Saturday, September 24, 2011

Warblers fly an unconventional route into the Pacific

Over twenty-thousand islands dot the 63.8 million square miles of the Pacific Ocean, their land surface area covering only half a percent of this massive expanse of water. Yet, land plants and animals have managed to cross thousands of miles of ocean to reach even some of the most remote of these islands.  How did they manage this?  The simplest explanation is that plants and animals started on the larger continents of Asia and Australia, and gradually worked their way east, hopping sequentially from island to island until they eventually reached the far-flung loners in the eastern Pacific.

A paper in this month’s Journal of Biogeography refutes this “stepping-stone” hypothesis for species of reed-warblers living on islands across the Pacific. Cibois and colleagues analyzed mitochondrial DNA from birds in the reed-warbler genus ranging from Australia to Hawaii and Guam to the Pitcairn Islands. Many of the DNA samples came from the toe pads of dead birds in museums that had been collected up to a century earlier, which allowed the researched to use eight warbler species that had already gone extinct. The scientists compared the DNA sequences of birds on different islands to see how closely related they were; the more similar two sequences of DNA, the more recently that two birds shared a common ancestor and therefore came from the same place. If reed-warblers followed the ‘stepping –stone hypothesis’ and colonized the Pacific Islands by gradually hopping eastward from island to the next closest island, then birds living on Australia and other islands close to the Asian continent should be least closely related to birds living on the islands farthest east.

What emerged from the phylogenetic tree of relatedness that Cibois and colleagues constructed was nothing quite so simple. Among the islands of Micronesia, the amount that birds from two islands were related was not correlated with the distance between the islands. And, instead of a gradual progression of relatedness stretching eastward into the Pacific, they found two waves of colonization departing from the island of Guam. One wave spread far out toHawaiiand from there south to the Eastern Polynesian islands, the other traveled southeast into Micronesia and at some point birds from these islands recolonized the continent of Australia! The two waves met up far out east in the Marquesas islands, where today birds that came originally from Polynesia live on the northern islands of this archipelago, while birds from Micronesia lives on southern islands; these 2.2 million-year-distant cousins are separated by just 41 kilometers of water.

All this flying around probably took place over the course of the last couple million years when ice was expanding and contracting across the face of the globe stirring up ocean currents and changing the direction of prevailing winds, which may have facilitated the transportation of birds to remote islands. Although no fossils of reed-warblers exist on Pacific islands, the researchers were able to construct rough estimates of the times when populations on different islands evolved away from each other by counting the number of changes to the DNA and assuming that 2% of the DNA sequence would change every million years. Contrast this to the colonization of the world by modern humans who evolved in Africa 250,000 years ago, reached Australia ~60,000 years ago, and had spread out into the farthest Pacific islands by 1,700 years ago.

So why didn’t the reed-warblers follow the seemingly easier route of jumping from islands to next closest island? No one knows, but it certainly wasn’t intentional on the birds’ part. The colonization of the Pacific islands by various groups of plants and animals is a sizzling topic in biogeography. As scientists study the colonization patterns of a wider variety of organisms, it will be interesting to see whether animals like the reed-warbler that don’t follow the more conventional stepping-stone hypothesis share similar life-styles or other aspects of their biology.

You can find this article at:

Cibois A., Beadell J.S., Graves G.R., Pasquet E., Slikas B., Sonsthagen S.A., Thibault J.C. & Fleischer R.C. (2011). Charting the course of reed-warblers across the Pacific islands, Journal of Biogeography, 38 (10) 1963-1975. DOI: