Searching for Hope in the Family Tree
Within the genetic code of Hawaii's honeycreepers, scientists may find clues to help save the world's most imperiled group of birds
Susan Champlin Taylor
Rebecca Cann could hardly believe what she was seeing.
The evolutionary geneticist was looking at the results of blood samples taken from 311 birds, 42 native and 269 nonnative, on the Hawaiian island of Oahu. The samples would show whether the creatures had avian malaria--a key culprit in the devastation of Hawaii's native birds. But on this day, Cann found what was possibly the last thing she expected: While some species of nonnative birds were showing evidence of malaria in counts as high as 60 percent, none of the 41 native Oahu amakihi in her study tested positive for malaria.
"I was shocked," says Cann. "We know it's not that these birds can't be infected--we've caught 'amakihi with malaria on Kauai and the Big Island of Hawaii. There are other, nonnative birds in the same habitat that have this disease. The disease is there. The vector [carrier] is there. But the 'amakihi aren't getting it. Why?"
That's the $64,000 question in Hawaii these days. And for scientists such as Rebecca Cann, the answer may be found inside the animals themselves: in the genetic makeup of the birds that thrive despite the odds against them. Within their genetic code may lie the seeds of hope for saving many of Hawaii's other, endangered endemic forest birds.
Most stories about Hawaii's native species are horrifying tales of extinction, endangerment and encroachment by nonnative species (humans among them). The stories are particularly poignant in regard to the islands' brilliantly colored and highly vulnerable endemic forest birds. Jack Jeffrey, a biologist with the U.S. Fish and Wildlife Service, describes a "wave of death that's been moving uphill" in Hawaii: a variety of alien threats eliminating the islands' native birds from the lower elevations.
"When I first came to Hawaii in the mid-1970s, you would find many native birds living at 3,500-foot elevations," says Jeffrey. "Now you rarely find them below 4,500 feet. And look at the lower-elevation islands: On Lanai and Molokai, the birds are almost gone. The terrain on Kauai only goes up to about 5,000 feet, and even at higher elevations, the birds are disappearing."
Grazing cattle, introduced 200 years ago to provide meat for European settlers, consumed the landscape as if it were a giant salad bar, says Jeffrey; in doing so, the cattle wiped out the food and nesting habitat for many of the forest birds. Today, feral pigs chomp the forest plants and provide breeding grounds for mosquitoes that carry avian malaria. Alien rats prey on the native birds' eggs and young. Having evolved with no mammalian predators and therefore no defenses, the birds are easy targets. "Essentially," adds the biologist, "the only forest area in the entire state that has never been impacted by feral creatures is Olokui, a half-square-mile isolated plateau on Molokai, and a few small ridges on steep slopes in West Maui." The Hawaiian birds also must compete for habitat with the 50 or so nonnative bird species that have been introduced into the islands in past decades and taken over their new territory like houseguests from hell.
Of these threats, perhaps none has been as devastating to the native birds as disease, carried by the mosquitoes that inadvertently were first brought to Hawaii on a whaling ship in 1826. "There's plenty of evidence that introduced diseases and introduced mosquitoes have been responsible for the decline and extinction of Hawaiian birds in the last 100 years or so," says Leonard Freed, a field biologist and evolutionary ecologist who works with his wife, Rebecca Cann, both at the Hakalau Forest National Wildlife Refuge on the Big Island of Hawaii and at the University of Hawaii on Oahu.
"It's smoking-gun kind of evidence--because none of the extinct birds were tested for disease--but it's pretty massive," observes Freed. The 'akialoa, he points out, is a classic example of an extinct native Hawaiian bird for which disease would have been relevant: It ranged below 5,000 feet where mosquitoes abound because the plants it depended on did not grow in higher locales. "It was a sitting duck for the mosquitoes because its specialization limited it to lower elevations," says Freed.
Half of the recorded native bird species are now extinct in Hawaii and nearly half of the remaining populations are endangered. Warns Rebecca Cann, "If we don't solve this problem in 20 years, we're only going to know about many of these birds from fossil records."
Adding to the intrigue--and the urgency--from an evolutionary geneticist's point of view is the fact that many of Hawaii's native forest birds are members of the same family: honeycreepers, which descended from finches that colonized Hawaii three million years ago. Honeycreepers are widely noted for displaying the most dazzling array of adaptive radiation of any group of birds in the world. From one ancestor, at least 54 different species of honeycreepers evolved.
