The Amazon basin is well known for its wide variety of species, but the rainforest might owe some credit to the mountains as a source for that rich diversity.
A new study found that populations of poison frogs made their way from the Andes to the Amazon about a dozen times over the last 10 million years. Scientists suspect that the mountains have long been supplying the jungle with other species of plants and animals, too.
Besides weighting a long-standing debate about the source of biodiversity in the Amazon, the new work suggests a need to rethink the way we go about protecting the region.
"The history of what's in the Amazon is complex and coupled to the evolutionary history of the same groups in other regions," said evolutionary biologist David Cannatella, of the University of Texas, Austin.
"What that means is that if you want to understand what you need to do to protect the Amazon," he said, "You need to understand what's going on in the Andes."
Led by his graduate student Juan Santos, Cannatella and colleagues chose to study poison frogs as a test case of Amazonian biodiversity because they are diverse and widespread. Some 350 species of poison frogs live throughout Central America, from Andean highlands to lowland rainforests. About 70 species live in the Amazon itself.
The team used DNA analyses and new modeling techniques to figure out exactly when each of more than 200 species branched off from an ancestral family tree, and where each ancestor lived when the branching happened. The genealogy stretched back more than 25 million years.
Published in the journal PLoS Biology, the tree showed a series of pulses, most in the last 10 million years, from the Andes to the Amazon. With each wave of immigration, frogs split into new species in the highlands before dispersing to the lowlands, where they evolved further.
"This provides the first evidence of a major involvement from the Andes as a source of diversity for the Amazon and northeastern forests of Colombia," said herpetologist David Wake, of the University of California, Berkeley. "It is really the first pretty good demonstration that the mountains are acting as a species pump."
Alternate theories have proposed that the Amazon is like a museum that has been stable for a long enough time to support an excessively high number of species.
"They've deeply enriched our understanding of what causes tropical diversity to be so over the top," Wake told Discovery News. "It's not just a little bit more diverse. It's horrendously more diverse."
Now that they have such an extensive and detailed family tree for poison frogs, researchers can also begin to prioritize conservation efforts. It may make more sense, for example, to put effort into saving a species that is ancient and unique rather than one that evolved recently and has lots of close relatives.