Climate change causes the extinction of the ‘Purple Snail’.August 2007. 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.
Island populations have long been of particular interest to evolutionary biologists due to their role in the development of the theory of evolution. Numerous studies have demonstrated that rates of colonization and extinction can be high in some islands, which may lead to a rapid rate of species turnover in these cases, though more isolated or larger islands may have comparatively stable systems
Long term research
Few islands have been the focus of research interest for long enough for the accumulation of useful datasets for long-term monitoring (notable exceptions being Barro Colorado and the Galapagos Islands). Aldabra atoll in the Seychelles group has been one of the most intensively studied small islands over the last 100 years, with particularly intensive research since 1968. These studies have been focused on the recovery of the tortoise populations and their impact on the atoll’s habitats and the impact of invasive species.
Land snail collections on Aldabra were first made in 1895, with occasional collections since then and extensive surveys in 1997, 2000 and 2005. Examination of collection material allows the evaluation of distribution and abundance change for the largest and most distinctive Aldabran snail species, the banded snail Rhachistia aldabrae. Rhachistia aldabrae is endemic to Aldabra atoll where it has been recorded from the islands of Picard, Malabar, Polymnie, Esprit and Grande Terre. Last known specimen
No detailed studies of R. aldabrae have been carried out, but several visitors to the atoll since 1895 have collected specimens deliberately or incidentally in general invertebrate sampling. The last specimen was collected in 1997, since then extensive searches have been made for this species in many parts of the atoll (including all localities where the species had been recorded previously) in 2005 and
2006 but only old shells (estimated to date back at least 5 years) could be located.
Recent shells of live snails were recorded on the main islands of Picard, Malabar and Grande Terre during
1907–1976; after that only two isolated fresh specimens have been found (1989 and 1997), both on
Picard. In 1907, the species was noted as being particularly abundant in the ‘Coroupa’ area of Grande Terre (Fryer 1911). This locality was visited in 2000 and no evidence of any recent survival of the species was found.
From the number and size of shells collected in the past, it has been calculated that there is a 95 percent chance that the species is probably already extinct
The absence of any recent shells or live specimens in those surveys further supports the view that this species is extinct. Significant correlations were found with annual rainfall. No R. aldabrae were recorded in years where rainfall was low. Of 30 years of rainfall data, 17 were below 1023 mm. Such years occurred as isolated dry years or two consecutive dry years until 1999; since then all 6 years for which data are available have been dry. Both the frequency of low-rainfall years and the length of dry periods have increased. During 1970–1980, there were three low-rainfall years compared with seven during 1994–2003.
No specimens of R. aldabrae have been located since 1997, and no juveniles since 1976, despite extensive searching and as the most recent remains appear to date from approximately 2000, this species is considered to be extinct.
No general changes in vegetation cover or structure have been identified on Aldabra and the algal browsing diet of this family of snails would imply that habitat and food availability are unlikely to have changed significantly over the past 100 years. The species was recorded in mixed scrub on Picard and in areas of ‘mixed vegetation, partly of ‘open country’ plants but mainly of ‘shore-zone’ forms’ suggesting that it did not have any special association with any restricted habitat. The decline in abundance correlates with increasing prolonged dry periods.
During periods of drought, R. aldabrae aestivated on the branches of shrubs. Decreases in rainfall would have reduced the length of activity periods. 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. This is supported by the demographic change in the population, from a high proportion of juveniles and neonates during 1974–1976 to only adults after 1976.
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.
Courtesy of Justin Gerlach, Nature Protection Trust of Seychelles. Taken from Biology Letters, a journal of the Royal Society.