When it comes to sheer resilience, few, if any, species can match the tiny Devils Hole pupfish.
Cyprinodon diabolis, as the species is known, has the most ruthlessly circumscribed natural habitat of any vertebrate: Devils Hole, an exceptionally deep, water-filled cave in a limestone formation in the unforgiving Nevada desert, where the fish mostly stay on a rock shelf little more than 200 square feet. Not only that, but the pupfish are believed to be one of the most inbred of all species, a lack of genetic variation that makes it difficult for the creatures to procreate and thrive.
And yet, improbably, Devils Hole pupfish are thriving. Late last month, the National Park Service announced that the spring population of the species had grown to 191, the highest in 25 years, according to a count conducted twice a year by scuba divers. Because of seasonal fluctuations in food sources, fall counts tend to be higher, meaning that this year’s tally could be a watershed.
“If, this fall, we have over 300, I’ll be really ecstatic,” said Kevin Wilson, an aquatic ecologist at the National Park Service who has studied the Devils Hole pupfish for more than two decades. (Devils Hole is officially part of Death Valley National Park, most of which is in California.)
If the pupfish census does not seem especially impressive, consider that there were only 35 pupfish left in Devils Hole in 2013, prompting worries about extinction. For now, that danger has receded ever so slightly.
“This is a tremendous success story,” said Christopher Martin, an evolutionary biologist and pupfish expert at the University of California, Berkeley. “Ten years ago, we couldn’t have expected this level of success.”
Biologists have been feeding the pupfish frozen food to supplement their regular diet of algae since 2007. In 2019, the biologists finally arrived at the optimal formula of mysid shrimp, water fleas and blood worms. “This change in the supplemental food probably did enhance that increase in population numbers we’re seeing,” Dr. Wilson said.
Hurricane Hilary, which hit last summer, also helped. Even though the storm caused flooding and damage to the park, it benefited the pupfish living in Devils Hole by “adding nutrients that washed off the surrounding land surface in a fine layer of clay and silt,” according to the National Park Service.
The tiny pupfish, usually about an inch in size, is believed to have lived in Devils Hole for at least 10,000 years and probably much longer, Dr. Martin said. Its name alludes to a playful, puppylike disposition.
How the pupfish ended up in the Nevada desert is not known for certain. Much of Nevada was once underwater. The waters eventually receded, but somehow the pupfish found a refuge in the vast expanse of scrubland and sand.
To this day, no person is known to have completed an exploration of the lowest depths of Devils Hole, which is hundreds of feet deep. (A submersible would never fit into the narrow cavern, Dr. Wilson said.) In a notorious accident in 1965, two young men died during a dive in Devils Hole.
Not much for deepwater exploration, the pupfish stay at depths of 80 feet or less. There, the temperature is 93 degrees Fahrenheit, potentially even hotter near the surface.
Because the pupfish are effectively perched on a shallow underwater ledge, changes to the water table can harm prospects for survival. Dr. Wilson worries that the profusion of enormous solar panel farms in the surrounding desert could drastically increase water usage, damaging the delicate Amargosa River system. Mining is booming again in Nevada. Pahrump, a desert town near Ash Meadows, has seen its population explode.
“There’s increasing pressure on groundwater,” Dr. Wilson said. A well could inadvertently tap the Devils Hole aquifer, causing a drastic drop in the water level there.
Perhaps the greatest danger to the species is its lack of genetic diversity, which increases the incidence of harmful genetic mutations and thus makes it harder for the population to grow. In a classic Catch-22, the pupfish have only one way of inbreeding less: by growing their population.
To prepare for potential catastrophe, the U.S. Fish and Wildlife Service has been breeding Devils Hole pupfish in captivity since 2013. Introducing bred pupfish into Devils Hole is unfeasible for a variety of reasons, but should something happen to the wild pupfish, the species will live on.
For now, however, the wild pupfish are hanging on in Devils Hole. Christopher Norment, a vertebrate ecologist and the author of “Relicts of a Beautiful Sea,” a book about Death Valley, said that although he was “somewhat jaundiced” about the long-term prospects of the Devils Hole pupfish, he was impressed by its tenacity.
“It’s the story of survival in the face of overwhelming odds,” he said.