Ecological risk assessment of amphibians in the Phongolo River floodplain

Abstract

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 socioecological balance and as such has been the subject of socio-ecological assessments in the past. Previous studies identified a gap in available knowledge on the ecological risk to amphibians within the floodplain. Specific focus was drawn to malaria vector control pesticides and the associated toxicity to amphibians. This study used a tiered assessment approach to generate required data and assess the risk to amphibian well-being in the Phongolo River floodplain. Ecosystem services were incorporated into the assessment alongside the risk to amphibian wellbeing 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. In this phase of the study it was concluded that close proximity to spraying sources in the Phongolo River floodplain resulted in amphibians from a conservation region in the floodplain actively accumulating DDT at sub-lethal levels. The next tier in the assessment involved sub-lethal toxicity data generation. This was done through laboratory based acute exposures of Xenopus laevis to vector control pesticides measuring behaviour, metabolomics and pesticide accumulation as effect outcomes. Behavioural changes were seen in frogs exposed to a mixture of DDT and deltamethrin. Metabolomic changes were also determined, but were mostly attributed to a general stress response as similar metabolic pathways were affected in all exposures compared to control. The next tier of the study involved a simulated field exposure assessment where pesticide residue accumulation, metabolomic response and aquatic invertebrate community effects were measured. Metabolomic responses 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 massive loss of invertebrate diversity. The final risk assessment incorporated data generated in this study to determine risk levels and current risk impacts on amphibians using a relative risk model. Using source–habitat effect interactions the relative risk to different habitats in the floodplain were determined.

Overall amphibian well-being was at moderate risk, driven by the likelihood of chronic or sublethal effects from pesticides and high amphibian biodiversity in the region. Based on both ecosystem service and amphibian well-being risk outcomes, the aquatic habitats in the floodplain (river, temporary pans, and permanent pans) were identified as priority habitats for conservation management and development of protection goals in order to benefit total socio-ecological functioning and maximise amphibian wellbeing management in the process. The outcome of this study supports the use of amphibian health and well-being as a monitoring tool for the Phongolo River floodplain. Long term pesticide monitoring alongside amphibian well-being and biodiversity monitoring will serve as sensitive indicators of ecological change and allow intervention prior to ecosystem service impacts being observed.