Pacific Island Ecosystems at Risk (PIER)


Report on invasive plant species on Rota,
Commonwealth of the Northern Mariana Islands


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Introduction

Dangerous species not known to be on Rota

Species that are invasive elsewhere and are likewise invasive or have the potential to become so on Rota

Species that are mentioned or listed as being weedy or invasive elsewhere and are common or weedy on Rota

Native and naturalized species exhibiting aggressive behavior

Strategies for dealing with invasive species

Recommendations

Appendix 1.  Background material and references

Appendix 2.  Species by category

Appendix 3.  Invasive species present on Saipan, Tinian or Guam but not present in Rota

Appendix 4.   Scientific name synonyms


 

Invasive Plant Species on Rota, Commonwealth of the Northern Mariana Islands

James C. Space, Barbara Waterhouse, Julie S. Denslow and Duane Nelson

 

U.S.D.A. Forest Service

Pacific Southwest Research Station

Institute of Pacific Islands Forestry

Honolulu, Hawai‘i, USA

 

25 October 2000

 

Invasive plant species on Rota, Commonwealth of the Northern Mariana Islands

James C Space, Barbara Waterhouse, Julie S. Denslow and Duane Nelson (1)

This is a continuation of the survey of islands in Micronesia and American Samoa for invasive plant species requested by the Pacific Islands Committee, Council of Western State Foresters. A survey of other Micronesian islands was conducted in 1998 and was discussed in a previous report. This report summarizes a survey of the island of Rota, Commonwealth of the Northern Mariana Islands, on 5 April 2000. The objectives, as with the previous survey, were three-fold: (1) To identify plant species on the island that are presently causing problems to natural and semi-natural ecosystems; (2) to identify species that, even though they are not presently a major problem, could spread more widely or spread to other islands where they are not present, potentially causing problems; and (3) to confirm the absence of species that are a problem elsewhere and, if introduced to Rota, could be a threat there.

During our visit James Manglona of the Department of Lands and Natural Resources showed us sites of known infestations. We also had available copies of various botanical and weed surveys conducted in the past (see Appendix 1, References). A visit of such short duration does not permit an exhaustive survey of the weed biota of the island. However, the intent was to conduct an overall survey. Additional surveys of individual species or sensitive areas can and should be conducted as needed. This report summarizes our findings and makes some suggestions for further action.

For convenience and to be consistent with the Micronesia report, invasive species occurring in or of threat to Rota have been grouped into four categories:

1.  Species that are invasive elsewhere in similar ecosystems but were not seen on our visit and are not reported in the literature as being present on Rota (230 species).
2.  Species that are invasive elsewhere and are also invasive or potentially invasive on Rota (12 species).
3.  Species that are invasive or weedy elsewhere and are common, weedy or cultivated on Rota (63 species).
4.  Native species that exhibit aggressive behavior (10 species).

These species are listed in Appendix 2. Additional information about each species is located on a World Wide Web site, http://www.hear.org/pier/, and on the PIER-CD.

There are a number of other species that are invasive weeds in gardens, fields and pastures and along roadsides but don’t seem to pose a particular threat to wildland ecosystems. These species are not included.

1. Dangerous species not known to be on Rota

A large number of invasive species have yet to reach Rota. These are listed in Appendix 2, Table 1. The worst of these include the following:

Cecropia obtusifolia and C. peltata, invasive tree species that are a problem in Hawai‘i and French Polynesia, respectively.

Cinnamomum verum (cinnamon), found to be very invasive in American Samoa and elsewhere.

Clerodendrum chinense, a species that can form dense thickets that exclude other species. It tolerates shade well and suckers profusely.

Clidemia hirta (Koster’s curse), a serious problem species in Hawai’i and in other locations. In Micronesia it is only present in Palau, but would be a serious problem if introduced as it grows well in the shade of closed forests.

Coccinia grandis (ivy or scarlet gourd), a smothering vine that is showing potential for serious damage to the forests of Saipan. The vines climb over trees and form such dense cover that the forest underneath is completely shaded out and destroyed. It is also invasive in Guam and Hawai‘i. The proximity of the plant in Saipan poses a grave threat to Rota.

Cordia alliodora, introduced as a forestry tree to Vanuatu and demonstated to be quite invasive. It should not be planted.

Dieffenbachia seguine [maculata] (spotted Dieffenbachia or dumb cane), a house plant that often escapes cultivation (primarily through the dumping of plants or cuttings) and is potentially a very bothersome species as it reproduces vegetatively and can thrive in the dense shade of an intact native forest canopy.

