Foreign flora are adapting so successfully to the Hawaiian Islands that they are crowding out indigenous tree species and making it nearly impossible for those species to recover on their own, according to measurements from a new combination of airborne-sensing technologies. Researchers hope the new strategy will help other efforts to assess ecosystem changes wrought by invaders.
Hawaii is ground zero for invasive species. Scientists estimate that in recent years more than 10,000 alien varieties--plant and animal--have been brought there. About 120 of those are known to be wreaking havoc on native ecosystems despite efforts to stop them. Much of the problem has to do with the nature of the islands themselves. Their moderate climate, plentiful rainfall, and fertile soil cause most immigrants to live long and prosper. And because native species haven't had time to adapt to these new arrivals, many stand defenseless against the invaders.
Assessing the impact of these trespassers has been difficult. Hawaii's rugged mountains are covered with dense jungle, which makes them difficult to study in the field or by satellite. So a team led by ecologist Gregory Asner of the Carnegie Institution for Science in Stanford, California, developed a hybrid technology. The system uses radar to peer through layers of tree canopy to create three-dimensional images of the forest and spectroscopy combined with software that can identify individual plant species by how their leaves reflect light.
The team flew the sensor array over more than 220,000 hectares of the Big Island in January 2007. The worst invader, Asner says, is strawberry guava (Psidium cattleianum), which was imported for its fruit. It can grow up to 15 meters tall and is "fundamentally changing" the forest architecture, he says, altering the hydrology and nutrient balance. Guava also restricts the light reaching the forest floor, preventing other plants from taking root. The tree has rapidly encroached into the protected areas established by Hawaii’s Department of Land and Natural Resources to preserve indigenous forests, the team reports online this week in the Proceedings of the National Academy of Sciences. As a result, Asner says, P. cattleianum is "destroying native forests on the Big Island." He says the team has detected similarly severe invasions by kahili ginger (Hedychium gardnerianum), an ornamental tree whose rhizomes, or hard root caps, create a barrier in the topsoil that no other plants can penetrate.
The technology is "a revolutionary advance," says conservation biologist Sam Gon III of The Nature Conservancy in Honolulu. Research scientist Julie Denslow of the Institute of Pacific Islands Forestry in Hilo, Hawaii, adds that the study "dramatically confirms what ground-based researchers and managers long have known" about the effects of invasive species on Hawaiian rain forests. It provides a "chilling illustration of the enormous challenge facing forest managers in their attempts to maintain the health of protected areas" in the islands, she says.