Justin Gerlach
Research & conservation Red Listing invertebrates Publications Phelsuma Press

Climate change and the Aldabra banded snail

In 2007 I published a paper attributing the extinction of the Aldabra banded snail Rhachistia aldabrae to climate change. In 2014 the banded snail was rediscovered. So why had I thought it was extinct?

Short-term climate change and the extinction of the snail Rhachistia aldabrae (Gastropoda: Pulmonata). Biol Lett.

Abstract: The only known population of the Aldabra banded snail Rhachistia aldabrae declined through the late twentieth century, leading to its extinction in the late 1990s. This occurred within a stable habitat and its extinction is attributable to decreasing rainfall on Aldabra atoll, associated with regional changes in rainfall patterns in the late twentieth and early twenty-first century. It is proposed that the extinction of this species is a direct result of decreasing rainfall leading to increased mortality of juvenile snails.


When I did my research on this species I compiled all the records for the snail, going back to its discovery in the late 1800s. It was very abundant in the past (in fact visitors to the atoll in the 1950s came back with jars full of shells and in the 1970s traps that were set for insects ended up with loads of snails in them). Sometime after 1976 there was a dramatic drop in abundance, the only shells I knew of from the 1980s were a couple that extremely experienced naturalists who had visited the atoll dozens of times and had spent months there brought to me because they had never seen them before. In the 1990s the scientific officer of Aldabra collected snails very carefully all over the atoll, this systematic collection had Aldabra banded snails in, but only old shells. I visited Aldabra only a couple of times but looking for this snail was one of my priorities. Again, all I could find were very old shells that had probably been dead for years. The last one to be collected alive was found by a ranger in 1997, who collected it because no-one on the atoll had seen one before.

It was clear that there had been a dramatic decline in the snail's population. What had caused it to decline? Looking at the ecological data I could find nothing that explained the dramatic disappearance except for an increase in the length of the dry season. There were still large areas of good habitat, but apparently no snails. No new predators had arrived on Aldabra (and no evidence of predation on the shells I found). I hypothesised that the increasingly dry climate was a stress on the juveniles. Adults of this type of snail are very well adapted to dry conditions and it seemed unlikely that they would be killed by a relatively subtle change. Baby snails though need high humidity to survive and would be unlikely to tolerate excessively long dry periods. It was notable that the insect traps in the 1970s had a high proportion of juveniles whereas all the shells after that time were of adults (they last longer but the lack of any juveniles at all was notable).

Fortunately, despite appearances the snail was not extinct. In August 2014 one of the Seychelles Islands Foundation's rangers spotted a snail when visiting a relatively inaccessible part of the atoll. Knowing of the supposed extinction of the Aldabra banded snail he realised that this seemed to be a living example ofb the same species. Seven snails were found in that area, including juveniles, demonstrating that a population does survive.

The Aldabra banded snail survives, but it is just one tiny population when it used to be spread across the atoll in profusion. While these survive there is room for optimism, but the species is just clinging to existence. They may be able to adapt to the changed conditions and slowly recover as long as there are no further changes. With ongoing climate change and with rising sea levels around the atoll this may be a vain hope.

Other climate change impacts
The potential effects of climate change on the status of Seychelles frogs (Anura: Sooglossidae). Journal of Threatened Taxa 3(6).

Abstract: The status of the Seychelles frogs of the family Sooglossidae was investigated, using monitoring data from 1993–2010, climate data from 1998–2010 and studies of populations and local climate effects. Climate monitoring at each plot covered rainfall and temperature, with leaf wetness and soil moisture being monitored additionally at one site. Analysis of the data and ecological modelling of the distribution identify geographical patterns in climate which explain the present distribution of the different sooglossid species. In addition it identifies a drying trend in the first quarter of the year which corresponds to frog population declines in mid-altitude forests. This is interpreted as evidence of an ongoing deterioration in the suitability of habitats for the frogs, with declines recorded in areas of marginal suitability. By extension it is assumed that currently optimal frog habitat is also undergoing a decline in suitability, due to early year decreases in moisture. Projected changes in climate were used to predict changes in ranges of the sooglossids over the next 90 years. This predicts significant declines, with the possible extinction of the palm frog Sooglossus pipilodryas by 2100. Accordingly all four sooglossid species should be categorised as Endangered, rather than their current status of Vulnerable.

Will climate change affect terrapin (Pelusios subniger paritalis andP. castanoides intergularis) conservation in Seychelles? Phelsuma 17A

Abstract: We report a modeling study on habitat suitability and predicted distribution shifts of two speciesof Seychelles’s freshwater turtles (Pelusios castanoides and P. subniger) under a climatechange scenario. We utilized data from the entire species distribution for modeling habitat suitabilityof the two species under current and future climate conditions, by using the MAXENT algorithm.At the continental scale, it appeared that P castanoides will shift its range towards more coastalareas, whereas P. subniger will move towards more southern sites. In the Seychelles archipelagoscale, habitat suitability for P. castanoides will decrease significantly, mainly in the interior areasof Mahé Island. On the contrary, the climatic conditions are predicted to remain suitable for P.subniger, which will enjoy a significantly increased habitat suitability in Seychelles.


Climate change, species extinctions and ecosystem collapse. Phelsuma 17A

Abstract: Climate change models have predicted many environmental impacts, but there have beenrelatively few published studies of ecosystem or population changes. Three studies of ecosystemsand populations are reported here from the Seychelles islands. A study of a seagrass and lagoonecosystem on Silhouette island, shows that rising sea-levels are causing changes in current patternsover the reef. Local increases in current speed act as a stress on the seagrass, leading to death ofthe plants over much of the reef-flat and loss of its stabilising function. Silt iseroded off the reef-flat and into the lagoon, removing habitat for some lagoon-dwelling animals.Significant changes in fish populations have been recorded, including the possible extinction of thegoby Asterropteryx gubbina; this is the first report of possible species extinction for which sea-levelrise appears to be the primary cause. Sea-level rise is also causing increased marine incursion intoestuarine habitat, this has led to declines in populations of the dartfish Parioglossus multiradiatus,which should be considered to be Critically Endangered. Climate change impacts are also apparentin terrestrial systems: the destablisation of a hybrid zone, leading to the extinction of the snailPachnodus velutinus in 1994 followed a period of reduced rainfall. This is in accordance withpublished models that predict increased frequency of extreme weather patterns in the region. Thesestudies indicate that rapid population and ecosystem changes are occurring. Climate change may already be one of the primary drivers of extinction.