A comparison of the Mollusca diversity in the Mooi River (North-West Province) as found during surveys conducted in 1963 and again 50 years later

Abstract

The highest number ofdocumented extinctions of any major taxonomic group is ascribed to the Mollusca. Factorsmainly responsible for this include environmental changes and anthropogenic impacts whichresult in habitat destruction. Furthermore, the conservation status of less than 2% of the moreor less 7000 mollusc species, known worldwide, is still not properly assessed. The aim ofthe study done in 2014 was to assess the current status of the mollusc diversity in the MooiRiver and to compare the results to the study done in 1963. The eight sites selected for thissurvey correspond with sites surveyed during the 1963 study, with surveys conducted duringboth an early and late low-flow period. The pH, electrical conductivity (EC) and temperaturewere also measured at each locality. Of the 15 mollusc species collected in 2014, altogether14 were the same as in the 1963 survey. However, an alien invasive species Physa acuta wasadditionally collected during the current survey, which is probably due to an increase inorganic enrichment. During the 1963 survey Burnupia mooiensis and Pisidium langlyanum werepresent at most of the sites, while Gyraulus connollyi and Ferrissia cawstoni were found at sixof the eight sites during the 2014 survey. Most of the molluscs previously recorded, were stillpresent in 2014. With regard to the abiotic factors, the pH values recorded in 2014 showed amoderate to large decrease as compared to the 1963 survey. This acidification is ascribed tothe development of gold mines in the adjacent area and apparently had little influence on themollusc diversity. The increase in EC measured in 2014 did not have any influence on thediversity of the molluscs, probably due to their wide tolerance for this factor. As expected,the temperature measured in 2014 was markedly higher during the late low-flow period ascompared to the early low-flow period. In conclusion, the differences discussed above revealedno obvious influence on the diversity of the freshwater molluscs over a 50 year period, despiteincreased anthropogenic impacts.