A native Tahitian land snail Partula
otaheitana sinistrorsa, sampled by
J. B. Burch in 1970.Photo: Gene Lindsay
Like many remote oceanic islands, the Society Islands house unique species of plants and animals. Introduced species can give them a real whacking. Just ask the native land snails - if you can find any.
These vials contain freeze-dried samples of Tahitian
tree snails, collected in 1970 and stored in a freezer at the University
of Michigan. Courtesy J.B. Burch
Zoologist John Burch on a collecting expedition in
Tahiti in 1970. A tough site for research, eh? Burch's long-forgotten
should help efforts to rejuvenate populations of snails in Tahiti.
Courtesy J.B. Burch
Land snails are having a tough time in Tahiti and the rest of the Society Islands in the South Pacific. Considered prime examples of evolution at work, the 61 unique species of land snails, like many critters native to remote islands, had pathetic protection from predators. They quickly disappeared after a predatory snail was introduced to the islands; only six species survive in the wild.
In French Polynesia, native people used the shells of these snails for jewelry. Somewhat smaller than the average garden snail, they lived on trees during the day and came down at night to graze on vegetation.
But all was not well in this tropical paradise. About 1969, someone brought giant African snails to the islands, as a slow-moving source of food. These intruders had the poor taste to develop a taste for vegetable and fruits, so in the 1970s, local agricultural scientists brought in the rosy wolf snail to eat the giant African snails.
It sounded logical. But while the rosy wolf proved an inept predator of the giant snails, it lambasted the local species, sending them into a tailspin.
That kind of species decline is an old story on remote islands, where biological diversity is both more profound, and more imperiled, than on continents.
Remote islands are jewels of evolution because the few species that survived long ocean journeys found a paradise with little competition and plenty of open ecological niches. As a result, they evolved into forms found nowhere else through a process called adaptive radiation.
It's pretty, but the rosy wolf snail proved to a deadly
predator of native snails in the Society Islands. Courtesy The Jacksonville Shell Club.
As the local snails became dinner for the rosy wolfs, do-gooder conservationists, led by the Zoological Society of London captured some samples, and, helped by zoos in Europe and North America, started breeding 15 of the 25 species that still survive.
Now, the conservationists have started returning some of those snails to Moorea, a smaller island near Tahiti. The first returnees are living in predator-proof pens, but eventually, they may be set free in their old home turf, says Diarmaid O'Foighil, curator of mollusks at the Museum of Zoology at the University of Michigan. "The hope is that once the predators have eaten everything they can, they will die off from starvation, and then the locals can safely return. The rosy wolf is becoming very rare in Moorea... and may be in the process of dying out."
As scientists try the tricky task of reintroducing animals to their old habitat, however, they face a yawning data gap. Which snail species lived there, at what numbers and in which habitats, and how were they related?
How many species, in fact, were actually in the Society Islands? Since the die-off began, our understanding of species relationships has been revolutionized by the study of DNA, which gives a far more accurate picture of evolutionary relationships.
Now, courtesy of an old museum collection, the gaps are starting to fill in. It turns out that John Burch, a retired zoologist from the University of Michigan, went mollusk collecting in Tahiti in 1970. He brought back and freeze-dried hundreds of them. Logically, he stashed the samples at the university's zoology museum, where they sat, undisturbed and unstudied, until a chance conversation last year between Burch and O'Foighil.
Those snails, says O'Foighil, are the only good documentation for Tahiti's snails in the days before the rosy wolf started its long, expensive feast. The collection will play both practical and scientific roles, he adds. "The conservation angle is very simple. There has been a sustained effort to save as many lineages as possible... but our estimate of what we are saving is based on essentially 19th-century species definitions," which were based on shell characteristics. "We know they're not accurate, and a rational conservation process depends on knowing what you had."
Using DNA analysis, O'Foighil, Burch and colleagues are trying to decipher exactly how many species were once living on Tahiti, the largest of the Society islands. Already, he says it's clear that some distinct "species," based on the shells, are actually the same species.
The collection should also help shed light on fascinating questions of island biogeography - the study of how species appear and disappear on limited scraps of land. "From a scientific point of view, we can test evolutionary hypotheses that would explain how you could get X number of snail species on a young island. Did they all evolve at the site, or through multiple colonizations?"
DNA studies to date indicate that particular species arrived several times, and then expanded into associations of related species, O'Foighil says.
In terms of scientific progress, the story proves the merit of collecting for its own sake, he concludes. "Yes, people complain, say it's going to cost a lot [to keep the old samples]. The real value of museum collections only becomes apparent in the long term. Jack [Burch] had no idea these things would go extinct. The value of the collection he made has increased greatly because of the sad fact that most of the populations have gone."
-- David Tenenbaum
The Society for the Preservation of Natural History Collections