Among them are specialists that feed on a particular food, and generalists that eat a variety of plants and insects. The distinctive scarlet i'iwi is a classic example of a specialist. Its dramatic curved bill is specially adapted to reach efficiently deep into tubular flower blossoms such as lobelias for nectar: The i'iwi perches delicately on the twig next to the flower, then turns its head upside down to reach up into the flower's blossom. A perfect fit. The i'iwi also feeds on the smaller blossoms of the ohia tree.
The 'amakihi--of which there are three species on the different islands--are generalists that range freely from low to high elevations. All have shorter bills than the i'iwi and eat a variety of insects as well as the nectar of many flowers, including introduced plant species.
Today, approximately 23 honeycreeper species remain--less than half the number of species that once populated the islands. Most of the specialists have been wiped out, either due to disease or the loss of their food supply. To help determine why some of the surviving species are less susceptible to disease than others, Cann analyzed DNA structures to establish a family tree of endangered and nonendangered honeycreepers--"who was the closest genetic relative of who else," she says. She found that, despite differences in their appearance, some of the species are very similiar genetically.
For Cann, such information is "good news in terms of the disease threat. It means that if we can find the gene or genes in some nonendangered birds like the 'amakihi that help them to resist or tolerate the disease, then we'll find important clues for locating those genes in other species."
According to Cann, previous studies on animal and plant resistance to pesticides indicate that a species generally requires 50 generations to develop individuals with genetic mutations that make them resistant to the effects of the chemicals. If only those individuals survive and reproduce, their offspring may also carry the mutations and thus will survive as well. "It takes time for a once-rare mutation to become the predominant type," says Cann.
The same principle may apply to pathogens. For an 'amakihi, a generation is just one year because the species reaches sexual maturity in a year. Given that mosquitoes were introduced to Hawaii in the 1820s, the scientist notes that "there's been enough time for 'amakihi that survived the initial epidemic to have evolved so that a few individuals would have the genes to help them resist avian malaria."
The surprising results with the Oahu 'amakihi have led Cann and Freed to pursue new tests to determine if captive 'amakihi can indeed resist the effects of malaria, as blood tests appear to indicate. If they can, says Freed, "we'll try to find a genetic marker that would indicate where appropriate genes for resistance are found."
A genetic marker usually is a particular sequence of DNA that is unique in size. The scientists could use such markers to see whether similar, appropriate genes exist in other, endangered honeycreepers. Though such research would require a lot of what Freed calls "genetic scut work," the findings could enable scientists to use captive-breeding programs to produce generations of disease-resistant birds, which eventually could be reintroduced into the wild.
Cann even envisions a day when experts could transfer genes from resistant to nonresistant birds. "If you can do it in a mouse, which isn't the best model, there's no reason why you couldn't do it in a bird," she says.
Jack Jeffrey agrees that the genetic work of Cann, Freed and other researchers has practical applications for Hawaii's birds--both now and in the future. "What can we do to prevent these bird populations from continuing to decline and to protect them for future generations?" he asks. "We can monitor the heck out of the place, but it doesn't do anything for the birds. However, this is the kind of research that's going to give us the information we need. Perhaps the next step is to try to make birds less susceptible to disease."
Of course, those birds need somewhere to go. "You need safe habitat to put the birds into," notes Jeffrey, who works with other U.S. Fish and Wildlife Service resource managers to protect and enhance habitat in places such as the Hakalau refuge by, among other things, replanting the native forest trees and eradicating the feral pigs that destroy habitat and create mosquito breeding areas.
For several years now, adds Jeffrey, "people have been saying, 'Where the pigs are, the mosquitoes are. And where the mosquitoes are, the diseases are.'" That observation was partially substantiated not long ago by U.S. Geological Survey biologist Carter Atkinson in Hawaii Volcanoes National Park.
Atkinson and his team chose the largest pig "exclosure" in the park--an area that had been fenced off for 10 years to keep pigs out. "We trapped mosquitoes during the warmest three months of the year over a two-year period," says Atkinson, "and we caught half as many of the insects inside the exclosure as outside it."
As in all fields of science, bird researchers in the islands often disagree over which methods work best for removing feral animals from the wild, and whether avian malaria is indeed spreading to higher elevations. Everyone, however, agrees about one key point: the urgency of protecting Hawaii's remaining native forest birds and helping future generations of the creatures to thrive.
"You may disagree with someone here about a point of management or strategy, but these people are extraordinarily committed," says Cann. "Without the things everyone is doing, there wouldn't be anything left to argue about. We're all generating knowledge. We don't know where the key piece of research is going to come from, so we all have to keep the faith."
California writer Susan Champlin Taylor traveled to Hawaii for this article.