Managing invasive species in Hawaiʻi

To the millions of tourists that visit each year, Hawaiʻi is known as the Aloha State or simply ‘paradise’. However, to conservationists it’s known as the ‘endangered species capital of the world’. Similar to many islands across the world, one of the key threats to Hawai’i’s rich and unique biodiversity is biological invasions.

An ‘alien’ species is a species introduced by humans outside of its natural range – those that spread and negatively impact native biological diversity are known as ‘invasive’. In Hawai’i, over 5,000 alien species have been introduced. While only a subset have become invasive, their impacts have been severe, causing serious habitat degradation and species extinction.

Feral pigs - a cross between domestic pigs from Polynesia and Europe - and strawberry guava - a fruit tree originating from Brazil - are among the most destructive invasive alien species in Hawaiʻi.

The effects of pigs on Hawaiian ecosystems are far reaching and their impacts cascade across multiple ecological levels. For example, the pigs frequently feed on tree ferns which are a dominant group in Hawaiʻi’s native forests. This kills the ferns, which causes drastic changes in forest ecosystem structure and function. When the pigs forage for food they disturb the soil, killing plants and facilitating the invasion of non-native plant species. Pigs also spread invasive plants by dispersing seeds in their faeces.

One invasive plant in particular, the strawberry guava tree, appears linked in a vicious positive feedback cycle with pigs. Pigs relish the fruits of these trees and have dispersed their seeds into native forests. Strawberry guava, like the pigs, is now common on most of Hawai’i’s main islands, forming dense stands, outcompeting native species, and destroying the habitats of native birds and insects.

Field biologists have relied on manual cutting and application of herbicide to control strawberry guava, as well as on trapping and hunting feral pigs. These methods are very labour intensive and expensive, and have not successfully addressed the impacts of these species on a large scale. However, the outcome of recent research efforts and comprehensive risk assessments is a promising biocontrol agent for strawberry guava: the Brazilian scale insect, which feeds exclusively on strawberry guava and can help reduce the growth and seed production of this invasive species. For the pigs, an improved and safer baiting method is being tested. Unmanned aircraft are also being tested to assist in invasive species monitoring, which can help inform and prioritise action on the ground.

Almost 30 Forum events at the IUCN World Conservation Congress 2016 will focus on prevention, control and eradication of invasive alien species and their impacts, some focussing exclusively on islands. These events will disseminate knowledge and best practice and participants are encouraged to develop strategic guidance to address biological invasions.

- Reese Brand Phillips, US Fish and Wildlife Service, Pacific Islands Fish and Wildlife Office