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Red Mangrove Eradication and Pickleweed control
in Hawaiian Wetland, Waterbird Responses, and Lesson Learned.

M. J. Rauzon and D. C. Drigot

Alien red mangrove (Rhizophora mangle) and pickleweed (Batis maritima) are major invasive plants in Hawaiian wetlands, including Nu’upia Ponds, a 195 hectare wildlife management area and historic Hawaiian fishpond complex on U.S Marine Corps Base Hawaii. These fishponds are also home to approximately 10% of Hawai’i’s endemic and endangered black-necked stilt (Himantopus mexicanus knudseni) population and at least 16 species of native fish. Invasive plants were changing the ecology and character of the fishponds from Hawaiian to Floridian. After 20 years of effort with thousands of volunteer hours, and over USD 2.5 million of contracted labour, over 20 acres of mangrove were removed. Mangroves were cleared by hand, shovels, and chain saws in archaeologically-sensitive areas and grappled with heavy tracked equipment in less-sensitive areas. Work was performed in the non-nesting season of the resident waterbirds. Prior to cutting, mature mangrove stands had been colonised by black-crowned night-herons and cattle egrets, causing work schedule alterations and the need for hazing permits. Pickleweed, an invasive ground cover, is annually plowed using Amphibious Assault Vehicles during “mud ops” training manoeuvres. The results show that stilts readily colonise mudflats cleared of alien vegetation, especially near established breeding areas. Lessons learned regarding waterbird conservation are discussed.
Keyword: Hawaiian stilts; cattle egrets; black-crowned night-herons; egg measurements; red mangroves; pickleweed; tilapia; wetlands.

Introduction of alien species has substantially changed the lowland landscape of Hawai’i. Alien plants displace native Hawaiian coastal plants, colonise unexploited habitats, trap sediments, and adversely affect water quality and hydrology. Alien animals consume primary producers, eliminate vegetative cover, foster erosion, and prey upon endangered species. Hawai’i’s coastal wetland areas have been extensively altered for aquaculture, agriculture, grazing and urban development (Cuddihy and Stone 1990). Consequently the remaining Hawaiian wetlands that still harbour a few adaptive indigenous species also face a constant onslaught of alien species encroachment from the surrounding, extensively-altered landscape.

In contrast to Hawaiian stilts, fecundity of cattle egrets is especially high. Within one year of establishing a colony, egrets produced over 200 nests with about 500 eggs, clearly demonstrating why the species has undergone such enormous global expansion. One reason for the birds’ success is its unique ability among Ardeidae to breed when they are one year old (Kohlar 1966). One clutch per year is usually laid but up to three has been recorded, with usually 2-6 eggs per clutch (Berger 1981). This fecundity is coupled with behavioural adaptability to anthropogenic disturbances and abundant food. By feeding on introduced species (cockroaches, centipedes, mice, etc.) that are exposed by large grazing ungulates and lawn-mowing machinery, egrets fill an unoccupied ecological niche in Hawai’i as elsewhere. Cattle egrets benefit the Base by eating many introduced pests. However, they pose a hazard to aircraft safety due to their propensity to forage in grassy airfield borders, and are reputed to carry avian diseases that could spread to native bird populations.

No native Hawaiian species have yet learned to adjust their behaviours so precisely to human’s ways, although the Hawaiian stilt may owe its survival to being able to forage on introduced food items as well. For example, stilts are commonly observed foraging on cockroaches in grassy inland areas of the Base, and forage daily at the polishing ponds at the Base water reclamation facility. Mangroves are essentially eradicated from the Nu’upia Ponds WMA, although seeds float into the ponds from the outer bays where mangroves remain uncontrolled in coastal areas outside MCBH jurisdiction. Sprouted propagules must be pulled up on a continual basis by volunteer service groups until an effective seed filter is in place at inflow channels. Future mangrove management may lie in biocontrol. The first steps in exploring a biocontrol strategy, albeit using another alien species, is underway. Mangrove propagules were sent from Hawai’i to Louisiana to test them for susceptibility to a beetle (Poecilips/ Coccotrypes rhizophorae) that reduces the production of viable seeds (Allen 1998).

A promising tool for regional mangrove management is a specialised amphibious excavator, recently purchased by the City and County of Honolulu in partnership with Ducks Unlimited and the U.S. Fish and Wildlife Service. Other state, federal and local landowners with similar wetland management responsibilities are evaluating possible ways to leverage their individually-limited resources through cooperative use of this equipment on alien species whose spread remains indifferent to jurisdictional boundaries. With the advent of such interagency partnerships to the arsenal of alien species management tools, it is hoped that one day soon the Hawaiian stilt may be removed from the Endangered Species list.

mangrove mangrove

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