Mangrove: the invasive marine weed tree


These days we read a lot about invasive plants and animals, but rarely are we reminded that not all of these make their home on dry land. Mangrove is a coastal tree that thrives in salt and brackish water, and creates its very own ecosystem, a mangrove swamp. The mangrove is not native to Hawai 'i, but like many other alien species it finds our island an ideal place to settle down and thrive.

The tide pools makai of Kapoho Vacationland are now a Marine Life Conservation District (MLCD) and protected by law. But don’t tell that to the mangrove seedlings which are floating down from, probably, private property along Kapoho Bay. They are establishing themselves throughout the tide pools and coral gardens, and if this continues we will no longer have tide pools and coral gardens (with the spectacular marine life that lives there now), but a mangrove swamp instead.

Poster - Big Island Red Mangrove Eradication Project - A Joint Partnership with the Big Island Invasive Species Committee (BIISC)

mangrove seedlingMangrove seedling still connected to fruit.


Archaeological Monitoring Plan for the Proposed Eradication of Mangroves at ‘Alula Bay

Tidepool Fish Assemblages at Wai`Opae Marine Life Conservation District, Hawai`i: Monitoring the effects of mangrove eradication on nearshore fish assemblages

Anchialine Pool Restoration and Mangrove Eradication at Alula Bay, Kealakehe, Kona

Restoration and Protection of Hawai‘i Island’s Coastline through Island-Wide Extirpation of the Alien Red Mangrove, Rhizophora mangle

Summary of Mangrove Impacts: Red mangrove in Hawai'i is a highly invasive alien that contributes to decreased water quality by dropping large amounts of organic matter, resulting in nutrient loading and anoxia, sedimentation, and hypersalinization, conditions favoring alien fish and excluding coral and associated organisms. Mangroves also exclude native terrestrial coastal vegetation and make coastlines inaccessible by their vast network of branches and prop roots. They destroy nesting habitat for all 4 Hawaiian endemic shorebirds.

Project Objective and Benefits: The overall objective is to eradicate mangroves from Hawai‘i Island, to restore the natural terrestrial and aquatic coastal ecosystems and protect the rest of the coastline from encroachment by invasive mangroves. Benefits include: elimination of annual shedding of mangrove biomass into tidal waters and resulting improvement in water quality; removal of direct threat to coral reef ecosystem from shading and physical obstruction by mangroves; expected ecosystem benefits include recovery and protection of native fish populations, recovery and protection of shorebird habitat (Hawaiian stilt* coot* and nene*), improvement of green sea turtle* and monk seal* habitats, protection and recovery of Ischaemum byrone, beach grass* populations. (* Indicates listed endangered species observed in project areas or vicinities.) Previous experience at Wai ‘Opae in Puna, Hawai‘i indicates that eradication is feasible. We propose to apply what we’ve learned at Wai ‘Opae to eliminate mangroves at the other sites of infestation, killing mangroves by direct injection of herbicide into trees and manual removal and/or foliar herbicide spraying of immature plants. Four additional sites have been identified for mangrove removal totaling approximately 15 acres. The project is a joint partnership with Malama O Puna and the Big Island Invasive Species Committee (BIISC).

Project Justification: The red mangrove (Rhizophora mangle) is a pan-tropical tree with native coastal habitat ranges in Florida, the West Indies, and South America. Red mangroves were first introduced to Moloka’i in 1902 and have since spread to all of the main Hawaiian Islands with major infestations on Molokai and Oahu. Red mangroves colonize salt marshes, protected reefs, tidal flats, fishponds, estuaries, embayments, canals, lagoons, streams, anchialine pools and other protected coastal areas (Chimner et al. 2006). They reproduce and disperse rapidly, which makes them highly invasive (Allen and Krauss 2006). “Although they have received less attention than many other alien species in Hawai’i, within the range of suitable habitat, mangroves have largely validated Egler’s (1947, p407) prediction that they would effect ‘a change as sweeping, as complete, and as striking as any which has occurred in the Hawaiian Islands’”(Allen, 1998).

mature mangrovesRed mangroves in Hawai’i have been found to grow to higher densities than in their native range (Cox and Allen, 1999), probably because Hawai’i lacks the species that attack the flowers and propagules. Litterfall from mangrove stands at Nu‘upia Pond, Oahu has been measured at 2.52 kg ha-1, which exceeds net primary productivity in its native range in Florida (Cox and Jokiel 1996, Odum McIvor and Smith 1982). These added organic inputs have led to detrital accumulations and algal blooms. Other influential ecosystem processes include water stagnation, soil sedimentation, anoxia and hypersalinization (Cordona and Botera 1998, McKee 1996).

