Pablo Escobar, the notorious head of the Medellín drug cartel, visited all manner of misfortune on Colombia, most notably corruption and violence. Less well known is Escobar’s curious environmental legacy, which scientists say is becoming a serious problem.

The capricious drug baron sought to create a menagerie of sorts at his Hacienda Nápoles, a 3,000-hectare (7,400-acre) estate containing 27 artificial lakes and a landing strip in Antioquia Department, about 170 kilometers (105 miles) east of Medellín. Among the exotic animals he brought to the site—zebras, elephants, kangaroos and many more—were four hippopotamuses he had purchased from a U.S. zoo.

After Escobar’s death in 1993, the Colombian government confiscated the estate and donated many of the animals to zoos. But the hippos remained—and multiplied. Some made their way off the estate grounds and took up residence elsewhere in the watershed of the middle section of the Magdalena River, which flows north through the western half of the country, emptying into the Caribbean. They have reproduced prodigiously, thriving in humid woodlands free of predators and hunters, in many respects enjoying more favorable conditions than in their native habitat of sub-Saharan Africa.

Alarms about the hippopotamuses’ impacts began sounding this January, when nine biologists from Colombia and Mexico published an article about them in the journal Biological Conservation. The scientists warned that the hippos’ current population, estimated at 100, could top 1,400 by 2034, eventually spreading extensively through northern Colombian ecosystems at elevations under 1,500 meters (5,000 feet) and producing “potentially long-lasting ecological and socio-economic effects.” The article cites impacts ranging from the destruction of vegetation to the large quantity of feces and urine the hippos release into the water. “The hippo is one of the largest terrestrial organisms on Earth and influcences natural systems at large temporal and spatial scales,” it says.

The co-authors recommended culling the hippos, though they recognized this proposal would draw wide public opposition. They also called for a campaign to “communicate to stakeholders and the general public the urgency of controlling this established exotic species”.

“The most appropriate decision would be to carry out a population-control hunt,” says Germán Jiménez, a biologist at Pontifical Xavierian University in Bogotá and one of the article’s coauthors. “This would not mean going out and killing hippopotamuses for fun, but instead would be a scientific exercise. Colombia has the opportunity to demonstrate that it does not provide more protection for an invasive species than it does for its farming communities and biodiversity itself.”

Ranging far and wide
Jiménez says hippos are currently found in a 2,000-square-kilometer (770-sq.-mile) portion of the Magdalena watershed and, if unchecked, could occupy over six times that area once their population reaches 1,500. “They thrive because they have limitless resources, since they have moisture year-round as opposed to in Africa, where there are frequent periods of prolonged drought,” the biologist says.

This month, Jiménez and other biologists discussed solutions in a series of meetings hosted by the Colombian Environment Ministry. The ministry did not respond to a request for comment, but an official with Cornare, the agency that oversees natural resources in the mid-Magdalena region, indicates the government is wary about culling the hippos. “It’s a complex decision,” says David Echeverri, Cornare’s coordinator for forests and biodiversity. “People like them, which is why we’re thinking about sterilization as an alternative.”

Rancor remains from a 2009 incident that occurred after residents of Puerto Berrío, a municipality about 130 kilometers (81 miles) from Hacienda Nápoles, complained about three hippopotamuses in the area. Expert hunters, accompanied by army personnel for security purposes, killed one of the animals. A photo of army members posing with the body circulated on the internet, prompting wide condemnation and an eventual court ruling against further hippo hunting.

Over the past two years, Cornare has sterilized 10 hippos, but the process is difficult. “We began capturing them in the open with tranquilizer darts,” says Echeverri. “Now we do it with corrals. We attract them with food. Sedating them is not easy; it is a tremendous risk for those taking part. We need resources.”

In February, Cornare announced that it is seeking to enlist help from Colombia’s ambassador in Washington to secure the donation of an animal-sterilization drug produced in the United States. Sterilization is expensive, totaling US$15,000 to $20,000 for each hippo when field-operation costs are taken into account.

Doubts about sterilization
But some scientists argue it is already too late to rely on sterilization. The Species Survival Commission, a network of experts associated with the International Union for the Conservation of Nature (IUCN), sent a letter in February to Environment Minister Carlos Correa calling the strategy “very unlikely” to succeed in the long term. The commission asserted sterilization “does not avoid the impacts caused by the animals [in future] decades, considering that hippos have a life expectancy of over 40 years.”

In its letter, the commission cited not only the damage hippos can do to aquatic ecosystems, but also their aggressive nature, noting they kill humans in Africa on an annual basis. Thus far no deaths due to hippos have been reported in Colombia, but last May a farmer was seriously injured in a hippo attack in the municipality of Puerto Triunfo.

“The hippo must be declared an invasive species so a management plan can be launched,” says Gonzalo Andrade, director of the Colombian Institute of Natural Sciences and president in Colombia of the Species Survival Commission. “But the scientific information to do that seriously is still lacking.”

Despite warnings of attacks, many farmers in the mid-Magdalena watershed now take paying visitors on hippo-watching walks and boat tours. That economic dimension cannot be ignored, says Jonathan Shurin, a biologist with the University of California, San Diego, who has done research in Colombia.

“For people who live there, hippos are a sort of resource,” Shurin says. “[The farmers] have to make a living there, and they don’t want to see the hippos removed. So I don’t agree hunting is the only solution. It will depend on how much Colombians are determined to try other solutions.”