When we think of invasive species, things like kudzu and zebra mussels come to mind, but Colombia may be facing a threat from a much larger species. Thanks to the fanciful whims of one of the most ruthless criminals in history, African hippos are running amok in the South American country.

Before Pablo "The King of Cocaine" Escobar was killed by police in 1993, he had the grand idea of starting a zoo on his luxury estate, Hacienda Nápoles. Escobar — whose cocaine cartel brought in $60 million a day during its peak — built a colonial mansion on nearly 8 square miles of land north of Bogota, complete with a private cart-racing track, an airport and a collection of exotic animals that mingled with giant concrete dinosaurs. Giraffes, ostriches, elephants, ponies, zebras, hippopotamuses and other assorted living novelties cavorted about the man-made lakes and fields of the complex.

Following Escobar’s death, the government turned the property into a theme park, but the cost of keeping the exotic animals was too high; most of them were placed in other zoos. But the hippos ... oh, the hippos.

Initially, Escobar illegally imported one bull and three females. Today, there are by some estimates more than 80 of them. They are living in at least four lakes in the area and are moving into nearby rivers; they have been sighted up to 155 miles away from Hacienda Nápoles. Yikes.

Hippos and the ecosystem

As the hippo population grows, researchers wonder what impact they will have on other animals. Perla Sofia/Shutterstock

Researchers have now determined the impact of these invasive animals on the ecosystems.

Over two years, they assessed water quality and oxygen levels, comparing lakes that were home to hippos to those without. They compared the microbiomes in the lakes and evaluated populations of insects, crustaceans and other organisms.

They found that hippos bring lots of nutrients and organic material from the surrounding landscape into the water. Because they are nocturnal animals that eat on land at night then spend most of their days in the water, their excessive waste alters the oxygen and the chemistry in the lakes.

"The effect of fertilizing all those bacteria and algae increases the productivity in the water," lead researcher University of California San Diego Biological Sciences Professor Jonathan Shurin said in a statement. "We found that the lakes are more productive when they have hippos in them. This can change the kinds of algae and bacteria and can lead to problems like eutrophication, or excess algae production that can lead to harmful algal blooms similar to red tides."

In the study, which was published in the journal Ecology, researchers estimate that the hippo population will continue to skyrocket. As their numbers grow, there are questions on how that will impact local animals, such as manatees, caymans and giant river turtles.

"If you plot out their population growth, we show that it tends to go exponentially skyward," said Shurin. "In the next couple of decades there could be thousands of them. This study suggests that there is some urgency to deciding what to do about them. The question is: what should that be?"

Why hippos are thriving

Hippos spend their days on a lake in Colombia. Perla Sofia/Shutterstock

In Africa, the dry environment keeps the population in check, but the warm, wet Colombian clime is like hippo heaven. There they can mate year-round, and they have started breeding at the young age of 3, while in Africa they don’t start until 7 at the earliest. The fertile females are thought to be giving birth to one calf every year.

Meanwhile the fisherman who frequent the lakes and rivers where the hippos roam are terrified, and damage reports about crops and even cows are increasing. No human fatalities have been reported, but with hippos responsible for an estimated 500 deaths per year, it seems only a matter of time. In fact, when experts from the World Wildlife Fund and the Disney Foundation visited the region in 2010, they described the situation as a "time bomb." So what to do?

The hippos can’t be sent to Africa because they run the risk of bringing non-native diseases with them; hippo castration is dangerous and difficult, and hippo-proof fencing appears to be prohibitively expensive. Colombians could follow the lead of Jamaica and begin eating them, but popular opinion seems poised to prevail against that solution.

Much of the public doesn’t understand the potential threat these gorgeous — but possibly dangerous and definitely out-of-place — creatures pose. When one rambunctious feral bull was shot in 2009 by authorities, the backlash was palpable, as it was when another was castrated. Aside from those who have had run-ins with the hippos, most people think they are cute.

Carlos Valderrama, a vet from the charity Webconserva who has been monitoring the situation, told the BBC that the reason nothing has been done about the hippos is that whatever decision the government makes, it will be controversial. "They already castrated one," he said, "and there are people saying, 'Oh why do you have to castrate them? Just let them be. Castrate the politicians.'"

Confirming the sentiment, one young girl told a local paper, "My father brought a little one home once, I called him Luna (Moon) because he was very sweet.”

With families bringing home baby wild hippos as pets, how the saga will end is anybody’s guess. But if the lumbering mammals become as problematic as other invasive species have elsewhere, Escobar's legacy may become even more complicated than it already is.