The positive fish habitat value of mangroves has been well documented in ecosystems where these trees are native (i.e., Caribbean, Australia, Florida). In Hawai’i, however, it appears that mangroves may be having a negative impact on fish community structure. The most extensive studies on mangrove impacts to fish have been done on Moloka‘i (Demopoulos et al. 2007, Nakahara 2007) and show similar results to studies on the island of Hawai‘i; mangroves seem to be be providing habitat for invasive fish species (e.g., poeciliids, snapper, tilapia) (Van der Veur 2006, MacKenzie et al. unpublished data); species that have adverse effects on habitat value for native fish communities as well as on native fish community structure. Elsewhere in the state, red mangroves have been shown to displace nesting habitats of the endangered Hawaiian duck (Anas wyvilliana), stilt (Himantopus knudseni), coot (Fulica Americana) and moorhen (Gallinula chloropus) (Rauzon and Drigot 2002, Chimner, et al. 2006). At Wai ‘Opae Marine Life Conservation District mangroves were observed over the past decade to exclude the listed endangered Hilo beach grass, Ischaemum byrone (David Paul, unpublished).

Once established in an area mangroves can spread down the coastline to areas where an initial colonization event had been unlikely, as has been seen at Wai ‘Opae. Fortunately, red mangroves are confined by their dispersal mechanism to shoreline areas and are thus good candidates for eradication as they are easier to locate than other aquatic or terrestrial invasive plants. That said, mangroves create their own inaccessibility, growing in thick networks of trunks, branches and prop roots that make movement through them difficult, either by water or land. Because mangrove propagules are only viable for 3 months off the tree, once they have been removed from an area for that long, repopulation can only happen if propagules wash in again from elsewhere. This is a huge advantage for eradication efforts, compared with other invasive plants that have seeds persisting for many years.

Mangrove swamps become most extensive on subsiding coastlines (Chapman, 1976), as they are able to continually form higher prop roots as the water rises and carries their propagules inland. The land at Wai ‘Opae is subsiding faster than any other in the state, 2 cm per year, which may explain why this area had the largest infestation of mangroves on Hawai‘i Island. Sea level rise would produce the same effect for mangroves as subsidence; that is, the infestations would become more extensive. We would be wise to prepare for sea level rise by eradicating mangroves now, before their accelerated spread further limits access to them and the coastline.

This project will not only improve the habitat value of coastal areas for native Hawaiian species through mangrove eradication, but will halt the spread of mangroves along the coast of Hawai‘i Island and significantly reduce the ecological, economical, and cultural impacts associated with the invasion of coastal areas by mangroves. The richness of coastal ecosystems, especially in Hawai‘i where endemism is so high, adds meaning to all our lives apart from any economic value. The destructive impacts of the red mangroves are just beginning to occur on the island of Hawai‘i and now is the time to stop the spread, before mangrove swamps become as developed as those on O‘ahu or Moloka‘i.


Description of the Project: Our pilot project site of Wai ‘Opae had 20 acres of mangrove infestation, which is now nearly eradicated, and we are replanting with natives: primarily hala, naupaka and kou. There is milo sprouting up naturally all over the site. There are an additional 15 acres of mangroves at 4 sites on the island. We are applying the methodologies we developed at Wai ‘Opae the additional sites: Poho‘iki and Paki Bay in Puna, Onekahakaha in Hilo. One other site, Alula Bay in Kona, the mangroves are threatening a heiau, so we plan to use volunteers to cut and remove them, and are working in collaboration with Keala Ching and Na Wai Iwi Ola Foundation (the cultural practioners who use the heiau) and Kaloko-Honokohau National Historical Park. There are 2 additional sites on the island where property owners are already engaged in eradication efforts.

