such a tiny creature may seem unusual. But wildlife experts say partulas are of critical scientific importance.

Their isolated locations and distribution across the Pacific makes them ideal subjects for studying evolution, just as the distribution of Galápagos finch species played a key role in Darwin’s development of his theory of natural selection.

“Partula snails are incredibly valuable to the scientific community,” said Paul Pearce-Kelly, a senior ZSL curator. “They have been studied for more than a century because their isolated habitat provides perfect conditions for studying evolution. They are the Darwin finches of the snail world.”

Partula snails also have a significant cultural importance in Polynesia. Their shells were used to make necklaces and crowns, and as each island has its own particular species of partula, these items had special importance as ceremonial gifts.

Snails like the partula also play a key role in recycling decaying vegetation in Polynesian islands. “Their ecosystems services are vital,” added Dave Clarke, of the ZSL. “For good measure, they have lovely long eye-stalks which makes them look very cute, I think.”

Partula are named after a Roman goddess of childbirth because they give live birth while most snail species lay eggs. Partula reach sexual maturity aged one year and live for up to 10 years, grazing algae from the surface of leaves in forests on Pacific islands.

Their demise in the wild was brought about by a botched land management scheme last century. “The French authorities who ran the islands in the South Pacific decided to introduce the African giant land snail as a foodstuff,” said Clarke. “The French like snails after all.”

Unfortunately, those African giant snails (Lissachatina fulica) escaped, bred and began destroying local crops. As a result, the authorities decided to get rid of these interlopers – by introducing a second snail, the rosy wolf snail (Euglandia rosea) to kill off the newly introduced pests.

“Rosy wolf snails are incredible predators,” added Clarke. “They move two or three times faster than other snails. They can also track other snails by following their slime trails. Then they move in and kill and eat them. They are the guided missiles of the snail world.”

Unfortunately again, the rosy wolf showed a distinct distaste for the African giant snails that they were supposed to digest and instead chose to eat local Partula snails. The impact was devastating. “There were around 120 different species of Partula snails in Polynesia,” Clarke added. “The rosy wolf has wiped out more than half of these species.”

The last few individuals from some of these ravaged populations were collected by Clarke in 1994. Others were already being studied at various universities round the world, and from these widely dispersed sources, breeding populations of several partula species that were now extinct in the wild, were established.

At the same time, populations of the rosy wolf snail started to drop. “They went through a boom and then a bust, and although rosy wolf snails have not completely disappeared there are not many left today,” added Clarke.

As a result, in 2015, it was decided to start returning partulas to the wild in Polynesia. More than 15,000 individuals – from a total of 14 species and sub-species of partulas – have since been shipped back to the islands on a journey of more than 15,000km.

“Breeding and nurturing more than 2,000 snails requires years of dedication,” said Dr Gerardo Garcia, a curator at Chester Zoo, which has been closely involved in the partula project.

“So to see these animals released into their historic homeland was a very emotional moment. Our planet’s biodiversity is under threat, so every single one of those tiny animals now represents a special symbol of hope.”