Two rubber trees, Funtumia elastica (African rubber tree) and Castilla elastica (Panama rubber tree), species that have proven very invasive in Samoa. While not known to be present in Micronesia (Castilla elastica is present in Hawai‘i) and thus posing a lesser threat, their highly invasive nature makes these species prime candidates for prompt eradication if found.

Lantana camara (lantana) has reportedly been eradicated from Rota and none was seen on our survey. It is very prevalent on Saipan and Tinian and is thus a candidate for possible reintroduction. Rota is to be commended for eradicating this pest species.

Merremia peltata, a native vine found throughout the Caroline Islands, is not present in the Northern Marianas. Even though it is native (or an aboriginal introduction) in the Carolines it is quite aggressive there. It would no doubt also prove quite invasive on Rota (as it has on other Pacific islands where it has been introduced), so it would be wise to be alert to its possible introduction.

A number of grass species, including:

Miconia calvescens (the purple plague), which has caused serious damage to the ecosystem of Tahiti in French Polynesia. It has spread to other islands in French Polynesia and has also escaped in Hawai’i where it is the subject of an intensive and costly eradication effort. Recently it was discovered in north-east Queensland, Australia, where eradication is also being attempted. It is an attractive garden plant and might be introduced this way or as tiny seeds on shoes or used equipment.

Paraserianthes falcataria [Falcataria moluccana] (Molucca albizia), a common introduced species throughout the Caroline Islands and present on Guam. It has the potential, over time, of spreading widely through the forest (as can be seen on Pohnpei). Introduction of this tree should be discouraged.

Passiflora foetida is present on Rota, but several invasive members of this invasive genus are absent, including:

Piper auritum (eared pepper, locally called "false sakau"), recently introduced to Pohnpei. This species suckers profusely and produces many small seeds that are spread by birds, rodents and bats. An attempt is being made to eradicate it on Pohnpei and it should not be introduced to other islands.

Psidium cattleianum (strawberry guava), a small tree that forms dense thickets and is a major problem species in Hawai‘i, Tahiti and elsewhere. It is presently only on Pohnpei and Palau in Micronesia.

Rubus spp., a number of which are problem species on tropical islands.

Spathodea campanulata (African tulip tree), a commonly planted ornamental present on Saipan and Guam. This tree has become a major problem in Fiji, the Hawaiian Islands and some other places. It was not seen on Rota and it would be good to exclude it from the island.

Tibouchina herbacea (glorybush or cane ti), another species that is a major problem in Hawai‘i.

Thunbergia alata, T. grandiflora and T. laurifolia, climbing vines that are present on Guam.

The best indicator that a species might be invasive is the fact that it is invasive elsewhere. However, each island ecosystem is unique, and invasiveness cannot be predicted with certainty. A good strategy is to be extremely cautious and exclude these and other species known to be invasive or weedy elsewhere (although an even better strategy, adopted by countries such as Australia and New Zealand, is to exclude all species not shown by risk analysis to be of negligible risk). Known problem species that have the potential to cause problems in tropical island ecosystems and are not yet present in Rota are listed in Appendix 2, Table 1. These species should be excluded through plant quarantine and, if establishment is detected, promptly evaluated for eradication. In addition, species that are present in Saipan, Tinian and Guam but are not present in Rota are listed in Appendix 3. There is high risk of introduction of these species because of traffic between these islands and Rota.

2. Species that are invasive elsewhere and are likewise invasive or have the potential to become so on Rota

Some known invasive plants that are causing trouble in similar ecosystems have been introduced into Rota (Appendix 2, Table 2). Some of them are already causing problems while others are not. Some are cultivated plants that have not (yet) escaped and their potential for causing damage is so far unknown. However, one of the best predictors of invasiveness is the behavior of the species elsewhere, and these are known troublemakers.

Adenanthera pavonina (the coral bean tree), invasive in secondary forests.

Antigonon leptopus (chain of hearts). This climbing vine has become a widespread pest on Guam. It favors soils derived from limestone and, thus, there is plenty of it on Rota.

Bidens pilosa (beggar’s tick) is widespread, as on many Pacific islands.

Chromolaena odorata (Siam weed), a highly invasive pan-tropical weed, is very prevalent on Rota. Biological controls are available that are effective in open areas, less so in shaded stands.

Clerodendrum quadriloculare, not seen but reported present, probably as an ornamental. It is an attractive shrub often planted in yards. It is suspicious because it appears to have the ability to invade intact or relatively intact native forests. This species is notorious for being a prolific producer of root suckers and, in fact, the plant is easily propagated by means of root cuttings. A dense, monospecific understory of this species was observed growing in full shade beneath the forest canopy on Pohnpei, making it a likely candidate to invade intact or only slightly disturbed native forest stands.

