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article imageHow amphibians crossed continents: A scientist tells the tale

By Megan Hamilton     Jul 29, 2014 in Science
The world's 7,000 species of amphibians can be found on six continents, but there has been few attempts at understanding how salamanders, frogs, toads, and caecilians had moved across time to these distant continents.
Until now.
DNA sequencing tells the story, via research conducted by Alex Pyron, an assistant professor of biology at the George Washington University. Dr. Pyron hoped to fit the puzzle pieces of their 300-million-year-old storyline, and how their journey fits within the timeline, Phys.org reports.
So he did something that hasn't been done before. He constructed a comprehensive diagram showing the geographic distribution of amphibians and the movement of 3,309 species between 12 global ecoregions. The phylogeny (a diagram of evolutionary relationships) includes nearly half of all living amphibian species in every taxonomic group.
"There have been smaller-scale studies, but they included only a few major lineages and were very broad," Pyron told Phys.org. "What we needed was a large-scale phylogeny that included as many species as possible. That allows us to track back through time, not only how different species are related, but also how they moved from place to place."
His studies included a rather surprising find: Certain groups of amphibians may have swam long distances from one landmass to another within the past few million years. The findings were published in the journal Systematic Biology.
Over the years, biologists have hypothesized the distribution of living lineages of amphibians has been driven by vicariance and dispersal — two key processes, Phys.org reports.
When a large-scale geophysical event — such as the fragmentation of the supercontinent Pangaea and the subsequent split of the Laurasian and Gondwanan landmasses occurred, some groups of amphibians were able to "hitch a ride," from one continent to another, Pyron explained. It's through this continental drift that the process of vicariance occurs. Pyron's biogeographic analysis supports this hypothesis, and shows that the movement of continents can explain much of the patterns in the distribution of living species of amphibians.
In his research, he also found that in the Cenozoic Era (66 million years ago to the present), that amphibians likely dispersed across land bridges, or across short distances of ocean.
Amphibians have a long history, so it's not surprising that they populated the continents through vicariance and dispersal, but there's a third part to this long journey that was an unexpected finding.
Earlier studies had assumed that long-distance dispersal over stretches of ocean was impossible for amphibians due to salt intolerance. However, as Pyron was completing his analysis, he noticed several cases of distribution that couldn't be explained by old age, Phys.org reports.
One group of frogs found in Australia and New Guinea known as pelodryadine hylids — which orginated somewhere between 61 million to 52 million years ago fits within a group of amphibians that are found only in South America. However, when the pelodryadines originated, all major continental landmasses were in their present-day positions, and South America and Australia had been long separated from Antarctica, Phys.org reports.
"They're 120 million years too late to have walked to Australia," he said.
So how in the heck can this group of South American amphibians be related to frogs on the other side of the planet?
"You wouldn't think that frogs would be able to swim all the way there, but that seems like one of the more likely explanations for how you could have such a young group nested within South America and have it somehow get to this other continent," he told Phys.org.
In his study, Pyron notes other instances of long-distance dispersal across oceans.
"What you have is this mixture of processes. You have vicariance, which over 300 million years has put certain groups in Africa, some in Australia and others in South America," he said. "But even more recently, within the last few million years, you have these chance events of long distance dispersals across the ocean, which can influence distribution patterns."
The next research question that he wants to answer is whether there is any specific quality — maybe tolerance to salt water, for instance. This allows some groups of amphibians to be better at dispersing. Pyron is also conducting similar analysis with lizards and snakes in order to see if the same distribution patterns hold up. If new species are discovered, Pyron will continue to revise his analysis.
All of this comes at a time when these amazing creatures are threatened with extinction, according to Amphibiaweb.
Recent assessments of the entire group by the IUCN redlist found that nearly 32 percent of the world's species of amphibians are threatened with extinction. That's some 1,856 species, Amphiaweb reports. Tragically, in the last two decades, some 168 species are believed to have become extinct. This is extremely sad, when you consider that amphibians have existed for 300 million years.
The biggest factor leading to population declines in these creatures is habitat destruction, but what is most troubling is that even in cases where habitat is protected, amphibians are still disappearing. While there are likely many causes for declining populations, the two most serious causes that are emerging are the disease chytridiomycosis, followed by global climate change. Chytridiomycosis is a fungal disease that has been associated with the loss of hundreds of species of amphibians worldwide — a particularly devastating loss of biodiversity that some say is the worst in recorded history, Amphibiaweb reports.
Costa Rica's golden toad is perhaps the most famous story of amphibian extinction. These beautiful little toads were known from a small area of montane cloud forest in Northern Costa Rica. This region is the Monteverde Cloud Forest Reserve.
In 1987, the population of golden toads crashed, and only a few individuals were found up until 1989, but the little creatures haven't been seen since. Some 20 out of 50 species of frogs and toads that occurred within a 30km squared area in Monteverde disappeared in these population crashes and have shown no signs of recovery, Arkive reports. This is an area of pristine forest that is free of direct human influences. However, it's thought that fungal diseases and climate change may have played a role in the extinctions.
Long before the time of the dinosaurs, amphibians walked, crawled, and swam all over the planet. Perhaps in finding out how these remarkable creatures conquered the planet, we can find ways to save them.
If we can't save the world's frogs, toads, salamanders and caecilians, what does that say about us as a species?
More about Amphibians, physorg, cenozoic, South america, Australia
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