Imagine looking up at a sky so full of birds, they block out light from the sun. Passenger pigeons (Ectopistes migratorius) used to fly in flocks of hundreds of millions, maybe even billions, of birds that took hours to pass overhead. Then, we started shooting them.
Humans began commercially hunting passenger pigeons in the 19th century, and by 1914, they were extinct, according to Audubon magazine. These birds are a prime example of how quickly and efficiently humans can wipe out even the most common species. But is it just us, or can nonhuman animals drive other animals to extinction?
Sort of, but humans are usually involved. Some animals are capable of interspecies decimation if humans put them in the wrong place and they become invasive — species that cause ecological or economic damage to their non-native environment. For example, Burmese pythons (Python bivittatus) from Asia are gobbling up anything that moves in the Florida Everglades. The python population started out as released and escaped pets, according to the Florida Museum of Natural History.
Species that can't recognize or appropriately respond to a new species in their environment are called "naive," or said to suffer from ecological naïveté. It's not their fault; animals don't evolve to flee or defend themselves against aliens lickety-split, and adaptations don't spring up overnight.
"The primary way that alien species wipe out natives is through consumption — so, predators introduced to areas where there have been no predators before, or the types of predators that were there were different," Tim Blackburn, a professor of invasion biology at University College London in the U.K., told Live Science. "That gives them a sort of inbuilt advantage that allows them to eat their way through naive faunas [animals of a particular region]."
Blackburn's go-to invasive-species example is the domestic cat. "They have contributed to the extinction of dozens of species of bird," he said — the Stephens Island wren (Traversia lyalli) in New Zealand, which went extinct in 1895, is one example. Cats are the leading direct human cause of bird mortality in the U.S. and Canada, according to the American Bird Conservancy. In other words, American birds are under greater threat from pet cats than from guns.
Humans are responsible for moving feline hunters and giant snakes around the planet. Anything they do after that is on us. But what about when animals naturally migrate to a new area? According to Blackburn, animals tend to naturally disperse to nearby areas, where the kinds of species are generally similar and, therefore, respond appropriately to one another — so there are usually no unfair matchups.
Occasionally, the movement of land forces an interspecies shake-up. The Great American Biotic Interchange (about 10 million to 10,000 years ago) is a prominent example of this; tectonic plates pushed North America and South America together, and species from each continent met via a central American land bridge. South America was introduced to many new animals, including predators such as bears and big cats, while North America received species like ground sloths and armadillo relatives called glyptodonts in return.
The diversity of animals that moved from North America to South America was higher than the reverse, so South America gained more new residents. A 2020 study published in the journal Proceedings of the National Academy of Sciences proposed that this was due to a disproportionately higher extinction rate of South American mammals. In other words, more species in South America went extinct during the exchange and less were able to colonize North America.
"Perhaps the South American native mammals were more susceptible to the new predators," said Juan Carrillo, a paleobiologist at the University of Fribourg in Switzerland and lead author of the 2020 paper. Predation by North American carnivores is just one hypothesis for what drove the asymmetric interchange.
"The ground sloths and the glyptodonts were probably large enough to escape these predators," Carrillo told Live Science. "And that may be one of the reasons they were able to migrate to the north and we found them in the fossil record in many parts of North America."
But while the impact of modern invasive species on extinctions is clear, the interchange paints a more complicated picture. "It was not just one instant in the Earth's history but actually took over several millions of years and had different phases," Carrillo said. The South American extinctions took place during a period of climate change when Earth was cooling, which also likely had an impact.
But is it still fair to assume that at least some prey species in South America went extinct because of a North American predator coming in? It is possible, but it's difficult to disentangle that cause from climate change and other factors, Carrillo said.
Animal traits are forged in an evolutionary battleground, but that doesn't mean predators rise up to dominate their prey. Carrillo noted that if a predator were to eat its prey to extinction, it wouldn't have anything to eat and thus also would become extinct. If a predator has multiple prey, then it could, in theory, survive wiping out one species, but extinctions usually involve multiple factors.
Blackburn doesn't know of any examples of a natural invasion in which a species consumed another to extinction. "The natural world is just inherently incredibly complicated, and it takes an enormous amount of work to tease out the processes that are generally going on," he said.
Humans are clearly driving species to extinction through activities such as overhunting, destroying habitat and introducing invasive species. "The fact that those effects are so dramatic is itself almost strong evidence that these processes are real and very different to what's gone on before," Blackburn said.
Originally published on Live Science.