Imperata cylindrica (cogon grass), a very invasive grass species present on Guam and Saipan as well.

Mikania micrantha (mile-a-minute weed) is widespread. It is very invasive at a number of locations in the Pacific.

Mimosa diplotricha [=invisa] (giant sensitive plant) and Mimosa pudica (sensitive plant) are both present on Rota. Mimosa diplotricha is a particularly nasty plant as it is covered with thorns and forms dense tangles that are difficult to walk through. Mimosa pudica, a smaller plant with only small prickles, can form dense mats. Both will undoubtedly spread further unless control action is taken.

Momordica charantia (bitter-melon), a member of the cucumber family, is a climbing vine and its fruit is a host for fruit flies. It is quite prevalent on Rota

Syngonium angustatum, a climbing aroid, is probably another escape from cultivation. Like Dieffenbachia maculata, it has the ability to spread in the deep shade of intact forests. It is difficult to eradicate by hand as it is able to reproduce from a single node and bits and pieces of the stems or roots are easily overlooked. It is very common on Rota, on cliff faces as well as in the forest.

3. Species that are mentioned or listed as being weedy or invasive elsewhere and are common or weedy on Rota

A large number of other common or weedy introduced species were noted. Many of these species, which might best be termed aggressive weeds, are mostly prevalent along roadsides or on disturbed sites, although some species, particularly alien trees, can gradually spread into forested ecosystems. In the case of vines and plants that form dense ground cover, the regeneration of native species can be inhibited.

Some of these species could become a problem in the future, since there is often a long lag time between introduction and when a species begins to cause serious impacts. These species (listed in Appendix 2, Table 3) should be monitored for spread and possible control, if necessary.

Invasive or potentially invasive tree species include Acacia auriculiformis (earleaf acacia), Acacia confusa (Formosa acacia), Albizia lebbeck, Bauhinia monandra (orchid tree), Ceiba pentandra (kapok), Delonix regia (flame tree), Leucaena leucocephala (tangan-tangan), Moringa oleifera (horseradish tree), Pithecellobium dulce (Madras thorn), Samanea saman (monkeypod), and Tecoma stans (yellow bells). Some naturalization of Acacia confusa, Albizia lebbeck, Ceiba pentrandra and Pithecellobium dulce was noted and further planting of these trees should be discouraged. Delonix regia was common both as planted and naturalized specimens. Leucaena leucocephala, as is the case on most islands with limestone-derived soils, is very common.

A number of introduced grasses have become established, including Cenchrus brownii (bur grass); Chloris radiata (plush-grass, radiate fingergrass), Cynodon dactylon (Bermuda grass), Digitaria ciliaris (fingergrass, smooth crabgrass); Eleusine indica (goose grass), Paspalum conjugatum (Hilo grass), Pennisetum polystachyon (mission grass) and Pennisetum purpureum (elephant or napier grass). Digitaria ciliaris, Pennisetum polystachyon and Pennisetum purpureum are especially prevalent.

Other widespread weedy species include Blechum   pyramidata [brownei] (blackweed), Stachytarpheta jamaicensis and S. cayennensis [urticifolia] (blue rat's tail), Hyptis capitata (botones) and H. pectinata (comb hyptis) and Sansevieria trifasciata.

Carica papaya was noted as an adventive invader of recently burned and other disturbed areas.

Centrosema pubescens (centro), an introduced pasture legume, is quite prevalent on forest tracks and climbing on trees.

Melia azedarach (Chinaberry) was noted growing in the forestry nursery. Its seeds are bird-dispersed and it is a problem in South Africa, Hawai‘i and some of the islands of French Polynesia.

Some Triphasia trifolia (limeberry) was noted. It has become quite prevalent in forest edges on Guam and is likely to become so on Rota as well. Presence of intact populations of frugivorous avian dispersers, such as the Mariana Fruit Dove (totot), makes this more likely.

Wedelia [Spagneticola] trilobata (Singapore daisy) is not yet widespread, unlike most Pacific islands. Possibly it is a recent introduction that has not had time to become fully established.

4. Native and naturalized species exhibiting aggressive behavior

Some native species (or early introductions) exhibit aggressive characteristics. These species are listed in Appendix 2, Table 4.

Casuarina equisetifolia, if it is in fact native, seems to be spreading to a considerable extent.