tall dead mangrovesHabitat herbicide is approved for shoreline use and the mangroves are very sensitive to low concentrations of Habitat, whereas the native plants are less sensitive. Habitat’s active ingredient is imazapyr, which inhibits an enzyme found only in plants The enzyme, acetohydroxy acid synthase (AHAS), catalyzes the production of three branched-chain aliphatic amino acids (valine, leucine, and isoleucine) that are required for protein synthesis and cell growth (Cox 1996). Animals do not have this enzyme, as these amino acids are ones that we must take up in our food. Habitat therefore has low toxicity to animals, less than aspirin, and is biodegraded as well as broken down by sunlight. To minimize the risk of contamination of water we work at low tide in dry weather, and we have shown that the herbicide is inactivated by sea water. The herbicide is not active against submerged aquatic vegetation, eg, limu. Habitat also has the advantage of killing the propagules on the tree.

Two herbicide delivery techniques are being used in the eradication. It is our priority to eliminate the larger trees first because they produce propagules. In the first phase, herbicide injections are made into mangroves that exceed 1 meter in height. The delivery method we have demonstrated to be effective utilizes a cordless electric drill to make a hole or two into the tree, to which is added 0.5 to1 milliliter of herbicide (1/60 to 1/30 oz.). We systematically cover the entire area of mangrove infestation by treating all of the larger trees with doses of herbicide according to their size classes. Small trees that are too narrow to drill are broken and a drop of herbicide applied to the break. The second phase of the eradication involves foliar spraying of the shorter-stature mangroves, including seedling thickets and saplings up to 1 meter in height. We use a very low concentration of herbicide, 0.3%. An alternative to spraying is to have volunteers cutting and removing the smaller mangroves, so the more volunteers we get the less we spray.

We also have volunteers pulling and hand-cutting the isolated mangroves, especially those closest to the ocean. At Wai ‘Opae, we have hosted groups of children from the local schools. The children love to pull out and collect the small mangroves. This is a wonderful opportunity for kids to care for a place that will always be special to them, and they can then teach their families and friends about the problems of this invasive species, thereby greatly increasing the number of people out there intercepting mangrove propagules.

Permits and Approvals: We have informed the Army Corps of Engineers, Office of Conservation and Coastal Land (OCCL), and Department of Health Clean Water Branch of our intention to rid the 4 sites of mangroves. All of these agencies have informed us that we do not need to obtain permits from them, as long as we don’t use chain saws (OCCL stipulation). The County of Hawai’i does require that we get a Special Management Area (SMA) Minor Permit for each project site. We did not have to do an EA because on the State land we were exempted from needing a CDUP (conservation district use permit) and on the County land our project fell into the category of landscaping of a preexisting park, both conditions for exemption from needing an EA. The USFWS, who provided our funding, has made a determination that this project fell under a categorical exclusion from needing an EA.

PubIic Involvement: We gave presentations on the mangrove project at our last 2 MOP annual meetings, which were widely advertised, and 2 articles were published in the Big Island Weekly. We reported on the project at the Hawaii Conservation Conference for the past 2 years and covered it numerous times over the past 3 years in our monthly Puna News column. We have displayed posters at festivals at Poho‘iki, Puna Sustainability Expo, and UH Hilo. A prominent sign posted at the entrance to our Wai ‘Opae restoration site informs the public about our project. We have had numerous school and community work parties during which the volunteers learn some coral pool ecology while helping to care for a place. We welcome volunteers! Please call 965-2000 or email for more information or to help out.

Mahalo to all the students and community members who have helped with the work, to the USFWS for funding, and to you for your interest!

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Rauzon and Drigot, 2002. Red Mangrove Eradication and Pickleweed Control in a Hawaiian Wetland, waterbird responses and lessons learned. p 94-102 in: C.R. Veitch, ed. Eradication of Island Invasives: Practical Actions and Results Achieved. International Union for Conservation of Nature, Aukland, N.Z.
VanDer Veur, J. 2006. The Effects of Rhizophora mangle on Fish Assemblage of Kapoho, Hawaii. Student Thesis, UH Hilo.