A study published this week in the international journal Science of the Total Environment posed this very question. Are urban fringes poisoning local wildlife?
This is the first study to investigate persistent organic pollutants (POPs) in caracals, and their possible sources in a mammalian carnivore using an urban area in South Africa.
The study, conducted by the Urban Caracal Project, based at the University of Cape Town’s Institute for Communities and Wildlife in Africa (iCWild), focused on the city’s caracal population and the wild cat's exposure to harmful chemical pollutants found along the city’s urban fringes when foraging for prey.
The urban fringe, also called the "urban-wildland interface," is the area of land where a city or town meets the countryside or protected areas.
A glance at a map of Cape Town reveals that the city includes extensive green spaces and protected areas such as the Table Mountain National Park, and the Blaauwberg and Tygerberg Nature Reserves.
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The study tested caracal samples collected across the city’s peri-urban and agricultural landscapes for exposure to toxic organochlorines (OCs), including dichloro-diphenyl-trichloroethane (DDT, a neurotoxic insecticide) and polychlorinated biphenyls (PCBs, a carcinogen with many uses in household and industrial equipment).
Concentrations in both blood and tissue were analysed along with detailed spatial, dietary, demographic, and physiological data to assess OC sources and exposure risk.
I tracked down the lead author, Dr Gabriella Leighton, an urban ecologist with iCWild, and now a post-doctoral fellow at Rhodes University, to tell me more about the study.
“We found there was a high detection rate of PCBs and DDT in blood and fat tissue, occurring at similar concentrations to those found in wildlife in North America and Europe” explained Leighton.
Habitats that caracals venture into are also areas that increase their risk of exposure to these and other pollutants, potentially making these areas a “poisoned chalice” for these adaptable carnivores.
These include areas like vineyards, wetlands, and the urban edge where chemicals may move into the environment as a result of various human activities, including past and present use in agriculture and construction.
“We also found that caracals were exposed to multiple different toxic organochlorine compounds, which may result in interactive effects within individuals.
“Through our previous research, we also know that caracal in and around Cape Town are simultaneously exposed to rodenticides and novel blood pathogens, so the potentially harmful effects are of OC exposure is likely considerably worsened,” said Leighton.
The study explains that carnivores can play vital roles in ecosystem stability but are particularly vulnerable to bioaccumulation of pollutants.
Understanding the spatial and dietary predictors of these contaminants can inform the development of more-effective pollutant control measures, and carnivores, in their positions at the top of food webs, can act as useful indicators of species of environmental degradation.
Impacts on the health of wildlife due to indirect exposure, such as consuming contaminated prey animals, are usually more subtle but are concerning when these impacts act in concert with other threats, such as high mortality rates, genetic isolation, and disease observed in this caracal population.
“We found those caracals with higher OC concentrations also had an increase in infection-fighting cells, suggesting that there may be an immune response to exposure, which can have consequences through increasing susceptibility to disease.”
“Our findings reveal that organochlorine pollutant exposure is a contemporary issue for South African wildlife. At a national level, the ongoing legal use of DDT in malaria areas needs to be carefully considered and monitored. Our results also suggest this legal use may facilitate illegal use in other areas.
“Further, stockpiles of obsolete chemicals remain around the country and could be a source of some of the exposure we see.
“However, because these pollutants are so persistent and remain in ecosystems for decades, it is difficult to tell whether the exposure we detected is from recent or historical pesticide use.”
Dr Leighton supports the reinstatement of a nationwide pesticide disposal programme, incentivising farmers, and landowners to catalogue and properly dispose of pesticide stockpiles, with disposal efforts focused on agricultural areas, particularly those with vineyards.
Interestingly, the study found that PCB exposure was linked to the density of electrical transformers, where they are used in coolant fluids.
PCBs are prohibited in South Africa by the Stockholm Convention (2001) and are slated for elimination by 2025. Eskom and the City of Cape Town are in the process of phasing out outdated equipment, but leaks do sometimes occur, and these pollutants are at risk of ‘spilling over’ into local wildlife. Removing these old transformer units should be expedited.
Dr Leighton argues that local conservation authorities can promote caracal conservation by prioritising the maintenance of connectivity between protected areas, and in so doing ensuring their access to non-contaminated prey.
Improving the health of urban wetlands is another mitigation action that will benefit local wildlife, for example, restoring wetlands (which are natural filters), or reducing wetland pollution through bioremediation techniques.
On an individual level, citizens can help by reducing poison use and lowering the demand for toxic rodenticides and pesticides.