In Colombia’s enormous Magdalena River basin, an ecological anomaly has triggered an extraordinary debate among ecologists. Ought some invasive species – in this case hippos – be tolerated, or even welcomed, for the ecological role they play as proxies for prehistoric keystone species lost thousands of years ago?

In the early 1980s, infamous trafficker and kingpin Pablo Escobar smuggled four hippopotami – one male, three females – from an American zoo to his private menagerie at Hacienda Nápoles. Years later, on 2 December 1993, Escobar was shot dead by members of the Colombian national police’s search bloc in a shootout in Medellín. After his death, Escobar’s collection of exotic animals was dispersed to zoos around South America. The hippos, however, were deemed too large, too dangerous and too costly to relocate. They were set free and made their way down into the nearby Magdalena River, where they settled and began to breed.

Today, their descendants are thought to number over 200, roaming a 3,500 square miles of wilderness, roughly the size of America’s Yellowstone National Park, within a vast tropical river basin spanning 40,000 square miles. There they can be observed grazing riverbanks, wallowing in oxbow lakes, and reshaping wetlands in ways no South American native species has in several thousand years.

Hippos are true ecosystem engineers

At the outset, amid rising institutional alarm, environmental authorities declared Escobar’s hippos to be an invasive species and even a ‘biological pollutant’, calling for their immediate eradication. In 2009, a young male dubbed Pepe was shot by hunters sent out by local authorities. The mutilated hippo, draped over a tarp, made front pages. Colombians recoiled, protests erupted and a planned cull was suspended. The hippos, once symbols of narco excess, became popular heroes. Left alone, Escobar’s hippos didn’t just survive, they have thrived. Female hippos breed at three to four years old, producing a calf every 18 to 24 months. Population models now project 1,000 hippos by 2035.

In a landscape scarred by deforestation, gold mining and industrial cattle ranching, some ecologists have begun to observe that these African giants may be fulfilling a vital ecological role missing since South America’s catastrophic late-Pleistocene megafauna extinctions took place between 13,000 and 11,600 years ago. The continent experienced the most complete megafauna loss of any, losing forever giant ground sloths, sabre-toothed cats, giant armadillos, elephant-like creatures known as gomphotheres and a host........

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