BISBEE, Ariz. —Dusty boots, bagged lungs, Dr. John Wiens searched the rocks of a desolate Arizona mountain for the last survivors of a 3-million-year-old lizard population—then spoke the words that confirm his life's work and break his heart.
“They're not there,” he said. “It seems that the species is already extinct.”
Chance Horner/CBS News
The loss of plant and animal species on Earth is occurring at a rate never seen in human history, according to the United Nations. That includes the likely extinction of the lizards Wiens has studied for 10 years: the Yarrow spiny lizard population found in the Mule Mountains of southern Arizona.
“There are a lot of species on Earth, and we're going to lose a lot of them because of climate change,” said Weins, an evolutionary biologist at the University of Arizona. “It's catastrophic.”
Going up with the elevator to extinction
Over the past 3 million years, a million years longer than humans, the spiny lizards from the Yarrow to the Mules adapted to live in cold mountain climates called the Sky Islands.
Because the desert floor is too hot, the lizards were essentially marooned at higher elevations, as if on an island, and separated from other yarrow populations in southern Arizona and northern Mexico.
These lizards were also easy to find in the wild, unlike many other species. They often sunbathed on large rock outcrops. This behavior allowed Wiens and his colleagues to regularly count their population to see how climate warming was affecting them.
In 2014, the team was unable to find any lizards below 5,700 feet. Up to this elevation, the temperature in the mountains had become too hot. In 2021-22, they returned to Mules to count lizards at the same site. They had gone.
“They're dying at lower elevations,” he said.
At that time, the lizards could only be found living much higher, at 7,100 feet, a cooler elevation. In a scientific paper, Wiens and his colleagues calculated the rate at which the lizards were dying, and concluded that it is among the fastest rates ever recorded.
But since the highest peak in the Mules is 7,700 feet, the spiny lizards of the Yarrow quickly ran out of elevations with cooler air. Based on his calculated rate of decline, and with nowhere else to go, Wiens projected that the lizards would become extinct here by 2025, a phenomenon scientists call riding the “elevator to extinction.” .
In March of this year, a survey trip to the mountains with CBS News proved their hypothesis correct, a year ahead of schedule. Wiens was unable to find any more lizards, although it will take several more trips before he can draw a conclusion.
“It looks like the species has gone extinct, this distinct lineage that's been separated for about 3 million years,” he said. “This is the future. This is a climate-related extinction.”
According to Krista Kemppinen, a senior scientist at the Center for Biological Diversity, who was not involved in Wiens' research, the implications are dire for other species in the Sonoran Desert, where the mules are found, as they may already be in the upper part threshold of how much heat they can tolerate.
“The region is really like a ticking time bomb in terms of climate change,” he said.
Humans 'bear some responsibility for this'
According to a comprehensive 2019 UN report, there are 1 million plant and animal species threatened with extinction around the world.
Wiens concluded that the number is likely much higher in a more recent research paper, published in Global Change Biology. He estimates that 3 to 6 million species will be threatened with extinction in the next 50 years, driven largely by climate change, which will make it too warm for many species to survive.
“As human beings, in the developed world, we all have some responsibility for this,” Wiens said.
Although the 3-million-year-old population of Yarrow's lizard species is presumed extinct in the Mule Mountains, its distant relatives still exist in other mountainous locations in Arizona and Mexico, although many are also in decline .
Still, across the country, 1,700 plants and animals are listed as threatened or endangered under the Endangered Species Act, which provides resources to help protect species and their habitat.
The act is widely seen as an environmental success story. Some high-profile species on the list include:
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The California condorthe largest flying bird in the US, with about 90 adults remaining in the wild.
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The iconic florida pantherwith about 200 animals remaining.
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The massif of the North Atlantic right whale, which crosses the Atlantic Ocean; only 250 individuals remain.
Even so, the Endangered Species Act only covers part of the species at riskin part because the process of classifying a species can be long, bureaucratic and political.
“It can take an average of 12 years, when legally it should only take two,” Kemppinen said.
Not enough time for the spiny lizards of the Mule Mountains.
