Before his fall, notorious cocaine kingpin Pablo Escobar was the owner of an expansive Columbian ranch complete with private zoo home to four resident – albeit illegally smuggled –  hippos. Following his death in 1993, the hippos were left to their own devices, eventually populating the wetlands and rivers that surround the property by the dozens.

For decades, ecologists have denounced the minivan-sized herbivores as invasive pests responsible for the destruction of local ecosystems. Indeed, previous studies have found that the hippos change wetland structures simply by walking through it.

But new research challenges the theory that the human-introduced hippos, and other animals in similar situations, are detriments to ecosystems. Instead, an international team of conservation biologists and ecologists argue that the introduction of herbivores may compensate for extinction losses caused by humans over the last 100,000 years. Writing in the Proceedings of the National Academy of Sciences, researchers compared key ecological traits like body size, diet, and habitat of introduced herbivores like Escobar’s hippos to herbivore species from just before the Late Pleistocene extinctions to 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 PhD student at the University of Technology Sydney's Centre for Compassionate Conservation, in a statement. "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."

Nearly two-thirds of introduced herbivores are more similar to extinct species than to endemic species, suggesting that animals introduced may serve as “surrogates” for those that have gone extinct. Over the last 100,000 years, humans have caused the extinction of several large mammals – wooly mammoths, elephant-sized sloths, and saber-toothed cats, to name a few – but have also introduced numerous species that have, in a sense, rewilded parts of the world. For several million years, giant mammalian herbivores dominated much of Earth’s ecosystems following the extinction of dinosaurs, but early humans’ hunting of them resulted in widespread ecological changes.

"While we found that some introduced herbivores are perfect ecological matches for extinct ones, in others cases the introduced species represents a mix of traits seen in extinct species," said study co-author John Rowan, Darwin Fellow in organismic and evolutionary biology at the University of Massachusetts Amherst.

"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."

The authors conclude that their work supports the need for “renewed research” on herbivores introduced to nonnative ecosystems, suggesting that focus may require a shift from eradication to protection.