What makes a narrow-range taxon? Insights from Australian freshwater snails.

Abstract

A combination of intrinsic and extrinsic factors, such as dispersal capabilities or opportunities, habitat preferences, life history attributes, physiological attributes, habitat availability, biotic and abiotic interactions and historical factors, determine not only the geographic distribution of a taxon, but its propensity for population differentiation and speciation. Freshwater hydrobiid snails have little intrinsic ability to disperse outside their immediate habitat and opportunities for accidental dispersal are limited. Long-term permanency of habitat is critical and the existence of many taxa is highly dependent on local hydrological conditions and/or rainfall, water chemistry, geology and the structure of the non-aquatic environment. Dispersal success is apparently related to habitat, including factors such as size and riparian vegetation and canopy, as well as proximity to other populations and the intrinsic behaviour of the snails. Speciation patterns for complex clusters of narrow-range taxa are generally not well understood. Allozyme data for four genetically distinct taxa were analysed to determine the patterns of differentiation of populations at Wilsons Promontory, Victoria. Two of these taxa showed high levels of local differentiation largely concordant with geographic location, suggesting long in situ evolution. The other two taxa showed much less divergence between populations, suggesting relatively recent dispersal. Narrow-range hydrobiid diversity is concentrated in parts of south-eastern Australia, including Tasmania, and in artesian springs associated with the Great Artesian Basin. These areas of high diversity unfortunately largely coincide with some of the least protected areas. Human-induced changes to habitat will undoubtedly threaten the viability of highly localised taxa or populations. The loss or fragmentation of suitable habitat may have caused the extinction of some taxa or may be contributing to the accelerated differentiation of some populations.