Skins of the extinct Akialoa, with their impressive bills
Hawaii: extinction capital of the world
28 January 2008
Captain Cook's crew had no idea how useful the birds they collected in Hawaii would be to scientists studying extinction, says Jim Groombridge.
If you want to study extinction, Hawaii is a great place to start. Aptly named as the extinction capital of the world, half of the islands' 140 historically recorded native bird species are extinct. And some have vanished so recently that resident field biologists can recall them being common when they were children. Some of these extinct species have common names like 'O'o, 'O'u, and 'O'o'a'a - surely these are birds worth getting to know.
Known as the extinction capital of the world, half of Hawaii's 140 native bird species are extinct
Figuring out why species have gone extinct is a real challenge. They probably provided a ready meal for invasive species such as rats and cats. These introduced mammal species escaped from ships such as Cook's Discovery and now thrive on almost all of the world's islands, gorging on the naïve island wildlife.
But geneticists can explain extinction differently: as populations decline to a small size they rapidly lose the chance element of genetic diversity. Together with associated problems of inbreeding between related individuals, these genetic factors are believed to contribute to extinction. Perhaps especially so for endemic species isolated on small islands.
The problem, ecologists argue, is that introduced mammals such as rats and cats are likely to exterminate a bird population well before genetic factors figure. Recent work has shown how we can explain historical bird extinctions on some islands purely by the presence of introduced mammalian predators. This suggests that rapid decline of bird populations by predation leaves little time for genetic effects of small population size to be felt.
Untangling extinction's causes
Humans might also have contributed by hunting birds for their valuable feathers. Indeed, on Hawaii early Polynesians prized feathers as currency to pay taxes to royalty, and, later, European collectors shot thousands of birds for the feather trade. Disentangling which of these factors drive species to extinction is complex. But such knowledge is crucial for conservation biologists working to restore endangered species.
An 'Apapane, 'Himatione sanguinea', shown fitted with a radio-transmitter, is a very common species on Hawaii
So, has genetics played a role in recent extinctions? To answer this question for extinct species like the Hawaiian 'O'o and 'O'u, we can measure the genetic diversity in samples from historical museum specimens. For this we use a technology called microsatellite genotyping to examine their historical genetic signature.
A remarkable genetic resource exists: feathered cloaks made by early Polynesians.
Doing the same for closely related species that are exceedingly common on Hawaii today such as the 'Apapane and 'Iiwi (pronounced eee-eeevee) provides an ideal yardstick to compare extinct with extant. Thankfully, these four species caught the eye of naturalists and hunters alike throughout the 1800s and 1900s. They are extremely well-represented in museum collections, with many hundreds to thousands of dated skins available worldwide.
We used 20 genetic markers to measure the level of genetic variation in close to 150 museum skins collected between 1887 and 1945 and in samples from modern populations. The emerging picture so far suggests that some extinct species retained levels of genetic diversity broadly similar to those detected in the extant species during the 1800s and 1900s that are still common today.
The levels of genetic diversity in the extinct 'O'u population seems to be well within that found in the common species, and we know from field records that the 'O'u population experienced a rapid decline to eventual extinction by 1980. Conversely, we found less diversity in the 'O'o population, which we believe reflects this species' lengthier demographic decline rather than hunting pressure by feather collectors.
Cloaks of many feathers
But, do these different historical genetic signals reflect a true genetic picture? In many historical studies that use DNA, museum-aged specimens collected around the 1800-1900s are often all that's available, leaving us to wonder about the genetic make-up of populations before this. Cook's 'discovery' of Hawaii in 1779 marked the heavy ecological footprint of Europeans, but Polynesians inhabited the islands for well over a thousand years before this.
The cloak is made from feathers taken from many thousands of idividual birds
Fortuitously, a remarkable genetic resource from Hawaii exists in the form of feathered cloaks made by early Polynesians and worn in battle by Hawaiian royalty. A priceless sample for any geneticist, these remarkable cloaks are constructed from feathers taken from many thousands of individual birds and designed with striking patterns using red, yellow, green and black feathers from the extinct 'O'o and 'O'u and the common 'Iiwi and 'Apapane.
Deriving DNA and genetic data from these cloak feathers means we can extend our genetic understanding of extinction in Hawaii to far more ancient bird populations. Feather and skin samples from some of these cloaks have already yielded the entire genetic make-up of some birds. Radiocarbon dating of feathers is now adding an exciting new perspective. It allows us to independently date our genetic data. For instance, one cloak of 'O'o and 'Iiwi feathers is between 550 and 600 years old, one of the world's oldest feather artefacts.
DNA from cloak feathers means we can extend our genetic understanding of extinction in Hawaii.
The next step is to extend this historical genetic work to a larger set of Hawaiian bird populations. This will mean we can look for broader trends between historical genetic diversity and differences in population trajectory. Luckily, almost 30 different island populations are available, each with a detailed demographic history and a rich collection of carefully catalogued and dated museum skins.
The ancient Polynesian feather collectors on Hawaii and early explorers like Captain Cook whose collections now fill museums, have paved the way for an exciting new approach to our understanding of the genetic processes that drive extinction.
Dr Jim Groombridge is a lecturer in Biodiversity Conservation at the Durrell Institute of Conservation and Ecology (DICE) at the University of Kent and was awarded a NERC New Investigator's Award for this genetic work on Hawaiian avifauna, in collaboration with Dr. Robert Fleischer of the Smithsonian Institution and Professor Richard Nichols at Queen Mary, London. Dr Sarah Anderson carried out the genetic work. The project used the NERC Molecular Genetics Facility at Sheffield and the NERC/Arts and Humanities Research Council Radiocarbon Accelerator Unit in Oxford. Jim is an Associate Editor of Conservation Genetics and a member of the Editorial Review Board for Endangered Species Research.
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