Hawaii's aliens mimic natives

Japanese White-eye, seen here on a Heliconia in Maui, is the spcies which appears to be changing the most between islands. Photo: Forest and Kim Starr (commons.wikimedia.org)
Japanese White-eye, seen here on a Heliconia in Maui, is the spcies which appears to be changing the most between islands. Photo: Forest and Kim Starr (commons.wikimedia.org)

The slew of endemic bird species found on the Hawaiian island chain is well-known, with such groups as the honeycreepers having evolved into dozens of widely-differing species after an emberizine 'finch' ancestor arrived there no more than five million years ago.

The arrival of mankind on the islands had a devastating effect on the island's native wildlife, but also served to introduce more species which have since become widely established, particularly in the last 150 years.

Now it has been discovered that at least five of those alien species are likely to have undergone a selection process themselves, and are beginning to diverge between different islands in the chain.

A team from New Jersey sampled six successful introduced species from the four largest ioslands, Hawaii itself, Oahu, Kauai and Maui, from live caught birds and museum specimens. Of these species, only two - the popular cagebirds, Red-billed Leiothrix and Nutmeg Mannikin - showed no significant change in its biometrics since being introduced. The other five, House Finch, Northern Cardinal, Japanese White-eye and House Sparrow, all showed intra-island changes in their dimensions that could not be accounted for merely by the well-known phenomenon of passive genetic drift. This latter effect causes gradual change in an isolated population by reproducing from the limited gene pool of a few founding individuals, showing change from the main population by exaggerating the differences of a small sample of individuals.

At least one of the measured traits - mass, body length and wing length - changed between islands in the four more plastic species. The causes of the changes are as yet uncertain, but it is speculated that the changes may be adaptive, occurring among largely sedentary populations changing the dynamics of their flight capabilities to suit the unknown idiosyncratic environmental and ecological conditions on each island. However, the founder effect could not be entirely separated as a aprtail cause of the intra-island variation. Bill dimensions remained largely unchanged among all six species, indicating that no dietary adaptations had taken place.

Though such changes have been observed before among introduced species on other islands, this is the first time such multiple substantial changes in body form have been demonstrated among a relatively slow-evolving vertebrate group like birds. It also throws light on how the native birds may have radiated and on why they appear to have done so at different rates. Whether anything eventually comes of these subtle divergences in deep time, Hawaii has proved once again that islands are nature's evolution laboratories.

Reference: Mathys, B A and Lockwood, J L. 2011. Contemporary morphological diversification of passerine birds introduced to the Hawaiian archipelago. Proceedings of the Royal Society Bdoi:10.1098/rspb.2010.2302

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