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When it comes to colonizing new habitats, island species tend to get the short end of the stick. Typically, organisms from the mainland invade an island and take over—pushing the natives to near extinction. But sometimes, colonization can go the other way. In a rare case of “reverse colonization,” researchers in Brazil found that the short-faced bats now living in South America originally arrived from nearby Caribbean islands.
Previously, scientists had unearthed some evidence that island fruit bats had managed to invade the South American mainland. But those findings were weak and ambiguous says Valeria Tavares, a zoologist at the Federal University of Minas Gerais in Belo Horizonte, Brazil. So she and her colleagues turned to genetic analysis of present-day bats and fossil research of long-lost species to bolster the original findings. Their research shows that between five million and 2.5 million years ago, island bats did indeed move from the islands to the mainland.
Reverse colonizations are rare because island species generally evolve in relative isolation. Like Darwin’s finches, island animals tend to be much more specialized than their continental counterparts—adapting to new habitats becomes difficult. Also, secluded habitats feature fewer competitors, which makes island species ill-equipped to outcompete novel species.
But Tavares says the islands’ short-faced bats had a few key physical attributes that allowed them to spread into mainland habitats. “South America has lots of fruit bat species, but they were able to create a new niche,” says Tavares.
In particular, short-faced bats have an extremely powerful bite, so they can eat harder fruits than native bats. They also have a transparent “window” in their wings, which Tavares speculates they use to keep an eye out for danger while roosting. With a peephole, the bats can safely roost in the open—unlike other species that sleep in caves, or in little tents made out of leaves.
Robert Ricklefs, an evolutionary ecologist at the University of Missouri–St. Louis who was not involved in the new research, says that while reverse invasions are rare they probably happen more often than scientists think—especially in places like the Caribbean where the distances between islands and the mainland are not too great. In his research on Caribbean birds, Ricklefs has found roughly equal proportions of colonizing species going each way.
“Biologists don’t pay enough attention to this issue, partly because reverse colonists mostly get lost in diverse continental biotas, and also because the issue is dominated by discussions of invasive species introduced to continental areas,” Ricklefs says.
Tavares thinks there are more reverse invasions out there waiting to be discovered, once biologists take the time to look. “The idea that there would be none is logical, but biology is more complicated than that,” she says.