A 1998 paper by U.S. Forest Service researcher James Allen explains that red mangroves arrived on the island of Molokai in 1902. Most of us have probably tried a quick fix at some point that solved one problem but ended up causing another. Hawaii did something like that, but with a tree. They were mainly planted to stabilize coastal mudflats. What followed became a case study of how a well-meaning project can turn into a long-term headache.
Hawaii didn’t have mangroves before the early 1900s, so the tree was a newcomer.
D’Iorio’s chapter agrees that introduced mangroves have contributed to habitat loss for the four waterbirds, including the endangered Hawaiian stilt. The Hawaiian stilt might be the most affected species. Its favorite feeding places are shallow flooded marshes and open tidal flats where mangroves grow easily. Managers often manage pond water levels for the birds, but mangroves also often block drainage outlets. The story is not all bad news and both sources agree on that. The USGS chapter describes the impacts of the mangroves on Molokai as both positive and negative. They hold coastal land in place, create new fish and bird habitat , although Allen found that none of the four native waterbirds use it, and slow the flow of sediment onto the reef. At the same time, they also fill old fishponds, creep onto reef flats and beachfront property and shrink wetland habitat that is needed by native waterbirds.
But the 1902 planting can be a helpful reminder that introducing a species to a new place can end up setting off changes nobody planned for.
Allen also says the trapping of sediment can be seen as a benefit. Both works are older, so the numbers today may differ.
