Persistent organic pollutants: There are about 50 to 60 caracals on the Cape Peninsula. There’s a link between electrical transformers, which use polychlorinated biphenyls, and levels detected in caracal. Photo: Stu Ports
Cape Town’s urban edge may represent a “poisoned chalice” for caracals, who are unwittingly being exposed to toxic persistent organic pollutants (POPs) in their hunting grounds.
A new study by researchers from the Urban Caracal Project has shown that organochlorine pesticides are widespread in caracals, with selected hunting areas such as vineyards and wetlands linked to their exposure.
“We found that caracals around Cape Town were exposed to several types of environmental pollutants,” said lead author Gabriella Leighton, of the Institute for Communities and Wildlife in Africa at the University of Cape Town. “The chemicals we are interested in are called POPs and we found that DDT and polychlorinated biphenyls [PCBs] were the most important of these.”
DDT is a toxic insecticide banned in most countries but still used in South Africa, particularly in malaria-prone areas. “PCBs are carcinogenic synthetic chemicals generally used as mixtures of several ‘congeners’, which are different forms of PCBs, and they have varied uses in household appliances, such as lights, paint and oils, but are mainly used as coolants and lubricants in electrical transformers and capacitors,” Leighton explained.
The team assessed pollutant exposure in samples collected from 56 individual free-ranging caracals across the study area, finding a high occurrence of organochlorines. “Specifically, we detected PCBs in 100% of blood samples, while DDT exposure was recorded in 83% of blood samples,” the paper states. Adipose — body fat — samples showed 100% occurrence of both PCBs and DDT, with higher concentrations than in blood.
They investigated how caracals were being exposed to organochlorines by examining the areas they were using and the types of prey they were consuming. “We found that the areas where caracals hunt, such as vineyards, wetlands and areas closer to human development, are largely the same as those areas linked with high risk of pollutant exposure.
“These areas may represent an ‘ecological trap’ for wildlife around Cape Town — areas that are very attractive for foraging but that are, in fact, suboptimal habitat that could contribute to a decline in the population.”
Further work is needed, Leighton said, to test the idea that the urban fringes of the city are an ecological trap, including how exposure to these pollutants may translate to harmful effects on caracal health and reproduction, and thereby potentially affect their numbers over time. Estimates indicate that there are between 50 to 60 caracals on the Cape Peninsula.
“These pollutants are ‘magnified’ in organisms because when they are consumed they are stored in animal tissues and therefore accumulate over time, and up the food web when one organism consumes another contaminated one,” Leighton said. “Those animals feeding higher up the food web are therefore more likely to be exposed to higher concentrations.
“We found that caracals feeding on prey higher up the food web, such as other carnivores, like large spotted genets, mongoose and snakes, had higher concentrations of pollutants in their tissues. Similarly, caracals feeding on exotic prey, including species introduced by humans, such as domestic cats and livestock, were also more likely to be exposed.”
Cape Town’s urban edge is also highly variable, Leighton said. “The edge of developed areas can abut protected nature reserves, greenbelts, vineyards and the ocean. Our analysis found that caracals favour the urban edge and use it frequently, likely because it provides abundant prey.”
Although the Stockholm Convention, effective since 2004, restricted the use of persistent organic pollutants in South Africa, it remains among the largest importers of pesticides in sub-Saharan Africa. Legacy organochlorines such as DDT are still detected in the environment at some of the highest concentrations globally.
The sources of these pollutants are varied, which means the issue of wildlife exposure needs to be addressed with multiple approaches, Leighton said. “For both DDT and PCBs, stockpiles of obsolete chemicals need to be addressed and removed. There have been national attempts to retrieve these outdated and restricted chemicals in the past, but results from studies like ours suggest that stockpiles likely still exist and these efforts should be renewed with a specific focus on agricultural areas, particularly those with vineyards.”
She said that caracals using areas changed by people, such as vineyards and areas with higher human population and electrical transformer density, as well as wetland areas, had higher organochlorine burdens.
“Wetlands were selected foraging areas for caracals but are also sinks of pollutants and should be the focus of any clean-up measures, especially around cities … For PCBs specifically, we found there was an association between the density of electrical transformers and levels detected in caracals.”
This relationship has also been reported for black harriers, an endangered raptor in the Western Cape.
PCBs have restricted use in South Africa and are slated for elimination by 2025 under the Stockholm Convention.
“To reduce contamination, Eskom and the City of Cape Town urgently need to update any electrical transformer units and substations that contain old, PCB-contaminated equipment. While Eskom and the City are both fully committed and involved in removal plans and upgrading, these need efforts to be expedited to reduce further leakage and ‘spill over’ into wildlife.”
The researchers also found that caracals that had high measures of pollutants in their blood also had increased measures of infection-fighting cells, which suggests there may be an immune burden associated with exposure, she said. “While further research is needed to better understand the patterns we have observed in caracals, over time this stimulation of the immune system may have negative consequences, including increased susceptibility to disease.”
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