Hawai‘i’s many offshore islets may be eye candy to visitors and residents alike. But, as Chris Swenson of the Fish and Wildlife Service noted during last summer’s Hawai‘i Conservation Conference at the Convention Center in Waikiki, they are also ecologically self-contained systems hosting large seabird colonies and native coastal plant and insect species. What’s more, he said, they are ideal outdoor laboratories where researchers and managers can test restoration techniques that could one day be used on the main Hawaiian islands.
The islets range from large rocks jutting out of the ocean to the 284-acre Lehua islet, and include everything from flat, sandy pads to rugged sea stacks. Seabirds are by far their best-known inhabitants. Boobies, black-footed and Laysan albatross, shearwaters, petrels, and terns all make their homes on the islets scattered around the main islands. And they also are safe havens for rare native plants, bugs, and birds. One islet, for ex ample, has the only remaining intact loulu forest, giving researchers an idea of what parts of Hawai‘i must have looked like be fore the arrival of humans.
Ken Wood, formerly of the National Tropical Botanical Garden, noted that the Offshore Islet Restoration Committee, es tablished in 2002, has surveyed 42 offshore islets in waters around the Main Hawaiian Islands, and has documented 243 vascular plant species: 51 endemic, 50 indigenous, 137 non-natives, and five Polynesian introduc tions.
At the annual Hawai‘i Conservation Con ference held July 28 and 29, organizers dedi cated a session to islet restoration. The fol lowing summarizes some of the highlights:
Hawai‘i’s islet ecosystems face a variety of threats, including fire, predators, and hu man disturbance. At Lehua islet, located one kilometer north of Ni‘ihau and 30 kilome ters west of Kaua‘i, the top priority is to remove rats and rabbits, Swenson said.
Lehua supports about 25,000 nesting pairs of 11 different species of seabirds, said Eric Vanderwerf, also with the FWS. The islet is the largest nesting colony in Hawai‘i for brown- and red-footed boobies, and is the only site in the Main Hawaiian Islands where both Laysan and black-footed alba tross nest.
In addition to having to compete with introduced cattle egrets for space, Lehua’s native seabirds must also contend with barn owls and Polynesian rats that prey on the birds and their eggs. Vanderwerf said that Dave Burney, a paleoecologist with the National Tropical Botanical Garden, ex amined owl pellets found deep in the sand beneath a roosting area and found evidence that the owls were at one time eating brown noddies, which Vanderwerf said had been extirpated from Lehua. Burney found that the owls were eating wedge-tailed shearwa ters as well. Near the owl roosting sites, Burney found rat and house mice bones. As there are no house mice on Lehua, Vanderwerf said, the owls probably hunted the mouse-infested Northwestern Hawai ian Island of Nihoa before resting at Lehua.
Rabbits also impinge on the birds’ habi tat by nibbling up the foliage that some of the seabird species use for nesting. Vanderwerf states that the Offshore Islet Restoration Committee plans to eradicate or control invasive species on Lehua and other islets, adding that Kaho‘olawe also has a lot of potential for seabirds.
It was an unpleasant sight, but necessary to illustrate the harm ants can cause to native seabirds. University of Hawai‘i zoology graduate student Sheldon Plentovich pro jected onto the room’s massive screen a slide of a Mokulua shearwater chick whose foot had been decimated by fire ant stings. The chick, and another with similar inju ries, did not live past fledging, she said.
Sixteen alien ant species can be found throughout Hawai‘i’s islets, where the ter restrial arthropod communities are domi nated by ants. Five of those species, Plentovich states, are particularly harmful to native communities.
Ants are traditionally impossible to con trol. Plentovich, under UH Zoology De partment dean Sheila Conant’s supervi sion, conducted ant control experiments on two pairs of islets off O‘ahu’s windward coast: the Mokuluas off Lanikai, and Goat Island near Malaekahana and Flat Island in Kailua.
The Mokuluas are dominated by crazy ants (Paratrichina longicornis) and fire ants (Solenopsis geminata), while Goat and Flat islands are dominated by big-headed ants (Pheidole megacephala). Plentovich treated one islet from each pair with AMDRO granular pesticide, monitored them for sur vivors using peanut butter, honey and Spam as attractants, and compared them to the untreated islets.
At the northern Mokulua islet, which had been treated with AMDRO, fire ant populations decreased from 24.6 individu als per bait card to .56 individuals, and crazy ants went from 39.5 individuals down to 7.8 individuals per bait card.
On Mokuauia (Goat island), big-headed ant populations went from 135 individuals per bait card down to zero. And after two years, no big-headed ants were found in samples. Populations of other ant species, including fire ants, grew, but their densities remained lower than that of the big-headed ants prior to treatment.
Plentovich found no evidence of injury from big-headed ants on any of the birds on Goat and Flat islets. However, she did find evidence of fire ant injuries on shearwaters at the Mokuluas. She found many birds with ant stings, which lead to holes or the complete loss of parts of their feet.
Plentovich concluded that the pesticide treatment not only reduced ant numbers, but also, in the case of the treated northern Mokulua islet, led to a reduction in fire ant injuries to chicks. She added that leaf scales on that islet, which may be tended by ants, were also reduced, leading to an increase in ‘ilima leaf cover.
Across the Pacific lies the Marianas archi pelago, which faces many of the same threats that Hawai‘i’s islets do, and more. Earl Campbell, invasive species coordinator for FWS, described both the common and unique challenges managers there face.
Snakes, rats, cats, monitor lizards, ungu lates and people are the main threats to the Mariana islets. On Managaha, a 10-acre islet off the west coast of Saipan that attracts tourists and supports a number of conces sionaires, cat and rat eradication efforts have allowed the local shearwater popula tion – the only known shearwater colony in southern region of the Commonwealth of Northern Mariana Islands – to flourish, Campbell said.
Sarigan, a 500-hectare islet that has been ravaged by pigs and goats for 100 years, underwent an aggressive predator control effort from January through March of 1998, led by Curt Kessler of the FWS. Kessler organized aerial and dog-team hunts to reduce the animals’ population. Campbell said the islet is home to a number of listed species and they “were losing everything”because of the ungulates. Within two months, 904 goats and 68 pigs were killed and the population of the endangered Micronesian megapode – a dark, grey-headed chicken-like bird – in creased, as did that of the threatened fruit bat. Overall vegetation also rebounded.
Restoration efforts on Anatahan, which is near Sarigan, were again led by Kessler and started in May 2002. The 3,200-hectare islet had been trampled by pigs for 100 years and goats for 40. Between 1944 and 1999, the forest canopy had decreased by 65 percent, Campbell said. And with Anatahan being home to a native hawk found nowhere else in the Marianas, as well as a fruit bat colony, Campbell said there was strong community sup port for eradicating the ungulates.
Managers eventually removed 3,000 goats and 100 pigs, but about 2,000 goats and 300 pigs re mained on the islet in May 2003, Campbell said. Then on May 11, 2003, the islet’s volcano erupted.
Although ash covered nearly everything on the islet, Campbell said fruit bats and vegetation survived. Kessler continued aerial shooting of pigs and goats in areas that the eruption had opened up, but ground work has ceased. As of 2004, fewer than 1,000 goats remained on the islet, Campbell said.
With a second eruption last June, Campbell said, “We’re very close [to eradicating the ungulates] and obviously the earth is helping.” He added that managers want to try to revegetate the island once the goats are gone.
— Teresa Dawson
Volume 16, Number 4 October 2005