Pablo Escobar’s Hippos Fill a Hole Left Since Ice Age Extinctions

Trilobites Invasive herbivore mammals seem to restore functions missing in some food webs and ecosystems since the Pleistocene era. When Pablo Escobar died in 1993, the Colombian drug kingpin’s four adult African hippopotamuses were forgotten. But the fields and ponds along the Magdalena River suited them. One estimate puts their current population at 50 to 80 animals: By 2050 there may be anywhere from 800 to 5,000 in a landscape that never before knew hippos.

They aren’t the only herbivores showing up in unexpected places. In Australia, feral camels roam the outback. Antelope are a common sight in rangelands from Texas to Patagonia. And feral hogs are everywhere.

Conventional wisdom holds that these animals are causing new, and potentially damaging, impacts to beleaguered ecosystems. But a study published Monday in the Proceedings of the National Academy of Sciences argues that the lifestyles of these and other exotic fauna may be restoring the ecological functions of species lost to extinction during the last ice age. “We found that, amazingly, the world is more similar to the pre-extinction past when introduced species are included,” said Erick Lundgren, an ecologist at the University of Technology in Sydney, Australia, and the study’s lead author. Beginning 100,000 years ago, during the Late Pleistocene, a wave of extinctions claimed large animals throughout the world: mammoths in Eurasia, horses and giant sloths in the Americas and a bestiary of giant marsupials in Australia.

Researchers have suspected that the loss of these megafauna may have left holes in the food webs and other cycles of the ecosystems where they lived, particularly in places like the Americas and Australia, where the extinctions were more intense. Mr. Lundgren and his colleagues wanted to test the idea that introduced herbivores were picking up ecological slack from their extinct counterparts. The team dug through scientific literature to create an enormous list of both living and extinct herbivore species from the last 126,000 years.

They categorized them by body size, anatomy, diet and how their guts ferment vegetation, then compared how the lifestyles of introduced and extinct herbivores in a region overlapped. The team expected to find that assemblages of purely native species would be closest to Pleistocene ecosystems. But mixtures of exotic and native animals were a closer match.