In the 1990s, a predatory flatworm was released in Samoa to control giant African snails; it later drove native island snails toward extinction

In the 1990s, Samoa and nearby islands introduced an invasive species to control another invasive species, but the result did not turn out as intended.

According to reports, the New Guinea flatworm, Platydemus manokwari, was introduced to Pacific islands in the 1990s as another attempt to control the giant African land snail, Achatina fulica. Instead, the predator began hunting native tree snails, including species of the Partula and Samoana genera.

The problem began decades earlier. The giant African land snail spread across Pacific islands in the mid-20th century, damaging crops and creating pressure for biological control. In the 1970s, the rosy wolfsnail, Euglandina rosea, was introduced to prey on the invasive snail. But it also attacked native land snails.


The introduction of P. manokwari became another chapter in the failed biological-control effort. Unlike a specialist predator that targets a particular pest, the flatworm is a generalist predator capable of feeding on a wide range of terrestrial molluscs.

Its ability to move through leaf litter and climb trees makes it particularly effective at reaching native snails that live in forest canopies. Once established, the small, nocturnal flatworms are extremely difficult to remove, allowing them to spread through tropical island habitats.

Research available through the National Institutes of Health describes Platydemus manokwari as a major threat to native Pacific land snails and an example of the risks associated with introducing generalist predators for biological control.

The consequences have been severe. Many Pacific island snails evolved in isolation for millions of years without terrestrial flatworm predators, leaving them poorly equipped to deal with the introduced hunter. Because some native snail species occupy extremely small ranges, even intense predation over a limited area can push an entire species toward extinction.

The US Fish and Wildlife Service has identified introduced predators, including P. manokwari, as major threats to several native snail species. Federal records published in the US Federal Register in 2015 led to a number of affected species in American Samoa being listed as endangered or threatened under the Endangered Species Act.

Native tree snails are also important to their rainforest ecosystems. Species from genera such as Partula and Samoana feed on decaying plant material and fungi, contributing to nutrient cycling in forest habitats.

Conservation efforts now focus on protecting the remaining populations through strict biosecurity, limiting the movement of soil and plants between islands, and maintaining captive-breeding programmes for some threatened species.

The incident highlights the dangers of interfering with natural ecosystems. Trying to solve one invasive-species problem by introducing another predator can create an even bigger ecological crisis, particularly on isolated islands where native species have evolved without similar predators.