In 1990s, Samoa released a predatory flatworm as a weapon against giant African land snails. 49 years later, the predator has outlived its original target-control mission and threatens native snails instead

A predator introduced to solve one invasive-species problem ended up creating another. In Samoa, the New Guinea flatworm was brought in during the 1990s as a biological control agent against giant African snails that were damaging crops. But the flatworm did not stay focused on its intended target. It also hunted native land snails, including tree-dwelling species that had evolved without this particular predator, as per a report by El Cronista.

The result became a warning about how quickly biological control can upset an island ecosystem.

Why was the flatworm brought to Samoa?

The original problem was the giant African snail, known scientifically as Achatina fulica or Lissachatina fulica. The large invasive snail feeds on a wide range of plants and posed a threat to agriculture across Pacific islands.

Authorities were looking for biological ways to reduce its numbers. The New Guinea flatworm, Platydemus manokwari, appeared useful because it is a predator of snails and had already been considered as a biological control agent.

In Samoa, the flatworm was introduced as part of efforts to control the giant African snail. Research later documented introductions on Savai’i during 1996–1998 and 2001, while the species was already present on Upolu by the late 1990s.

The problem was that the predator was not selective enough.

Why did native snails become vulnerable?

Platydemus manokwari is a generalist predator, meaning it does not rely on just one type of prey. Once it became established, native snails were also exposed to it.

The flatworm can move along the ground and climb vegetation. That gave it access to native tree-dwelling snails as well as species living closer to the ground.

For many Pacific island snails, this was a predator they had never evolved alongside. Their isolation had left them particularly vulnerable when a new threat entered their environment, as per a report by El Cronista.

Researchers subsequently observed declines in partulid tree snail populations in areas where the flatworm was present.

The U.S. Fish and Wildlife Service has identified Platydemus manokwari as a threat to native land snails and has noted that the disappearance of some Pacific island snail species has been attributed to the predator.

Did the flatworm cause every snail decline?

Not necessarily. Scientists have cautioned against blaming every disappearance of Samoan snails on the New Guinea flatworm alone.

Some populations were already declining before particular introductions of the predator. Habitat changes and other factors may also have played a role.

That distinction matters because island ecosystems can be affected by several pressures at the same time. The evidence does, however, show that native snail populations declined in areas where the flatworm was present, making the predator an important concern for vulnerable species.

The problem was not unique to Samoa, either.

What happened with other biological control attempts?

The attempt to control giant African snails was part of a wider history of biological control efforts across the Pacific.

Other islands introduced predatory snails, including species belonging to the genus Euglandina, for the same purpose. But those predators did not always concentrate on the invasive species they were supposed to control.

A scientific review of biological control programs in the Pacific found that some of these predators failed to adequately control giant African snails and instead became important factors in the decline and extinction of numerous endemic land snail species.

Island snails are especially exposed because many have extremely small geographic ranges. A species restricted to one island, or even a limited part of an island, can face an immediate threat when a new predator arrives.

Samoa's experience shows why biological control can be complicated. A species introduced to attack an agricultural pest does not necessarily remain within the boundaries humans intended for it.

Once released, it becomes part of the ecosystem and can interact with organisms that were never considered part of the original control plan.

That can be especially dangerous on islands, where many native species have evolved in isolation and may have few defenses against unfamiliar predators.

FAQs

Why was the flatworm introduced?
To help control invasive giant African snails.

Which snails did it threaten?
Native land snails, including tree-dwelling species.