Can environmental tolerances explain convergent patterns of distribution in endemic spring snails from opposite sides of the Australian arid zone

Patterns of distribution are influenced by species environmental requirements and limits, but experimental tests are needed to discern whether correlates of abundance directly affect survival and success. Springs in Australia’s arid interior support a wide diversity of gastropods only found in springs, and these species show dichotomous patterns of distribution. “Amphibious” species are broadly distributed across many springs and microhabitats, and “aquatic” species confined to the deepest pool areas within large springs. This pattern appears to be driven by the interaction between different environmental conditions in different microhabitats and the environmental tolerances of each endemic snail species. Factorial experiments were used to test whether conditions in the environmentally extreme and variable tail area of springs (considering pH, conductivity, temperature and desiccation potential, alone and in synergistic scenarios) elicited lethal or sub-lethal responses in spring snails endemic to springs on opposite sides of the Australian arid zone. All species restricted to spring pools were able to endure 24 h exposed to the average tail conditions, alone and in combination, but most suffered mortalities when subjected to extremes, and mortalities occurred sooner in the most restricted species when elevated pH and conductivity were experienced in combination. Responses of species from different locations are similar, but pattern of distribution in the field were not correlated with tolerance of environmental extremes—with the “amphibious” species from the sub-tropics being far more sensitive than its arid counterpart. These findings suggest that environmental variance within springs can influence patterns of distribution and abundance, particularly when extremes are experienced simultaneously over sustained time periods. But despite similarities in responses across species from these two spring complexes, no simple generalisations linking distribution and tolerance were discernible.