Tuesday, May 20, 2008

Hawai'i bird extinction: size matters, but also habitat, predators

Each time a new class of humans appeared in Hawai'i, the native birds suffered.

New research shows that native Hawaiian birdlife underwent two massive extinction events, coinciding with the first arrival of Polynesians in the Islands, and then with the arrival of Europeans. Half of all native bird species were killed off in these extinctions.

(Photo: Barren landscape on Rapa Nui. Rats are now implicated in the deforestation of that island, and are believed to have played a major role in Hawaiian landscape changes, which may have led to some Hawaiian bird extinctions.)

Alison Boyer, a biologist and Ph.D. Candidate at the University of New Mexico, studied fossil finds in the Islands and found that the timing is inescapable.

“Through the continuing accumulation of fossil evidence, it is clear that the avifauna of the Hawaiian Islands underwent a large-scale extinction event around the time of Polynesian arrival. A second wave of extinctions since European colonization has further altered this unique avifauna,” she wrote in a paper in the journal, “Diversity and Distributions.” (Avifauna means birdlife.)

But the question that intrigued Boyer was which kinds of birds—which classes of birds—disappeared.

She classified the Hawaiian birdlife by what they ate, what they looked like, how they behaved and how they were related to each other and other birds.

“Extinction of the unique Hawaiian birds began when people first colonized the islands over 1000 years ago,” Boyer said in an email. “During that time, I found that larger and flightless or ground-nesting species had a higher rate of extinction than other groups. Of course, many small species also disappeared, implicating a wide suite of human impacts including destruction of dry forest habitat.”

It is an amazing extinction record. Fossil discoveries show that at least 56 species of birds had become extinct during the Polynesian period before European arrival.

It is not clear to what extent direct hunting and eating resulted in the loss of larger birds.

“The relative importance of direct and indirect human impacts in the prehistoric extinction remains controversial,” she said in the paper.

But it's clear it had an effect.

“Direct hunting of native birds was widespread prehistorically. Charred bones of extinct birds are often found in prehistoric human middens in the Hawaiian Islands and elsewhere providing plentiful evidence that the ancient Polynesians used birds for food,” Boyer wrote, citing findings by numerous research teams.

That sort of activity tended to target larger birds. But smaller birds were a Polynesian target as well, in large part for their colorful feathers.

Still, that is not to say that humans were the primary cause. Habitat destruction for agriculture, and the arrival of rats, pigs and dogs could have played a role.

Rats are increasingly being recognized as an exceedingly important player in the loss of habitat. On Rapa Nui (Easter Island) for instance, while humans have traditionally been blamed for the loss of the native forest, new research is finding that loss of the dominant tree in the Rapa Nui forests may have been driven by rats eating virtually every seed—so the trees could not reproduce.

In Hawai'i as well, rats were key players, even when humans weren't even there.

“Much of the lowland forest of the ‘Ewa Plain region of Oahu appears to have been destroyed by rats even before human settlement in the area,” Boyer wrote, citing work by archaeologist Steve Athens and others.

Of all the flightless known Hawaiian birds, only one species survived to be seen by humans. It was a rail, Porzana sandwichensis, and it went extinct not long after European contact, with the arrival of new threats like roof rats, mongooses, cats and other species.

When Europeans arrived, a new wave of extinctions occurred, and it impacted a different group than the earlier wave. Among the factors implicated in the modern extinctions are further habitat loss, new predators, and the arrival of new diseases—generally ones carried by mosquitoes. Mosquitoes had not been present in prehistoric Hawai'i.

The loss of birds on the arrival of humans in a landscape is not unique to Hawai'i. Nor is the early disappearance of the large, flightless birds.

“Human colonization of the Hawaiian Islands initiated sweeping changes in the island environment including the extinction of more than 50% of native bird species. On every other island and continent examined so far, similar size-selective extinctions have followed human colonization,” Boyer wrote in her paper.

© 2008 Jan W. TenBruggencate