For every recognized vertebrate species, there are on average two unrecognized, or "cryptic" species, according to a new study led by University of Arizona researchers. The findings suggest that global vertebrate biodiversity has been significantly underestimated, which could have consequences for conservation efforts if these hidden species are not properly identified.
"Each species that you and I can see and recognize as distinct may actually be hiding two different species, on average," said John Wiens, senior author of the paper and a professor in the University of Arizona Department of Ecology and Evolutionary Biology in the College of Science . "This means that across vertebrates, there may be twice as many species as we previously thought, and many of these hidden species could already be at risk of extinction."
In the effort to classify and name animal species on Earth, scientists generally rely on distinct physical (morphological) features to differentiate between species. For example, different snake species may have different color patterns, scale patterns or body shapes. Cryptic species, however, are nearly visually identical to each other. Despite these similarities, they have evolved separately and their DNA shows they come from different genetic lineages.
"Many of these cryptic species have likely been evolving separately for a million years or more," said Wiens. "So, their DNA tells us that they've been distinct for a long time, even if they look identical."
Uncovering cryptic species
Advances in molecular sequencing have made it easier and cheaper for scientists to compare DNA across populations. As a result, researchers continue to uncover cryptic species.
Even more surprising was how consistent the pattern was across different groups, according to Wiens. "On average, morphologically based species of fishes, birds, mammals, reptiles, amphibians and other vertebrate groups all seemed to be hiding around two cryptic species."
One well-known example comes from Arizona itself: the Arizona mountain kingsnake. For many years, it was thought that kingsnakes in different regions in Arizona belonged to the same species, since they look similar. However, in 2011, molecular data revealed that populations in Northern Arizona were distinct from those in Southern Arizona. The southern mountain snakes were then classified as a different species called Lampropeltis knoblochi instead of a subspecies of the northern counterpart, named Lampropeltis pyromelana.
"If you compare those two mountain kingsnakes, they all look pretty much the same with their red, black and yellow-white stripes," said Yinpeng Zhang, a graduate student in the Department of Ecology and Evolutionary Biology and the paper's first author. "But the molecular data show that there are distinct but cryptic northern and southern species."
Zhang began thinking about this project three years ago when he noticed that many taxonomy papers were uncovering cryptic species that look identical but are genetically distinct. He quickly realized that no one had ever tried to measure how common this was across vertebrates.
The team synthesized results from more than three hundred studies published by researchers around the world.
"There aren't many research groups focused purely on cryptic species," Zhang said. "Most people discover them as a byproduct of other biodiversity or taxonomy studies rather than as the main goal."
The researchers also compared different methods for estimating cryptic species to provide a framework for other scientists.
Informing conservation efforts
According to Zhang and Wiens, their findings extend far beyond taxonomy. They believe that conservation efforts may need to adapt in light of the increasing number of vertebrate species that may be affected by habitat loss or other threats to their environment.
When what was once considered a single widespread species is divided into multiple cryptic species, each newly recognized species has a smaller geographic range.
"People have generally found that the smaller a species' range size is, the more likely that species is to go extinct," said Wiens.
Wiens believes that the first step in protecting these cryptic species from extinction is formally naming them as distinct species.
"Even though hundreds of molecular studies have uncovered hundreds of cryptic species, very few have been formally described or named," said Wiens. "That leaves these species without official recognition or legal protection."
Additionally, if cryptic species aren't correctly identified, conservation managers looking to boost population numbers might breed members of different species unintentionally, according to Zhang.
"Hidden diversity is an important consideration to make in our conservation efforts," Zhang said.
For the researchers, the lesson is clear.
"If we don't know a species exists, then we can't protect it," Wiens said.
