A new study led by researchers at University of Arizona has found that the number of vertebrate species on Earth may be far higher than previously estimated.
The study shows that for every known vertebrate species, there are on average two additional “cryptic” species that have not yet been formally identified. These hidden species look almost identical to known species but are genetically different.
Researchers say the findings suggest global vertebrate biodiversity has been significantly underestimated, raising concerns that many species could be at risk of extinction without scientists even knowing they exist.
According to John Wiens, a professor in the Department of Ecology and Evolutionary Biology at the University of Arizona and senior author of the study, many species that appear to be a single group could actually represent several distinct species.
“Each species that you and I can see and recognize as distinct may actually be hiding two different species, on average,” Wiens said. “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.”
Scientists traditionally identify species by studying their physical characteristics, such as color patterns, body shape, or size. However, cryptic species often look almost exactly the same, making them difficult to distinguish using only physical features.
Researchers say DNA analysis has helped reveal these hidden differences. Genetic evidence shows that many cryptic species have been evolving separately for long periods of time.
“Many of these cryptic species have likely been evolving separately for a million years or more,” Wiens said. “So, their DNA tells us that they’ve been distinct for a long time, even if they look identical.”
The research team reviewed results from more than 300 scientific studies from around the world to estimate how common cryptic species are among vertebrates. These include fish, birds, mammals, reptiles and amphibians.
According to Yinpeng Zhang, the study’s lead author and a graduate student in ecology and evolutionary biology at the university, the pattern appeared consistently across different groups of animals.
“On average, morphologically based species of fishes, birds, mammals, reptiles, amphibians and other vertebrate groups all seemed to be hiding around two cryptic species,” Zhang said.
One example comes from Arizona, where snakes that were once believed to belong to a single species were later found to be genetically different. Researchers discovered that snakes in northern Arizona belonged to Lampropeltis pyromelana, while similar-looking snakes in southern Arizona were later identified as a separate species called Lampropeltis knoblochi.
Zhang said the discovery shows how species that appear identical can actually belong to different genetic lineages.
Scientists say the findings have important implications for conservation efforts. When a species is divided into several cryptic species, each newly identified species usually occupies a smaller geographic range.
Researchers note that species with smaller ranges are generally more vulnerable to extinction, especially due to habitat loss and environmental threats.
Wiens said one of the biggest challenges is that many cryptic species have not yet been formally named or described in scientific records.
“Even though hundreds of molecular studies have uncovered hundreds of cryptic species, very few have been formally described or named,” Wiens said. “That leaves these species without official recognition or legal protection.”
Zhang added that failing to identify cryptic species could also affect conservation programs. For example, conservation managers trying to increase animal populations could unknowingly breed individuals from different species.
Researchers say the study highlights the need for more scientific work to identify hidden biodiversity.
“If we don’t know a species exists, then we can’t protect it,” Wiens said.
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