An invasive Japanese bird has wiped out tens of thousands of native birds at the Hakalau Forest National Wildlife Refuge on Hawaii island, two University of Hawaii researchers have found.
The mejiro, or Japanese white-eye, Zosterops japonicus, has surged in population in recent years to the detriment of eight native species: the akepa, akiapolaau, amakihi, iiwi, apapane, elepaio, omao and Hawaii creeper.
Those findings by UH researchers Leonard Freed and Rebecca Cann appeared Wednesday in the journal NeoBiota.
Ironically, the Hakalau reserve on the windward slope of Mauna Kea was established in 1985 in part to protect those eight species.
But in 1989 the refuge and its volunteers began an extensive forest restoration project, planting nearly 400,000 seedlings on an abandoned cattle ranch above old-growth forest.
This area was soon colonized by mejiros, and their population grew exponentially once the trees grew enough to develop a canopy that provided bugs to eat and warmth at night at a high elevation. This increase was followed by significantly more white-eyes in the forests adjacent to the restoration area.
Freed and Cann, who have been studying the Hakalau forest birds for 20 years, found that as the number of mejiros grew, young birds of the native species showed stunted growth, indicating that food was scarce, and birds of all ages had trouble replacing their feathers.
Each problem was unprecedented, they say.
By contrast, native birds deeper within the forest — where mejiros were still rare — had normal growth and feather replacement, they found.
“The natives have been neither able to reproduce successfully, nor replace their feathers as they wear out, in the same amount of time as they had previously,” Cann, a geneticist with the UH John A. Burns School of Medicine, said by email. “The population of natives that remains is dwindling down, and may enter a demographic spiral to extinction as older and older birds become the only survivors.”
Their research, based on survey data collected on 8,330 acres of open forest and 4,937 acres of dense forest, showed a dramatic increase in white-eye population between 2000 and 2007. Over the same period, the population of native birds dropped by one-third, they estimate.
There was a more gradual increase of mejiros in the dense forest downslope, where about 10 percent of the native birds disappeared.
The news is particularly bad for one species, the akepa, Freed said in a telephone interview.
The akepa in their four-month breeding season lay a single two-egg clutch.
Early in the season, the eggs typically hatch into males. Late in the season, they hatch into females — but that’s when they must compete not only with adult mejiros, but their offspring as well.
As a result, females now comprise just 13 percent of the akepa population, said Freed, an associate professor in the UH Department of Zoology and Cann’s husband.
“That’s why we can project extinction,” he said.
In fact, the only species that is doing well in the refuge is the mejiro, Freed and Cann say.
“Conservation scientists cannot predict the outcome of a restoration effort,” Cann said. “We knew the birds needed new habitats to recover from earlier population declines — disease, predation, etc. — but the unintended consequence of the restoration effort, in the meantime, is a growing area of young trees that is a better habitat for the invasive alien species than the native species. An attractive nuisance has been created.
“The areas of good old growth forest where the native birds are making their last stand now have to be managed to selectively remove white-eyes back below the levels before they expanded from the adjacent restoration areas.”
Jim Kraus, who manages the Hakalau refuge for the U.S. Fish and Wildlife Service, said Thursday by email that he has not had a chance to read the NeoBiota article.
But he said the refuge expects to finish an analysis of the past five years of bird survey data in the next year or so.
“Until that is done, we’ll continue on the management course prescribed by our plan,” he said. “Of course, the service welcomes new scientific information relevant to management.”
The Hakalau refuge grows 20,000 plants a year at its on-site greenhouse, according to its website. These include koa, ohia and understory species such as akala, kolea and ohelo.
Seeds are collected on site, germinated, planted and transplanted in target areas by volunteers under the supervision of the staff horticulturist.
“The forest habitat is coming back, and the ‘proof in the pudding’ is the return of the native forest birds to an area that was open pasture just 20 years ago,” the refuges website says. “Many of the common species, apapane, iiwi, elepaio, and amakihi are seen regularly within the replanted areas.”
