Hawaii's tree snails return to wild after 33 years, with just 11 left in 1991

A creature barely three-quarters of an inch long has taken more than three decades of scientific effort to bring back to the forests of Hawaii. The O‘ahu tree snail, a species once considered lost in the wild, is slowly getting another chance after conservationists rescued the last 11 known individuals in 1991 and spent years breeding them in captivity.

The species, Achatinella fuscobasis, returned to its natural habitat in December 2024, but its comeback also highlights a troubling conservation mistake: a predator introduced to control an invasive garden pest became one of the biggest threats to Hawaii's native snails.The first group of these endangered snails was released on December 10, 2024, into a specially prepared enclosure in the Honolulu Watershed Forest Reserve in the Ko‘olau Mountains on O‘ahu.

The reintroduction, announced by Hawaii's Department of Land and Natural Resources (DLNR) on December 18, marked the end of more than 33 years in captivity. It was also the result of nearly half a century of conservation partnerships, research and efforts to prevent the species from disappearing permanently.

How a pest-control experiment turned into an ecological threat

The story of the snail's decline is closely linked to another introduced species. In the 1930s, the giant African land snail arrived in Hawaii, reportedly brought in as a garden ornament.

It soon became an agricultural and garden pest, prompting authorities to look for ways to control its population. In the 1950s, the predatory rosy wolf snail was introduced as a biological-control agent. The predator was expected to help tackle the giant African snail, but its appetite was not limited to the intended target.

It also hunted native Hawaiian snails, including the colourful tree-dwelling species that had evolved without such predators.A 2021 research review published in the journal Biological Invasions examined the consequences of introducing predatory snails and flatworms to control the giant African land snail. Researchers found that the claimed benefits of these biological-control efforts could not be substantiated, while the predators had contributed significantly to the decline and extinction of native island snail populations.The rosy wolf snail is particularly dangerous because it is a generalist predator. It can locate other snails and use a specialised feeding organ, called a proboscis, to reach inside their shells and consume them. Dr Michael Hadfield, emeritus professor of biology at the University of Hawaii at Mānoa, told the US Fish and Wildlife Service that researchers would sometimes find healthy populations of native snails suddenly reduced to empty shells after predators arrived.Rats and other invasive animals, including Jackson's chameleons and the New Guinea flatworm, have also contributed to the destruction of Hawaii's native snail populations. The crisis is not limited to one species or one valley.

11 snails that prevented their species from disappearing

By 1991, Achatinella fuscobasis had reached a critical point. The last 11 individuals from its only known wild populations on O‘ahu were collected by Hadfield and brought into captivity.

The decision was a desperate attempt to preserve a species that was unlikely to survive in forests increasingly occupied by introduced predators.Hadfield and other researchers began experimenting with ways to breed the snails in laboratory conditions. It was not as simple as keeping them in containers and providing leaves. The animals needed carefully controlled temperature, humidity and light cycles. Hadfield described the first environmental chambers as resembling household refrigerators, except they allowed researchers to regulate day and night cycles and temperature.

Since the snails became active in response to water, the chambers also needed sprinklers.Feeding them presented another challenge. The tree snails feed on fungi and algae growing on the surfaces of leaves. Researchers initially collected fresh branches from native ‘ōhi‘a trees, climbing into the mountains at least every other week. They eventually discovered that cultivating the required mould in petri dishes was more effective.

The snails could then feed on the growth inside their laboratory enclosures.The painstaking work paid off. By December 2024, the DLNR's Snail Extinction Prevention Program had raised the captive population to more than 1,000 individuals. The rescue had moved beyond simply keeping 11 animals alive to rebuilding a population that could potentially survive outside a laboratory.

A forest enclosure built to keep predators out

Returning the snails to the forest required another major challenge: ensuring that the predators responsible for their disappearance could not reach them again.

Conservationists prepared a quarter-acre protected area in the Ko‘olau Mountains, known as an exclosure. The habitat took approximately five years to prepare. Teams from the DLNR, the US Fish and Wildlife Service, the University of Hawaii and community volunteers worked to restore the area with native plants and remove invasive species.Volunteers and conservation workers spent around 600 hours searching for and removing predators from within the enclosure.

The structure was fitted with five-foot-high polyethylene walls, angled flanges, copper wire mesh and a hood barrier. These features were designed to keep out rats, Jackson's chameleons and the predatory rosy wolf snail.The new habitat was intended to give the animals a chance to experience natural conditions again, including trade winds, misty rain, cloud cover and sunlight, without exposing them to the same predators they had escaped in captivity.

The first release took place on December 10, 2024. Another group was scheduled for release in February 2025, according to the original DLNR announcement.

A warning beyond one species

Hawaiian tree snails, known collectively as kāhuli, are more than just small forest inhabitants. Their colourful shells have appeared in traditional Hawaiian poetry, hula, chants and lei-making. They also perform an important ecological role by feeding on fungi and algae growing on plant surfaces and contributing to nutrient cycling in forests.The wider situation remains concerning. The US Fish and Wildlife Service notes that Hawaii once had more than 750 species of kāhuli, but at least 60% have disappeared within the last century. Forty-four Hawaiian snail species are listed as endangered under the Endangered Species Act, with many others facing serious threats.Even the successful return of Achatinella fuscobasis does not mean the species is out of danger.

Its survival depends on maintaining predator-free habitats, monitoring the released population and continuing captive breeding. The animals also reproduce slowly, with adults producing only one to four live young annually and reaching reproductive maturity at around five to seven years of age.For now, the return of the tiny tree snail is an important conservation milestone, not a complete recovery. The 11 individuals rescued in 1991 helped prevent an irreversible loss, but the circumstances that pushed them towards extinction remain a warning about the consequences of introducing species into ecosystems without fully understanding their impact. More than three decades later, Hawaii has managed to give one of its lost forest jewels another chance.

Whether it can establish a thriving population beyond its protective walls remains the bigger test.