Red Mangrove’s Invasion of Hawai‘i’s Coastline: Why Does it Matter?

Red mangrove choking the Hule‘ia River next to the National Wildlife Refuge. Photo by malamahuleia.org.

On a recent trip to an estuarine restoration conference in Washington, D.C., I was inspired by all the hardworking efforts worldwide to restore coastal ecosystems. In a session on restoration of coastal mangroves in El Salvador, I was also reminded about how a “good thing” in one place can spell disaster in another! However, the call to action remains the same…

Take mangroves in Hawai‘i, for example.

Mangrove forests are found around the world in the “intertidal zone” of tropical estuaries, lagoons, and sheltered coastlines. Mangroves can occur naturally in tropical and subtropical latitudes between 25 degrees north and 25 degrees south. While Hawai‘i is within tropical latitudes (~21 degrees), due probably to its geographic isolation from other landforms, mangroves never occurred here naturally. Red Mangrove (Rhizophora mangle) was introduced from Florida by the sugar plantations in 1902 to stabilize the soil and provide forage for bees.

In most places where they evolved naturally, mangrove ecosystems provide important benefits to the environment and society as a whole. In places like Florida, Central America, and the Philippines, mangroves provide habitat for fish, crabs, shrimps, mollusks, nesting and migratory birds, reptiles, and mammals.

Mangroves also help keep coastal waters clean by trapping eroding sediments from the land, and form a protective barrier for storms.

Red mangroves choking Mo‘ikeha Canal in Kapa‘a. Photo by Ruby Pap

Unfortunately, in Hawai‘i, many of these benefits are not realized and in most cases the negatives far outweigh the positives. Red Mangrove has invaded many wet intertidal and riverine areas due to a lack of natural predators or competitors. It favors establishing itself in areas already disturbed by humans such as abandoned taro loi, fish ponds, and other areas where fresh sediment has been deposited. On Kaua‘i, a classic case of this occurs in the Hule‘ia watershed (Nawiliwili). It is also found along the canals in Kapa‘a town.

The introduction of red mangrove in Hawai‘i represents an entirely new lifeform in the Hawaiian wetland ecosystem. There is little wildlife found in Hawaiian mangrove forests except that they can provide shelter for predators that prey on waterbird chicks, such as cattle egrets, rats and mongoose. This means less habitat for Hawai‘i’s threatened fish and water birds, such as the Hawaiian Stilts, Hawaiian Coot, Hawaiian Ducks (Koloa duck) and Hawaiian Moorhens.

Further, the dense stands of trees and roots crowd out native plant species, and the sheer amount of decomposing plant matter leaves the water devoid of oxygen. These conditions might be OK for animal species adapted to such water elsewhere, but it is inhospitable for some native species. In the Hule‘ia River, for example, the sewage smell among the mangrove roots from these anoxic conditions is often apparent and alien fish species like tilapia are observed to be on the rise.

Like other invasive species, the impacts are much more than “environmental.” Red Mangrove has a strong foothold along the walls of historic fishponds, making their restoration very complicated and expensive, and it poses drainage and navigation problems along our waterways.

Ruby Pap

On Kaua‘i, a group of dedicated paddlers from Kaiola Canoe Club formed Malama Hule‘ia to tackle the mangrove infestation in the Hule‘ia River and Watershed. The Hule‘ia River is the largest stream and estuary of the Nawiliwili Bay Watershed; and in my opinion, one of the most beautiful and important watersheds on the island.

With the help of dedicated volunteers and foundation grants an area has been cleared adjacent to the Niumalu Beach Park. Help is still needed, as the infestation extends up the Hule‘ia River, into the Alekoko Fishpond, and adjacent to the National Wildlife Refuge.

Visit malamahuleia.org to learn more.

• Ruby Pap is a Coastal Land Use Extension Agent at University of Hawai‘i Sea Grant College Program. She can be reached at rpap@hawaii.edu.