Strategies for dealing with invasive species

The first line of defense against invasive species is to keep them out. Control at ports of entry is essential, and land management officials should work closely with plant protection and quarantine officials to make them aware of known and potential invasive plant species. Plant quarantine officers are familiar with most agricultural pests, but they may not be aware of some of the pests that threaten wildland ecosystems. In cooperation with the plant quarantine organization, a list of noxious species to be excluded should be developed and exclusion of these species should be backed by the force of law and regulation. Better yet is to go to the approach recently adopted by Australia and under serious consideration by a number of other countries. That is to exclude all alien species not shown to be of negligible risk. Risk assessment and management techniques can be used to assess the likelihood and effects of possible introductions and to develop exclusion and eradication strategies.

Education of the public about the danger of introductions and encouraging the use of native species can be helpful. People need to understand why they should follow the quarantine regulations, why they shouldn’t dump garden cuttings in the woods, and why they should report suspicious plants. What may just be a pretty flower to be planted in a yard or garden can turn out to be an invasive species. Education of schoolchildren is especially important, as this is the most impressionable age. Children can also have a notable effect on the actions of their parents.

The public also needs to understand that the immediate eradication of a small area of a problem species, even if it involves the use of herbicides, may be better than living with a problem species forever. There are many instances where you hear people say "I wish we'd taken action when this pest was first noticed". It may even be worthwhile to take people to a place where they can be shown the full extent of a problem if the infestation is allowed to spread so that they will accept and support eradication. For example, anyone visiting Tahiti would very likely come away convinced that Miconia calvescens is an ecological disaster and that it should be prevented from becoming established on other Pacific islands.

The public should also be encouraged to report suspicious plants. Public service announcements on television or radio can encourage this. Funding can be requested to prepare PSA's or "wanted" posters. Prompt follow-up to public reports is essential to maintain the credibility of such a program.

It is particularly important to work with any local nurseries, botanical gardens or plant importers, as these are often sources of new introductions. A positive approach is to work together to develop a "white list" of both native and non-native species that the public can be encouraged to plant.

Foresters, conservation officers, extension agents and others that spend time in the field should be alert to new species that exhibit invasive behavior. Often, these species first show up in urban or farm areas because they are usually introduced by people and tend to first become established in flower gardens and disturbed areas. Suspicious plant species should be promptly reported. Periodically scheduled surveys can also be conducted for new or expanding infestations. An evaluation should be conducted for any new species that appears to be invasive or is known to be invasive elsewhere. Assistance by an expert who is familiar with the species and methods for its eradication or control should be requested if needed. Prompt action is essential, since once a species becomes widespread, control or eradication can be extremely costly or impossible. Assistance is also available on-line from experts through the Pacific Pestnet and Aliens list-servers.

Laws and regulations should be reviewed to make sure they are adequate to deal with new introductions. This includes the ability of government to require the control of noxious species on private lands or to take action on private lands if the landowner cannot be located or does not take prompt action. Provision for emergency funds to deal with immediate problems should also be in place. Model laws and regulations can be obtained from states and countries that have implemented them.

The CNMI government is encouraged to take advantage of the Federal assistance programs in dealing with invasive plant species. Cost-share funding is available to provide locally available expertise in forest health protection. Experts are also on call from the Forest Service’s Forest Health Protection staff, or they can arrange for expert consultation. Finally, cost-share funding can be requested to deal with forest health problems, including plant pests, under the Cooperative Forestry and Hawaii Tropical Forestry Recovery acts. Funding is subject to recommendations resulting from a professional evaluation of the problem and the overall availability of funds. Funding also tends to be prioritized based on an economic evaluation of cost-effectiveness, so rapid eradication of a pest species while it is still confined to a small area would undoubtedly take precedence over the chronic problem of a species that has escaped control.

Recommendations

In addition to the above strategies, we offer the following specific recommendations:

To prevent the introduction of Miconia calvescens, quarantine officers should be alert to vistors from Hawai‘i who might have been in the woods or rural areas, especially on the Big Island of Hawai‘i, and inspect their shoes or boots for seeds. Any trucks or equipment coming from Hawai‘i that have been used in rural areas must be power washed or steam cleaned.

Appendix 1.  Background material and references

Appendix 2.  Species by category

Appendix 3.  Invasive species present on Saipan, Tinian or Guam but not present in Rota

Appendix 4.   Scientific name synonyms


(1)  Former Director, Pacific Southwest Research Station, USDA Forest Service (now retired); Botanist, Australian Quarantine and Inspection Service; Research Ecologist/Team Leader, Institute of Pacific Islands Forestry, USDA Forest Service and Forest Health Coordinator, Institute of Pacific Islands Forestry, USDA Forest Service, respectively.


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