'n Vergelyking van die Mollusca-diversiteit in die Mooirivier (Noordwes-Provinsie) soos gevind met opnames wat gemaak is in 1963 en weer 50 jaar later

Abstract

Die hoogste getal gedokumenteerde uitsterwings van enige belangrike taksonomiese groep word aan die Mollusca toegedig. Faktore wat hoofsaaklik hiervoor verantwoordelik is, sluit onder meer omgewingsveranderinge en antropogeniese impakte in wat tot die tot niet gaan van die habitat aanleiding gee. Daarbenewens is die bewaringstatus van minder as 2% van die ongeveer 7000 molluskspesies wêreldwyd bekend tot dusver nog nie behoorlik geassesseer nie. Die doel van die huidige ondersoek wat in 2014 uitgevoer is, was om die molluskdiversiteit van die Mooirivier met die resultate van 1963 te vergelyk. Dit is gedoen deur ’n opname uit te voer van die molluskdiversiteit by dieselfde lokaliteite as in 1963. Twee opnames is gemaak tydens ’n vroeë- en laatlaagvloeiperiode in 2014 by agt lokaliteite wat met dié van die 1963-opnames ooreenstem. Die pH, elektriese geleiding (EG) en temperatuur is ook by elk van die lokaliteite gemeet. Die identiteit van die 14 slakspesies wat in 1963 gevind is, het ooreengestem met 14 van die 15 in 2014. Die uitheemse indringerspesie Physa acuta is tydens die 2014-opname vir die eerste keer in hierdie rivier gevind, waarskynlik weens ’n toename in organiese verryking. In die 1963-opname is Burnupia mooiensis en Pisidium langlyanum by die meeste lokaliteite gevind, en in 2014 Gyraulus connollyi en Ferrissia cawstoni. Die meeste van die molluske uit hierdie gebied wat voorheen op rekord gestel is, is in 2014 gevind. Die pH wat in 2014 ’n geringe tot aansienlike verlaging getoon het, word aan mynversuring toegeskryf en het nie ’n merkbare invloed op die molluskdiversiteit gehad nie. Die hoër elektriese geleiding wat tydens die 2014-opname gemeet is, het vanweë die verdraagsaamheid van varswatermolluske in hierdie verband, ook geen verandering in die molluskdiversiteit teweeg gebring nie. Die temperatuur was in die laatlaagvloei deurgaans hoër. Ondanks die feit dat hierdie studies vyf dekades uitmekaar gedoen is en antropogeniese impakte toegeneem het, is geen noemenswaardige verandering in die molluskdiversiteit waargeneem nie

The highest number of documented extinctions of any major taxonomic group is ascribed to the Mollusca. Factors mainly responsible for this include environmental changes and anthropogenic impacts which result in habitat destruction. Furthermore, the conservation status of less than 2% of the more or less 7000 mollusc species, known worldwide, is still not properly assessed. The aim of the study done in 2014 was to assess the current status of the mollusc diversity in the Mooi River and to compare the results to the study done in 1963. The eight sites selected for this survey correspond with sites surveyed during the 1963 study, with surveys conducted during both an early and late low-flow period. The pH, electrical conductivity (EC) and temperature were also measured at each locality. Of the 15 mollusc species collected in 2014, altogether 14 were the same as in the 1963 survey. However, an alien invasive species Physa acuta was additionally collected during the current survey, which is probably due to an increase in organic enrichment. During the 1963 survey Burnupia mooiensis and Pisidium langlyanum were present at most of the sites, while Gyraulus connollyi and Ferrissia cawstoni were found at six of the eight sites during the 2014 survey. Most of the molluscs previously recorded, were still present in 2014. With regard to the abiotic factors, the pH values recorded in 2014 showed a moderate to large decrease as compared to the 1963 survey. This acidification is ascribed to the development of gold mines in the adjacent area and apparently had little influence on the mollusc diversity. The increase in EC measured in 2014 did not have any influence on the diversity 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 as compared to the early low-flow period. In conclusion, the differences discussed above revealed no obvious influence on the diversity of the freshwater molluscs over a 50 year period, despite increased anthropogenic impacts