Ecological risk assessment of amphibians in the Phongolo River floodplain

The Phongolo River floodplain in South Africa hosts the highest floodplain biodiversity in the country, while also being highly utilised for commercial and subsistence agriculture. The floodplain falls within the malaria risk region where vector control in the form of indoor residual spraying is still practised with dichlorodiphenyltrichloroethane (DDT). This region operates on a fragile socio-ecological balance. Previous studies identified a gap in available knowledge on malaria vector control pesticides and the associated toxicity to amphibians. This study used a tiered assessment approach to assess the risk to amphibian well-being in the Phongolo River floodplain. Ecosystem services were incorporated into the assessment alongside the risk to amphibian well-being to assess the relationship between these aspects. The first tier to this study involved identifying the data requirements, which included sub-lethal effects data regarding amphibians and malaria vector control pesticides along with a lack of current field monitoring data of these pesticide in amphibians. The second tier involved generating field monitoring data. This study showed amphibians from Ndumo Game Reserve in the floodplain actively accumulate DDT at sub-lethal levels. The next tiers in the assessment involved sub-lethal toxicity data generation. This was done through laboratory and simulated field exposures of Xenopus laevis to vector control pesticides. Behaviour, metabolomics and pesticide accumulation was measured as effect outcomes. Behavioural changes were seen in frogs exposed to a mixture of DDT and deltamethrin. Metabolomic changes were mostly attributed to a general stress response affected in all exposures compared to control. Metabolomic responses in the simulated field exposure had low overlap with those found in laboratory exposures, which was attributed to the addition of food sources in the simulated field environment. The mixture exposure of DDT and deltamethrin resulted in significant loss of invertebrate diversity. The final risk assessment incorporated data generated in this study to determine risk levels to amphibians using a relative risk model. Overall amphibian well-being was at moderate risk, driven by the likelihood of chronic or sub-lethal effects from pesticides and high amphibian biodiversity in the region. The aquatic habitats in the floodplain (river, temporary pans, and permanent pans) were identified as priority habitats for conservation in order to benefit socio-ecological functioning and maximise amphibian wellbeing in the process. The outcome of this study supports the use of amphibian well-being as a monitoring tool for the floodplain through amphibian well-being and biodiversity monitoring serving as sensitive indicators of ecological change.

Publication year:2021

Keywords:Doctoral thesis

Accessibility:Open