CLIMATIC WARMING DUE TO THERMAL-RADIATION FROM AN URBAN AREA AS POSSIBLE CAUSE FOR THE LOCAL EXTINCTION OF A LAND SNAIL

1. A field survey revealed that the land snail Arianta arbustorum had become extinct at 16 (55.2%) out of 29 localities in the surroundings of Basel (Switzerland) between 1908 and 1991. Habitat destruction by urban development was the reason for extinction of eight local snail populations. However, A. arbustorum also became extinct at eight localities, which were covered by vegetation suitable for the species and which still supported abundant populations of other helicid snail species including Cepaea nemoralis (all of them recorded in 1906-8). 2. Localities where A. arbustorum persisted and localities where it went extinct did not differ in exposure, inclination, percentage of woody plant cover, height of ground vegetation, distance to the nearest water body and number of other helicid snail species present. However, localities retaining A. arbustorum were situated on average at higher altitude than localities where it went extinct. 3. Remote sensing was used for mapping the surface temperature in the surroundings of Basel. During summer (when snail eggs were deposited) the surface temperature of the vegetation was significantly higher at localities where A. arbustorum went extinct compared to localities where the species still occurred. The surface temperature of the vegetation extracted from the satellite imagery was significantly correlated with the mean maximum temperature recorded in situ in potential oviposition sites of A. arbustorum at weekly intervals during summer 1992. The satellite imagery showed that built-up areas (central business district and industrial areas) affect adjacent vegetation areas by emitting thermal radiation. This suggests that local climatic warming due to extensive urban development might have caused the extinction of A. arbustorum populations. 4. One possible mechanism might be that egg development of A. arbustorum is sensitive to high temperature; embryos die if the ambient temperature exceeds a specific threshold. To test this hypothesis, we incubated eggs of A. arbustorum and C nemoralis in the laboratory at 19, 22, 25 and 29-degrees-C. Hatching success for eggs of A. arbustorum decreased from 81-5% at 19-degrees-C to 38.7% at 22-degrees-C, and no eggs hatched at higher temperatures. In contrast, eggs of C. nemoralis had a significantly higher hatching success than eggs of A. arbustorum at 22 and 25-degrees-C. A second experiment demonstrated that exposure to 25 or 29-degrees-C for only 24 h was enough to lower significantly the hatching success of A. arbustorum eggs. 5. These results suggest that temperature tolerance in egg development can determine the distribution of a snail species and, in case of climatic warming, cause its extinction.