Forty years ago, the Colombian jungle was home to just four hippos, living in captivity on a cocaine kingpin’s ranch in Colombia.
But after Pablo Escobar was killed in 1993 — leaving the hippos behind — the animals went wild. Since Escobar's demise, they have spent the intervening decades thriving. There are now as many as 100 of the hippos living in Colombia, wreaking their own special havoc on the land.
At least, that’s what we thought. But new research suggests this rogue bunch may not be so destructive after all.
Escobar’s hippos can certainly cause environmental damage — their poop and its fertilizing effect on water sources is a particular concern. But new research questions the notion that introducing big herbivores like hippos to new places is always problematic for the local ecosystem.
In more cases than not, non-native herbivores are actually filling a role left vacant by another, extinct species, according to a study published Monday in the Proceedings of the National Academy of Sciences.
"We found that introduced herbivores make the world more similar to the past."
The findings contradict another study published in January 2020 that found the hippos were doing significant damage to the environment. Study author Jonathan Shurin told Inverse at the time that the hippos present a "dilemma."
"On one hand, they're a local tourist attraction and curiosity. On the other, they pose a real risk to the public and the environment."
Replacing what humans wrecked
In the study, researchers analyzed the body size and shape, diet, and metabolic processes of 427 extinct and living large herbivores spanning the last 130,000 years. The ecology that includes non-native species is more similar to the historical baseline than looking at native animals only, the researchers discovered.
In the case of Escobar’s hippos, the creatures may replace several prehistoric animals that once roamed the area, said study co-author John Rowan, a postdoctoral researcher at the University of Massachusetts Amherst, in a statement.
The South American hippos have a similar diet and body size giant prehistoric llamas, he said, while their semi-aquatic behavior resembles “bizarre” extinct hoofed mammal group, called notoungulates.
“So, while hippos don't perfectly replace any one extinct species, they restore parts of important ecologies across several species," Rowan said.
That result wasn't a given, says lead author Erick Lundgren, a researcher at Arizona State University. Since the introduced herbivores come from the same pool of species that survived major extinctions, there was a chance they'd simply "amplify the extinctions," duplicating the same natural roles that are already well-filled.
"Amazingly though, by comparing total trait space (e.g. the types of trait combinations and thus effects on the environment), we found that introduced herbivores make the world more similar to the past," Lundgren tells Inverse.
Massive herbivorous mammals once dominated the planet, but began to decline significantly when humans arrived on the scene some 130,00 years ago, causing “widespread ecological changes due to the loss of their ecological functions,” the researchers write.
"We need to drop emotive terms like 'invasive.'"
Now, big herbivores entering areas where they died out millions of years ago may counteract that decline. In fact, 64 percent of introduced herbivores fill a role more similar to extinct species than modern native species, the researchers report. And 42 percent of introduced herbivores brought back key ecosystem roles that hadn’t been filled since the Late Pleistocene.
For instance, hungry hippos (and other introduced herbivores) influence the ecosystem by eating up shrubs that can otherwise expand unchecked, potentially increasing the severity of wildfires.
The chart below shows which modern-day animal may correspond to an ancient, extinct creature:
Who’re you calling invasive?
Lundgren says that it's our perceptions that make animals like Escobar's hippos seem like a problem.
"The idea that these hippos are problematic are based on an idea of 'should,'" he says. "That these ecosystems shouldn't have large animals grazing in riparian areas and defecating into the rivers; that they shouldn't because this hasn’t happened in recent human memory."
But just because we humans haven’t seen an animal in a given location before, that doesn’t mean they don’t belong, the researchers say.
"The idea of 'invasive species' is based on normative and cultural values of 'nativism,'" Lundgren says.
That stems from the idea "that organisms that were here before (European) human arrival have different intrinsic value than newcomers."
"I think we need to drop emotive terms like 'invasive' as they only diminish our ability to do science and instead have a broader perspective on ecological change."
The hippos may be unusual, but they are no outlier. The same is true of grazing animals in the southwestern United States.
"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 said.
"But this view overlooks the fact that horses had been present in North America for over 50 million years.”
“[Horses] 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," he said.
"Unless we evaluate these behaviors without a priori labels of 'invasive' — wherein anything an 'invader' does is by definition harmful — we will never understand them, or how ecosystems work," Lundgren says.
Shifting our mindset away from seeing these reintroduced animals as a problem to be eradicated can boost biodiversity, too, the researchers say.
"It would behoove us to focus on preserving the space for wild animals, regardless of their origin, to continue being wild. That what may appear novel - the trampling and digging and grazing, is actually not," Lungren says.
"Instead of putting our conservation dollars into killing, let's put them into fostering the space for life to continue to flourish."
Abstract: Large-bodied mammalian herbivores dominated Earth’s terrestrial ecosystems for several million years before undergoing substantial extinctions and declines during the Late Pleistocene (LP) due to prehistoric human impacts. The decline of large herbivores led to widespread ecological changes due to the loss of their ecological functions, as driven by their unique combinations of traits. However, recently, humans have significantly increased herbivore species richness through introductions in many parts of the world, potentially counteracting LP losses. Here, we assessed the extent to which introduced herbivore species restore lost—or contribute novel—functions relative to pre-extinction LP assemblages. We constructed multidimensional trait spaces using a trait database for all extant and extinct mammalian herbivores ≥10 kg known from the earliest LP (∼130,000 ybp) to the present day. Extinction-driven contractions of LP trait space have been offset through introductions by ∼39% globally. Analysis of trait space overlap reveals that assemblages with introduced species are overall more similar to those of the LP than native-only assemblages. This is because 64% of introduced species are more similar to extinct rather than extant species within their respective continents. Many introduced herbivores restore trait combinations that have the capacity to influence ecosystem processes, such as wildfire and shrub expansion in drylands. Although introduced species have long been a source of contention, our findings indicate that they may, in part, restore ecological functions reflective of the past several million years before widespread human-driven extinctions.