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MARGINALIA

The Rarest Snail in the World

Do development and scientific study conflict with conservation?

Pat Shipman

A Tenuous Existence

These observations raised a surprisingly complex question: What exactly does rare mean? Is it small in numbers, small in territory, highly specific in habitat—or some combination of those characteristics? Several scientific groups have addressed these questions. NatureServe assesses the rarity of a species by a combination of six factors: the population size, the extent of the species’ total range, the part of the range actually occupied by the species, the number of distinct populations, the percentage of the range with good viability or ecological integrity, and the variety of environments the species can inhabit. Because Maynard found only a single population of the species, with a very small range and high environmental specificity (it lives on one plant species), C. nanus is rare on several counts.

The International Union for the Conservation of Nature uses a different system to assess a species’ vulnerability to extinction, a measure that is roughly equivalent to rarity. A species is considered critically endangered—the category for the rarest organisms—if it is distributed over a range of less than 100 square kilometers or if it actually occupies less than 10 square kilometers within its range. Since Little Cayman has a total area of only about 26 square kilometers, all species restricted to that island automatically meet the first criterion and most meet the second. Mike Hounsome wrote the report that declared C. nanus to be critically endangered, not only because of its highly restricted range but also because of the ongoing loss of its shrubland habitat to development.

When Mike located Maynard’s study site on Little Cayman in 1975, he couldn’t find any living dwarf Cerion, only dead ones. The larger C. pannosum, however, was abundant. To Mike, it looked like Maynard’s hypothesis might be correct: Perhaps the large Cerion had outcompeted and replaced the smaller species. But at another location only about a kilometer away, Mike found that both species were alive after all—and their relationship was not so clear. C. nanus lived only on Evolvulus, sometimes alone and sometimes sharing a plant with C. pannosum. Occasionally the larger snail was the only species on an individual Evolvulus shrub, but it could also live on different plants altogether. Furthermore, 30 percent of the Evolvulus plants at Mike’s new site had no snails at all. It seemed there was food to spare, but the two species still appeared together on the same plants more often than would be expected by chance. Certainly, the dwarf Cerion had not (yet?) been displaced and gone extinct at this location, as it had at Maynard’s old site. Mike wondered “Why not?” but found no definitive answer.

A species that is confined to an island, as C. nanus is, can rapidly become much smaller in size when natural selection favors little individuals that thrive with limited resources. This biological phenomenon, known as island dwarfing, is fairly common among mammals and birds. But Mike’s observations suggested that, on Little Cayman, the larger snail may have had the advantage. When he found Maynard’s original site, it was crowded with snails—but rather than outcompete the larger snails, the dwarf species had disappeared entirely. And at the second site, why were one-third of the plants free of snails? If competition triggered dwarfing, why would the larger Cerion be more common than the smaller species, which could surely prosper on a smaller food supply?




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