After Pablo Escobar, the world’s biggest drug lord at the time, was killed by police in 1993, the Columbian government seized all of his assets. Among them, his luxurious Hacienda Napoles estate northwest of Bogota, which also served as Escobar’s personal zoo. Most of the exotic animals there were shipped away, but somehow four hippopotamuses — of which Escobar was especially fond — were left behind at the estate’s pond.

The hippos were resourceful enough despite their circumstances and they not only survived — they thrived. No one knows exactly many of them are, wallowing in the mud of Colombia’s main river, the Magdelena. Some believe their population could number as many as 80-100 individuals.

Many local residents close to their habitat are completely horrified.

“We have to lock ourselves inside with the children to try and avoid an accident,” primary school teacher Wilber Quinones told Sky News.

Then there’s their ecological damage. Many scientists have asserted that hippos, as an invasive species, are ruining the native flora and fauna. The water in which they defecate, for instance, causes algae outbreaks in Columbia’s rivers and lakes.

However, a new study published by researchers at the University of Massachusetts Amherst suggests the impact of Escobar’s hippos on the environment might be positive, counteracting a legacy of man-made extinctions.

The hippos aren’t alone, with the study finding many introduced herbivores actually share key ecological traits with extinct species across the world, generally providing balance to ecosystems.

Human impact has significantly altered the ecological landscape across the world over the last 100,000 years. We’ve caused the extinction of many large mammals through overhunting and habitat destruction, but also introduced numerous species to foreign environments often with severely negative consequences for the environment.

But, sometimes, our activity also leads to ‘happy accidents’, rewilding some parts of the world.

In South America, giant llamas once roamed, while the flat-headed peccary was once found all the way from New York to California. Even more mythical creatures once roamed the Earth, such as the tank-like armored glyptodons and two-story-tall sloths. This megafauna appeared millions of years after the demise of the dinosaurs but abruptly disappeared around 100,000 years ago.

Their ecological role has been filled, in some situations, by large herbivores that we’ve later introduced.

“For example, the feral hippos in South America are similar in diet and body size to extinct giant llamas, while a bizarre type of extinct mammal – a notoungulate – shares with hippos large size and semiaquatic habitats. So, while hippos don’t perfectly replace any one extinct species, they restore parts of important ecologies across several species,” study co-author John Rowan, an evolutionary biologist at the University of Massachusetts Amherst, said in a press release.

The researchers compared key ecological traits of herbivore species — things like body size, diet, and habitat — that lived before the Late Pleistocene to the present day.

“This allowed us to compare species that are not necessarily closely related to each other, but are similar in terms of how they affect ecosystems,” said Erick Lundgren, lead author and Ph.D. student at the University of Technology Sydney Centre for Compassionate Conservation (CfCC). “By doing this, we could quantify the extent to which introduced species make the world more similar or dissimilar to the pre-extinction past. Amazingly they make the world more similar.”

According to the findings, 64% of introduced herbivores are more similar to extinct species than to local native species. This includes mustangs (wild horses) in North America, which replaced pre-domestic horses that filled the same ecological niche but were driven extinct.

“Many people are concerned about feral horses and donkeys in the American southwest, because they aren’t known from the continent in historic times,” Rowan says. “But this view overlooks the fact that horses had been present in North America for over 50 million years – all major milestones of their evolution, including their origin, takes place here. They only disappeared a few thousand years ago because of humans, meaning the North American ecosystems they have since been reintroduced to had coevolved with horses for millions of years.”

There is a lot of talk about invasive species among scholars — as there should be, since invasive species have been associated with damage to natural parks and 13% of a total of 953 global extinctions recorded thus far. But in some cases, foreign species (at least some herbivores) can rewild ecosystems making the world more similar to its pre-extinction past.

The findings appeared in the journal Proceedings of the National Academy of Sciences. Other researchers who contributed to the study are affiliated with the University of Kansas and the University of California Davis and the Natural History Museum of Los Angeles County in the U.S., the University of Sussex in the U.K., the Universidad de Alcalá in Spain and Aarhus University in Denmark.