Millions of years before humans set foot in the Americas, a wave of alien animals began arriving in South America.
As the Isthmus of Panama rose from the waves, connecting the North and South American continents, llamas, raccoons, wolves, bears and many other species headed south. At the same time, the ancestors of armadillos, opossums, and porcupines headed north.
Paleontologists call the event the Great American Interchange. But they have long been baffled by one aspect: why did most mammalian immigrants go south, rather than the other way around? What happened to the southern mammals?
After the extinction of the dinosaurs and the gradual recovery of the Earth’s biosphere, there have been millions of years of animal trade between North America and Asia. During this time, the ancestors of modern horses, camels and cats crossed land bridges back and forth. But South America spent most of this time period, the Cenozoic, as an island continent, complete with its strange bestiary: giant sloths, bizarre native ungulates, tank-like armadillo relatives, and saber-toothed marsupial predators.
Then, 10 million years ago, a series of tectonic events gave rise to the Isthmus of Panama, connecting two very different faunas.
“This exchange was relatively balanced at first,” said Juan Carrillo, a colleague at the Smithsonian Tropical Research Institute in Panama and author of the study. “There was the same amount of mammals that migrated in both directions. But what we found is that five million years ago in the Pliocene there was a disproportionate decline in diversity. “
The source of that extinction event remains unclear, Dr. Carrillo said. The climate became drier in the Pliocene, repelling South American forests and initiating a global cooling cycle that eventually led to the ice ages. As habitats changed, South American ecosystems were likely subjected to considerable stress.
Competition from northern species likely also played a role, with northern predators such as dogs, bears and saber-toothed cats benefiting from relatively larger brains and more efficient teeth. Some of those southward migrants may have also brought parasites and diseases with them.
“The reason for the extinctions is probably quite complex and includes some biological interactions and habitat change,” said Dr Carrillo.
Whatever the causes of the extinction event, it appears that since the Pleistocene Ice Ages, when humans started showing up, North American species made up the best part of the interchange simply because fewer South American mammals were left heading north.
Even so, some South American species have been remarkably successful: giant sloths have made it to Alaska and terror birds have had one last gasp in Florida. But only a few, such as possums and armadillos, survived the extinction of the Pleistocene.
While the American exchange took millions of years, Dr. Carrillo said, the advent of industrial humanity has seen exotic species rush around the world, with no need for land bridges. Indian antelopes graze on Texan ranches. Wild Eurasian pigs are rampant throughout North America. African hippos have even found a foothold in Colombia with the help of Pablo Escobar’s drug empire.
The team’s research on the Great American Interchange suggests that no matter how exotic species arrive in a new environment, these kinds of introductions can lead to unpredictable changes in the shape of ecological communities.
“This movement of animals that humans do today could have some important long-term consequences,” said Dr. Carrillo.