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Hippos imported into Colombia by drug lord Pablo Escobar could have helped to restore ecological diversity in the surrounding area, according to a new study.

An international group of researchers, including Dr Chris Sandom and Owen Middleton at the University of Sussex, conducted a worldwide analysis comparing the ecological traits of introduced herbivores, like Escobar’s hippos, to those of the past.

Their results showed that such introductions can restore many important traits that have been lost for thousands of years.

Owen Middleton, PhD Student at the University of Sussex, said: “These results are exciting because they challenge the general consensus that non-native species are always ecologically harmful.”

“Identifying the similarity in characteristics of introduced species to recently extinct species was eye-opening. Yet, it’s important to recognise that their immediate effects on ecosystems are influenced by other factors, including habitat fragmentation and the presence of predators.”

The paper, published in Proceedings of the National Academy of Sciences, was authored by an international team of conservation biologists and ecologists from The University of Technology Sydney (UTS) in Australia; the Universidad de Alcalá in Spain; Aarhus University in Denmark; the University of Kansas, the University of California Davis and the Natural History Museum of Los Angeles County in the U.S, alongside the University of Sussex.

The researchers note that what most conservation biologists and ecologists think of as the modern 'natural' world is very different than it was for the last 45 million years.

Not long after the demise of the dinosaurs, rhino-sized wombat-relatives called diprotodons, tank-like armoured glyptodons and two-storey tall sloths ruled the world. They were driven extinct around 100,000 years ago – most likely due to hunting and other pressures from our Late Pleistocene ancestors.

While human impacts have caused the extinction of several large mammals over the last 100,000 years, humans have since introduced numerous species, inadvertently rewilding many parts of the world such as South America, where giant llamas once roamed, and North America, where the flat-headed peccary could once be found from New York to California.

This study has found that by introducing species across the world, humans have restored lost ecological traits to many ecosystems; making the world more similar to the pre-extinction Late Pleistocene and counteracting a legacy of extinctions.

Pablo Escobar, for example, brought four hippos to his private zoo in Colombia. Since he died in 1993, they have made their way into the surrounding waterways growing in number to between 80 and 100. Until now, they have been seen as an invasive pest – but this research might suggest otherwise.

Co-author Dr John Rowan, Darwin Fellow in organismic and evolutionary biology at the University of Massachusetts Amherst, said: "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.

“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 international team compared key ecological traits of herbivore species from before the Late Pleistocene extinctions to the present day, such as body size, diet and habitat.

Erick Lundgren, lead author and Ph.D. student at the UTS Centre for Compassionate Conservation (CfCC) said: "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. 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."

The research showed that around 64% of introduced herbivores are more similar to extinct species than to local native species. These introduced 'surrogates' for extinct species include evolutionary close species in some places, like mustangs (wild horses) in North America, where pre-domestic horses of the same species lived but were driven extinct.

Dr John Rowan explained: "Many people are concerned about feral horses and donkeys in the American southwest, because they aren't known from the continent in historic times. 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."

Senior author, Dr Arian Wallach from UTS CfCC added: "We usually think of nature as defined by the short period of time for which we have recorded history but this is already long after strong and pervasive human influences.

“Broadening our perspective to include the more evolutionarily relevant past lets us ask more nuanced questions about introduced species and how they affect the world."

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