South Africa is urbanising rapidly. By 2050, eight in 10 people will live in urban areas, significantly increasing the demands on basic infrastructure development and associated services.
In the country's Western Cape province, some 90% of the population is urbanised. Most of its residents live in the Cape Metropolitan Area. So it is truly remarkable that the city is still home to a population of between 60 and 100 wild caracals.
Hikers on Table Mountain's trails and greenbelts may have briefly spotted one of these elusive cats with their reddish-brown coat and tufted ears before they disappeared into the dense vegetation.
Having survived the eradication of larger carnivores like the Cape leopard and lion, this highly adaptable, medium-sized wild cat is now Cape Town's apex wildlife predator.
The Urban Caracal Project, a research and education initiative based at the University of Cape Town's Institute for Wildlife and Communities in Africa, is dedicated to studying Cape Town's caracal population. It aims to better understand the effects of urbanisation on the city's wildlife and to discover some of the secrets of how they are able to survive in this challenging landscape.
But surviving in a rapidly expanding city isn't easy. Indeed, it can be downright dangerous thanks to, among other issues, the increasing presence of environmental pollutants.
As conservation biologists, we are interested in how caracals become exposed to the multitude of pollutants associated with city-living. To do this, we tested the blood of caracals in Cape Town and found worryingly high numbers of different metal pollutants present. Exposure to these metals, including aluminium, arsenic, cadmium, copper, mercury and lead, most likely occurs via the prey species that caracals consume.
This raises important environmental concerns for all the city's residents - both wildlife and human.
Metal pollutants are a global biodiversity threat
Chemical pollution is a growing global concern. Cities and rapidly developing countries are disproportionately affected because they are characterised by high levels of both industrial and human activity. Metallic chemical elements are some of the most toxic and well-studied of these environmental chemical pollutants.
Most metals occur naturally in the Earth's crust. However, numerous human activities increase the quantity of, and rate at which, metals are released into the environment. Major sources of metal pollution include coal power plants, mines, agricultural activities, and waste disposal sites like landfills and illegal dumps.
The most dangerous of the metal pollutants are mercury, arsenic, and lead. These can all be extremely toxic to animals and humans, even in small amounts.
Both animals and humans are generally exposed to harmful metals through food and water. After entering lower down the food chain, metals accumulate over time in bodily fluids and tissues via a process called bioaccumulation. Pollutants then tend to move up through the food chain, becoming more concentrated through the process of biomagnification.
Consequently, animals occupying higher positions across an ecosystem's food web, especially top predators like caracals, are exposed to greater concentrations of pollutants than those lower down. Exposure to metal pollutants can reduce reproductive success. It also impacts the immune system, damages the nervous system, and increases the risk of cancer and cancer-related diseases. In acute cases it can lead to death.
Detecting toxic metals in wildlife
For our research, individual caracals were caught using cage traps and then sedated. A veterinarian then took blood samples. Caracals killed in vehicle collisions, and reported by the public to the project, were also opportunistically sampled.
Our blood analysis revealed that most metals detected were not present at toxic levels. However, the worrying exceptions were arsenic and chromium, both of which pose serious health risks. Hunting at the urban edge and in places with more human activity, such as near roads, vineyards and suburbs, exposes caracals to a greater number of metals and at higher levels than when hunting further away from these areas.
This is very concerning given that our research has also shown a similar trend for caracal exposure to other pollutant stressors, including groups of man-made chemicals, like organochlorines and anticoagulant rodenticides (rat poison), as well as novel pathogens.
Read more: Cape Town's caracals are exposed to harmful 'forever chemicals' through their diet
Dietary contamination through waterbirds
Our most interesting and unexpected discovery was that caracals hunting within or nearby coastal and wetland areas in Cape Town, where they enjoy a diet rich in aquatic-adapted birds, were more exposed to harmful metals like arsenic, mercury, and selenium than those on the urban edges.
This suggests that aquatic prey species - seabirds and waterbirds like Cape cormorants, gulls, Egyptian geese, and yellow billed ducks - are likely the main source of metal exposure in caracal.
Our findings highlight that Cape Town's freshwater and marine systems are likely more polluted than expected. Aquatic environments generally act as long-term sinks which accumulate a range of pollutants. Coal combustion, emissions from domestic fuel burning, natural fires and untreated city wastewater are all likely sources of metal contamination.
This may have implications for the health of other mammalian and avian predators in our study area, as well as human health implications for local fishing communities and wider seafood consumers.
Improving the city's ecological health
The City of Cape Town can do more to evaluate and mitigate this issue.
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The first step is appropriate monitoring of the problem - identifying the sources and understanding the scale. Monitoring should be focused on the urban edge, waste management sites, water treatment plants, road run-off, and agricultural areas.
It is crucial to develop a robust local, provincial, and national pollutant monitoring programme using a variety of indicator species. Such species, including small and medium-sized carnivores, like caracals, together with aquatic animals, are especially sensitive to the effects of bioaccumulation. Monitoring populations and regularly testing for levels of pollutants in their tissues will provide a clearer understanding of Cape Town's broader environmental health.
Other mitigation strategies include wetland and freshwater system clean-ups, implementing stricter regulations on fuel-burning emissions, improved treatment and disposal of city wastewater, and reduced use of agricultural pesticides. Taking these necessary steps will greatly improve both animal and human health.
Kim Helene Parker, a recent Masters graduate from the University of Cape Town, co-authored both this article and the research it is based on.
Gabriella Leighton, Post-Doctoral Fellow, Rhodes University
Jacqueline Bishop, Senior Lecturer in Conservation Ecology & Genetics, University of Cape Town
