Alien Habitat

In the Mangrove Jungle

The bad thing about alien invasive species is that they tend to take over and make over whatever habitats they move into. Red mangrove is especially good at creating its own habitat – one in which it can thrive and hardly anything else can survive. The dense jungle of roots and branches it produces slows the flow of water, traps silt and blocks out sunlight. The heavy litter of leaves and propagules it drops decay and rob the water of oxygen. When the bottom litter has grown thick and the water is shallow enough, then new propagules take root and increase the density of the mangrove. Meanwhile, prop roots descend from increasingly higher positions on the growing tree, expanding the tree’s footprint and enabling it to suck greater amounts of nutrients from the soil and oxygen from the air.

What, besides the mangrove itself, can survive in the mangrove jungle? While paddling on the river, we can see many adult tilapia swimming along the outer edges of the mangrove roots. So baby tilapias probably hide and grow in the shallower water among the roots. Based on observations from the canoe over the years, the population of this alien fish species does appear to be increasing in the Hulē‘ia River. We also see striped mullet (‘ama‘ama) jumping and swimming in the river. But this species is known to spawn offshore, and schools of their young can be seen in open shallow waters of the estuary. On Oahu, it has been reported that cattle egrets and night herons nest in the branches of mangrove trees. Although we have not observed such nesting along the Hulē‘ia River, it is possible that this occurs, because much of the mangrove cannot be easily seen, and the local population of cattle egrets, at least, is visibly growing. Where mangrove grows on dryer ground, the Kaua‘i feral chicken is known to roost in the trees, and feral cats may also live among the mangrove roots. However, none of the animals that benefit from the mangrove is Hawaiian in origin. All are alien in the same way that their mangrove habitat is alien.

Endangered Native Birds

As the red mangrove jungle spreads and increases in density, it covers the more open wetland spaces that native water birds depend on for feeding and nesting.  The U.S. Fish and Wildlife Service identifies five endemic water bird species that are endangered due to loss of their natural habitats: the Hawaiian stilt (ae‘o), the Hawaiian coot (‘alae ke‘oke‘o), the Hawaiian moorhen (‘alae‘ula), the Hawaiian duck (koloa maoli) and the Hawaiian goose (nēnē). In spite of the presence of the Hulē‘ia National Wildlife Refuge, which was created specifically to restore the natural habitats of these birds, only the koloa duck may be occasionally seen swimming on the Hulē‘ia River.  Ironically, one can get a better view of these birds around the Kaua‘i Lagoons Golf Course.  If they are living in the wildlife refuge, they cannot be seen there because of the tall mangrove growing along the river bank; and the one public view point allowed by the refuge is the same view point of the ‘Alekoko Fishpond that shows only red mangrove overgrowing everything. When looking out from that popular tourist stop, even the open waters of the fishpond appear to be devoid of wildlife.

There is cause for hope. At the Nu‘upia Ponds Wildlife Management Area, which is part of the U.S. Marine Corp Base in Kaneohe, Oahu, soon after the red mangrove was cleared, nests of the Hawaiian stilt were observed in the cleared areas. Other factors negatively affected the population growth of the stilt there, but at least the dense mangrove jungle was replaced by open wetland that the birds could use, and the herons and egrets that preyed on the baby stilts were gone. Removing the mangrove and planting native species is a good first step toward bringing back the populations of endemic water birds to the Hulē‘ia River and until we eradicate the red mangrove, these five birds will continue to be missing from our Hulē‘ia.

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