What's a species? Open your biology textbook and read: "Species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups."
Well, that sounds tidy.
But, as David Barrington, professor of botany, will tell you, this famous definition by Ernst Mayr has spawned as much controversy as clarity. And it's just one of dozens. Though there is broad agreement about the biological reality of species, where and why to draw the lines between them can be devilishly hard.
To see how hard, drive into the mountains of southwestern China with a jeep full of botanists and try to decide if the thin-leaved holly ferns at your feet are one species or two. Ready for a debate?
That's what Barrington will be doing in July. He and Zhang Li-Bing from Brigham Young University — both leading experts on the holly fern genus Polystichum — have assembled a team of American and Chinese fern scientists that will meet in Bejing and travel toward the Himalayas. It's the next step in Barrington's decades-long, worldwide study and cataloging of these quiet residents of forest floors and limestone outcrops.
And he's doing this trip under the banner that every guy with a new camera and waxed field vest dreams about; he's leading a National Geographic Expedition.
"It's not like the helicopter or the ropes-in-the-mountain thing at all," says Barrington, "it's a botanical expedition. Our rate of progress will be dismal. You spend all morning walking up one hillside."
A massive effort
Many of these hillsides are in geologically young Himalayan regions of Yunnan and Sichuan provinces as well as another Polystichum hotspot south of the Yangzte River. This rugged area of China has the world's greatest diversity of holly ferns, thanks in part to the evolutionary pressure put on plant lineages thrust into higher elevation habitats by rapidly rising mountains.
Here is the only place to find many of the holly ferns identified in the Chinese-language Flora of China, the painstaking account of the plants of the country begun in 1958 and completed in October 2004. Within its pages, 168 holly fern species, carefully described by Chinese botanists, include plant architectures and traits unknown anywhere else in the world.
And yet there is still more to be learned and research is brisk in several Chinese laboratories, delving deeper into the species history of holly ferns.
Which points to a central goal of Barrington's expedition: to work closely with Chinese botanists, particularly with Zhang, in writing a new, updated version of the Flora of China's account of the Polystichum genus. It's part of a massive effort to write an English-language edition of the whole book, since, as Barrington says, "English is the new Latin."
As co-authors, Barrington and Zhang’s new holly fern account will draw on modern techniques, including molecular analysis of DNA, for determining species — and will more tightly conform to the central idea of Mayr's biological species concept: species are distinguished by reproductive barriers, not by how they look.
But in practical terms, botanists often distinguish species based on a plant's form, its morphology, since these differences can serve as a pretty reliable surrogate for a true reproductive barrier. "The traditional approach to finding species boundaries is to look for clear interruptions in the spectrum of natural variation," Barrington says. "So, for instance, you could write a plant key that had a division between 'leaves round' versus 'leaves long and thin.' Now if there is every imaginable variation between 'round' and 'long and thin' there is no edge there."
The research will take place in the right place: the actual forests and limey rocks where the holly ferns grow. "You need to see the plants alive," Barrington says. Though a top goal of the trip is to make a collection of plants for herbarium study and laboratory research, there is no substitute for natural habitat. "You lose characteristics when you dry them and flatten them out," he says.
And making sense of species boundaries is only one piece of Barrington's larger goal of understanding the ecology and evolutionary history of the ferns. This means seeing what kind of soil they grow in, with how much light and moisture and with what kind of companion plants. It means mapping their locations across varying geographies.
"I've had a lot of success with this kind of work in the New World, especially in Central America, Mexico, Costa Rica, where I spent most of my time in the field," he says. "With a lot patience, I worked out Polystichum species lines very carefully and I now am at the point where I can talk about the history of the production of those species via evolution."
He hopes to do the same in China, supporting ongoing research in the country — before it's too late.
"The high-altitude forests where these ferns live are full of species vulnerable to water stress," he says. "And the big thing that is happening with climate change is increased water stress in these forests. It won't take much. My attitude is: I better get on this or there's not going to be anything left to study."