Effects of lead poisoning in South Africa's Gyps vultures

Abstract

Lead poisoning poses a significant global risk to wildlife and is widespread among scavenging raptors. The plight of African vultures calls for urgent evaluation of environmental toxins that may threaten their future survival. In this thesis, I combined a variety of field and laboratory techniques to identify the source and quantify the sub-lethal effects of lead poisoning in southern Africa’s Cape (Gyps coprotheres) and White-backed (G. africanus) Vultures. To identify the main source of lead poisoning in White-backed Vulture chicks, I quantified stable lead isotopic ratios in vulture blood samples and compared the ratios to those of several potential sources of environmental and anthropogenic lead. The results allowed me to exclude several potential sources, including lead from mining, industrial activity, air, water and soil, as well as legacy lead from fuel. I found the major source of elevated lead levels in White-backed Vulture chicks to be fragments of lead-based ammunition embedded in the carrion fed to them by their parents. I evaluated the sub-lethal impact of acute lead exposure on Cape and White-backed Vulture chicks by analysing its possible effects on key blood biochemistry parameters, immune function, packed cell volume and δ-aminolevulinic acid dehydratase (δ-ALAD) activity. The findings indicated the possibility of compromised hepatic function in White-backed Vulture chicks with severe clinical lead poisoning. The δ-ALAD analyses allowed for the definition of critical thresholds beyond which Cape and White-backed Vulture chicks are likely to undergo severe physiological stress. Because movement ecology may be impacted by lead exposure, I used GPS tracking technology to test whether White-backed Vulture chicks with blood lead concentrations above their species-specific critical threshold (as described above) display behavioural differences that may compromise their success as newly fledged juveniles. The data indicated that these birds display poor mobility, as evidenced by an apparent reduction in distances travelled, and a failure to significantly increase their home ranges beyond their natal area. I evaluated the impact of chronic lead poisoning on the cortical thickness indices and bone densities in the diaphyses of the long bones of Cape and White-backed Vultures. The study yielded valuable baseline data, which may be used in future research to evaluate bone health in Cape and White-backed Vultures with extreme bone lead levels. I assessed the accuracy of the portable LeadCare® II Blood Lead Analyzer in determining blood lead concentrations in White-backed Vultures, when compared to gold standard methods such as Inductively Coupled Plasma Mass Spectrometry (ICP-MS). LeadCare® II data demonstrated varying levels of negative bias when compared to ICP-MS. Efforts to define a universal conversion equation to ICP-MS equivalent values were unsuccessful. The outcomes suggest that the LeadCare® II system requires calibration prior to use, and that all LeadCare® II derived data should be validated via ICP-MS before use in lead impact studies. Collectively, the research presented in this thesis provides important new insights into the source and physiological effects of lead poisoning in Cape and White-backed Vultures, which will inform ongoing mitigation measures. The conclusions reiterate the importance of species-specific research in evaluating species sensitivities to lead, and the need to adequately evaluate the risk of lead as environmental contaminant across Africa.