Rats killing native snails on Molokai


POSTED: Sunday, September 27, 2009

A stunning 85 percent plunge in native Hawaiian tree snail populations began in 1995 at a Molokai site studied by University of Hawaii at Manoa researchers.

“;They were in pretty good shape. Then the big decimation in the snail meadow was really a shock,”; UH zoologist Michael Hadfield said in an interview. “;It happened so fast.”;

Shells piled up beneath the trees revealed the killers: Rats had been dining on the snails.

Hadfield and colleagues have monitored populations of the Partulina redfieldi species in four ohia trees in a Kamakou Preserve meadow near the mountain summit since 1982.

Their goals were to increase understanding of long-term dynamics of a healthy population and determine if laboratory-reared snails could be established in a tree without the native Achatinelline snails.

The snail populations increased significantly from 1983 to 1995 in the four study trees, with varying growth rates up to 926 percent (19 to 195 snails), Hadfield and research associate Jennifer Saufler (now at University College, London) reported in the August issue of Biological Invasions.

Then the number of snails at all the trees plummeted as rats moved in, the researchers said.

Hawaii once had 750 species of land snails found nowhere else and roughly 90 percent are extinct, reported Charles Lydeard of the Smithsonian Tropical Research Institute, Robert Cowie of the UH Center for Conservation Research and Training, and their associates in 2004.

Hadfield has focused research on the Achatinella species on Oahu. Predators and collectors of the beautiful patterned shells have wiped out all but 10 of the 41 species, he said. The remaining species are on the endangered list.

Rats aren't the only predators, although so far they're the only ones destroying the Molokai snails under study, Hadfield said.

The carnivorous euglandina snail imported by the Agriculture Department in 1955-56 to control the giant African snail is worse than rats in many areas, he said. A New Guinea flatworm eats snails in lower elevations and in recent months, Jackson's chameleons were seen in trees chomping on snails, he said.

The recent forest fire that burned portions of Kamakou Preserve would have been devastating for the P. redfieldi snails at the Molokai study site, but it didn't appear to reach the 4,000-foot meadow, Hadfield said.

The Nature Conservancy started a rat-poisoning program after the precipitous drop in snail populations in 1995, but snails continued to disappear, “;which we conclude was due to continued rat migration into the study area despite baiting, and a switch in rat food preference toward the snails,”; Hadfield and Saufler said.

Snails are long-lived and can spend their entire lives in one tree, the researchers said. Some observed in the Kamakou meadow were from 16 to 20 years old. Snails don't reproduce until 3 to 5 years of age and they have only about five offspring a year.

The captive-rearing laboratory established in 1989 on the UH-Manoa campus has more than 1,400 snails in 16 species that could be placed in the field to create new populations. “;But we've got to have some place to put snails back,”; Hadfield said. “;We can't just go feed predators.”;

He said enclosures on Oahu in northern Waianae and Pahole, above Makua Valley, have been effective predator barriers and one is needed in the north Koolau mountains for lab-reared snails.

“;We have now begun calling this a problem of 'extreme endemics,' which means species that have very, very small natural ranges of maybe a few kilometers, a single valley or single peak,”; he said.