Climate change claims a snail

Goodbye to a snail.

The Aldabra banded snail (Rachistia aldabrae), a rare and poorly known species found only on Aldabra atoll in the Indian Ocean, has apparently gone extinct due to declining rainfall in its niche habitat. While some may question lamenting the loss of a lowly algae-feeding gastropod on some unheard of chain of tropical islands, its unheralded passing is nevertheless important for the simple reason that Rachistia aldabrae may be a pioneer. As climate change increasingly brings local and regional shifts in precipitation and temperature, other species are expected to follow in its path.

Perhaps unsurprisingly for an inconspicuous snail living on an atoll 426 kilometers northwest of the northern tip of Madagascar and 1150 kilometers southwest of Mahé, the principal island of the Seychelles, the life history of the Aldabra banded snail is a mystery. What is known, is that since the late 1990s, extensive surveys of the Aldabran islands of Picard, Malabar, Polymnie, Esprit and Grande Terre have failed to turn up any Rachistia aldabrae individuals. The only remains of the species, last seen alive a decade ago, are scattered indigo blue and orange shells in the “mixed scrub” of Aldabra.

Unlike more charismatic species — the dodo, giant elephant bird, Tasmanian Tiger, the Baiji, or Bali tiger — that have made their exits, the demise of the Aldabra banded snail resulted not from overexploitation, destruction of habitat, introduction of alien species, accidental catch, or hunting, but from subtle changes in its environment.

NASA Astronaut Image of Aldabra Atoll (Seychelles) in the Indian Ocean. Aldabra Atoll, designated a World Heritage Site, is home to the world’s largest population of giant tortoises and the Aldabra rail, the last surviving flightless bird of the Indian Ocean region. Image courtesy of the Image Science & Analysis Laboratory, NASA Johnson Space Center.

Justin Gerlach, an Oxford University biologist with the Nature Protection Trust of Seychelles who has just authored the definitive paper on the extinction of Rachistia aldabrae, postulates that recent declines in rainfall spelled the end for the snail, which hibernates during dry periods.

“Decreases in rainfall would have reduced the length of activity periods,” he writes in the current issue of the journal Biology Letters. “This may not have been a major additional cause of mortality to adults, but the small juveniles would be less able to tolerate the desiccation. Consequently, long dry periods would be expected to reduce reproductive success, with complete failure in prolonged dry periods.”

Evidence, says Gerlach, comes from shell fragments. It seems that all shells collected since 2000 belong to adult individuals.

“From this, it seems probable that decreasing rainfall over 1980s and 1990s led to high juvenile mortality, and consequently an ageing population.”

Aging, he says, “may have led to the complete extinction of all populations between 1997 and 2000.”

The Aldabra banded snail’s extinction without direct interference from predators or mankind, make it a special occurrence, but one that is expected to become less rare in the future.

“This may be one of the few cases of extinction that cannot be attributed to a change in habitat, predators or diet, but may plausibly result from the direct impacts of climate on survival,” explains Gerlach. “Climate change has been proposed as a factor leading to the decline of many species, either directly or through indirect associations… There are few cases where this has been demonstrated directly (the golden toad Bufo periglenes), although indirect effects are reported for a large number of amphibian species.”

Panama’s golden frog (Atelopus zetecki) is highly threatened by the killer chytrid fungus (Batrachochytrium dendrobatidis), an infectious skin disease now found in frog populations around the world. In 2006, Dr. Alan Pounds and colleagues published a study which found that almost two-thirds of the 110 known harlequin Atelopus toad species of Central and South America went extinct in the 1980s and 1990s. The leading culprit for the demise was a type of chytrid fungus (Batrachochytrium dendrobatidis), an infectious skin disease. The researchers discovered strong correlations between changes in climate and the last known sightings of the frogs. According to the scientists, the Earth’s rising temperatures enhance cloud cover on tropical mountains, leading to cooler days and warmer nights, both of which favor the chytrid fungus which grows and reproduces best at temperatures between 63 to 77 degrees Fahrenheit (17 to 25 degrees Celsius).

Lately amphibians, especially brightly colored frogs and toads, have become the poster species for climate change-linked extinction. While scientists are just beginning to understand the rapid worldwide decline in amphibians, climate change is increasingly tied to shifts in habitats and the outbreak of a deadly fungal disease, which is killing frogs, toads, salamanders, newts, and caecilians on every continent. To date, more than 170 species are thought to have gone extinct since 1980 while the Global Amphibian Assessment, a comprehensive status assessment of the world’s amphibian species, classifies one-third of the world’s 5,918 known amphibian species as threatened with extinction. Some scientists believe that amphibians may just be the proverbial “canary in the coal mine” — the first group of animals to go before a wider die-off begins.

This trend among amphibians suggest that the Aldabra banded snail will not go alone. Thousands of species will join it, leaving Earth a poorer place.

Ironically even if drought had not done in the Rachistia aldabrae, its was likely doomed to extinction. With much of their elevation just inches about sea level, some islands in the Aldabra atoll are likely to become some of the first places inundated by rising sea levels. The culprit? Also climate change.

CITATION: Justin Gerlach (2007). Short-term climate change and the extinction of the snail Rachistia aldabrae (Gastropoda: Pulmonata). Biology Letters doi:10.1098/rsbl.2007.0316

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