Vicariance, dispersal and scale in the aquatic subterranean fauna of karst regions

Summary 1. The causes of distribution patterns of stygobionts (obligate subterranean-dwelling aquatic species) were examined with special emphasis on vicariance and dispersal. 2. Dispersal was investigated on the premise that if migration is important, then migration at small scales should predict patterns at larger scales. Data on the copepod fauna of epikarst in Slovenia were especially useful for the study of migration, because data on habitat occupancy could be collected at scales of individual drips located metres apart to the scale of individual caves to entire karst regions. Occupancy of drips in one cave was a remarkably good predictor of occupancy of caves in a region, although not of the overall range of a given species. These results were also supported by occupancy patterns of the general stygobiotic fauna of West Virginia caves, compared at different scales. 3. Vicariance was investigated by noting that proximity to marine embayments increases the likelihood of vicariant speciation. In the U.S.A., only the fauna of the Edwards Aquifer of Texas has a significant component of marine-derived species. Differences in shape of the relationship between species number and number of caves in a county indicated that the marine-derived component represented an addition to rather than a replacement of the other stygobiotic species. 4. Thus, we found evidence for the importance of both vicariance and dispersal. The techniques employed could be used to study these patterns more generally, as more data become available.