Notes from Jim Space on his survey of invasive plant species in Tonga
At the request of the Kingdom of Tonga, Jim Space of the U.S. Forest Service's Pacific Island Ecosystems at Risk (PIER) project and Tim Flynn of the National Tropical Botanical Garden recently completed a survey of invasive plant species of environmental concern. This is one of a continuing series of surveys conducted at the request of Pacific island governments. So far, surveys have been completed for American Samoa, Niue, the major islands of Micronesia, and now Tonga.
The major Tongan islands of Tongatapu, 'Eua and Vava'u were surveyed, along with the islands of Lifuka, Foa and 'Uiha in the Ha'apai Group. The island of Ha'ano was partially surveyed (bad weather and limited time did not permit a full survey).
A report has been prepared and is presently in review. It will be available in a month or so, but some highlights include:
Cordia alliodora (Ecuador laurel, salmwood, local name kotia) was introduced to Tonga as a forestry tree and is spreading where present (Tongatapu, 'Eua and Vava'u). Similar spread has been reported on Vanuatu. Foresters should be cautious about introducing this species to new locations.
Flemingia strobilifera (luck plant) was seen on Tongatapu, 'Eua, Vava'u and Lifuka/Foa. This species is a prolific seed producer and can form dense thickets. It is invasive in French Polynesia and is beginning to naturalize in Tonga. It has the potential to become a serious problem.
Indigofera suffruticosa (indigo, local name 'akau veli) is a major component of the weed vegetation of Tonga, the worst seen in the Pacific so far.
Panicum maximum (Guinea grass, buffalograss, local name saafa) is the most troublesome invasive grass in Tonga. It forms dense stands up to 2 m tall. The seeds are dispersed by wind, and it can survive long periods of drought. It spreads by seed and locally from underground rhizomes.
Pimenta dioica (allspice, pimento, local name sipaisi) is widely planted and naturalized in Tonga. We were shown one area on 'Eua where it had naturalized as a dense thicket of saplings. Given its demonstrated aggressive nature this species may well become a major problem in the future. The seeds are bird-dispersed.
Piper aduncum (spiked pepper, local name kava Hawai'i) was noted on all the islands visited and is probably present in other locations as well. Its tiny seeds are spread by birds and can also be introduced into new areas on machinery. Locally, it spreads by suckers, forming large clumps.
Pluchea carolinensis (sour bush) was seen on Tongatapu. The seeds are spread by wind and possibly by birds. It is a widespread pest species in Hawai'i. Since the infestation appears to be limited in area, eradication may be possible.
The dense stands of Psidium guajava (guava, locally known as kuava), especially on 'Eua, were the worst seen so far in the Pacific. This is a major invasive species in the Galapagos Islands and a problem in French Polynesia (Marquesas Islands), New Caledonia and Fiji as well. Frugivorous birds, as well as rats and feral pigs, disperse the seeds.
A yet-unidentified Solanum sp. was seen on 'Eua and Vava'u. Although small, it is quite spiny and would not be a desirable addition to the vegetation of Tonga. The specimens we saw were producing large amounts of small, tomato-like fruit. Spread may be by birds or pigs. This species would be a good candidate for eradication.
Solanum mauritianum (bugweed, wild tobacco, tree tobacco, local name pula) is quite prevalent throughout Tonga. It is a noxious weed in South Africa and is reported to be moderately invasive on Rarotonga, Cook Islands.
Solanum torvum (prickly solanum, devil's fig) is a large spiny species of disturbed areas and old fields that forms dense, impenetrable thickets. A fairly recent introduction according to local sources, it is becoming quite prevalent on Tongatapu and Vava'u. Its seeds are bird-spread.
Tecoma stans (yellow bells, yellow-elder, yellow trumpetbush, local name piti) is a serious invader of disturbed areas in Tonga, as in French Polynesia. It grows in dense stands, commonly with other weedy species. The seeds are wind-dispersed.
Wedelia trilobata (Singapore daisy) has become a serious pest on many Pacific islands as well as in Australia. On Tonga, it was only seen in a few locations. It forms dense mats along roadsides and in disturbed areas and is a problem in agriculture. Given its limited distribution and proven invasive nature, it is an obvious candidate for eradication.
Other common aggressive invaders include Lantana camara (talatala, talatala talmoa), Stachytarpheta urticifolia (blue rat's tail, blue mouse's tail, hiki ' i kuma, 'iku 'i kuma) and Mimosa pudica (sensitive plant, mateloi), often growing in a mixture with Indigofera suffruticosa, Psidium Guajava, Solanum mauritianum, Tecoma stans and other weedy species.
However, the good news is that Tonga is free of a number of serious pest species, including:
The rubber trees Castilla elastica (Panama rubber tree) and Funtumia elastica (African rubber tree), species that have proven very invasive in Samoa. Castilla elastica is present in French Polynesia as well.
Cecropia obtusifolia and C. peltata, invasive tree species that are a problem in Hawai'i and French Polynesia, respectively. Cecropia obtusifolia is also reported to be invasive on Rarotonga (Cook Islands).
Chromolaena odorata (Siam weed), a highly invasive pan-tropical weed.
Cinnamomum verum (cinnamon), very invasive in American Samoa. It is also present in Samoa, Fiji, French Polynesia and the Cook Islands (Rarotonga).
Clidemia hirta (Koster's curse), a serious problem species in Hawai'i and other locations. It was reported to be present in Tonga. If so, it is probably on one of the islands that we didn't survey, as it is a very distinctive plant and easily recognized. This is a very serious weed of the forest understory on many tropical islands and is present in Samoa, American Samoa and Vanuatu.
Cryptostegia grandiflora (rubber vine, India rubber vine), a climbing vine that has become a serious problem in northeastern Queensland, Australia. It is present in New Caledonia and Fiji.
Maesopsis eminii (musizi, umbrella-tree), a large African tree that has been introduced into other countries as a timber tree. Fruit-eating birds (and possibly fruit bats) spread its seed and it has become a problem in a number of countries. It has been introduced as a timber tree to Fiji, where it is starting to naturalize.
Miconia calvescens (the purple plague, velvet leaf), one of the most destructive invasive plants in the Pacific. It has been a disaster to the forest ecosystem of Tahiti in French Polynesia and has subsequently spread to other islands in French Polynesia and been introduced into Hawai'i, where a costly eradication project is under way.
Mimosa invisa (giant sensitive plant), a particularly nasty plant covered with thorns. It forms dense tangles that are difficult to walk through. It is present in a number of South Pacific locations (Cook Islands, Fiji, French Polynesia (Society Islands), Hawai'i, New Caledonia, Niue, Papua New Guinea, Samoa, Solomon Islands and Vanuatu) but, fortunately, has not yet reached Tonga. It has only recently been introduced to Niue and eradication efforts there are ongoing.
Psidium cattleianum (strawberry guava), a small tree that forms dense thickets. It is a major problem species in a number of island ecosystems including Hawai'i, Fiji, Tahiti and the Cook Islands (Rarotonga and Mangaia) well as La Réunion and Mauritius in the Indian Ocean. Birds and pigs (and possibly fruit bats as well) disperse the seeds.
Tonga is also free of Rubus species and the invasive grasses Pennisetum setaceum (fountain grass) and Imperata cylindrica (cogon grass).
The PIER project works closely with ISSG and will collaborate on the forthcoming Cooperative Initiative on Island Invasive Alien Species (further information on this initiative: Maj De Poorter).
Last modified 22 August 2001