Devils Hole pupfish, living within a single pool in Death Valley National Park, have clung to their existence under the watchful eyes of ecologists and public agencies. Numbers of the uniquely isolated and critically endangered species have ebbed and flowed with shifting conditions and nearby development. Now, researchers have identified a new factor that could cause fluctuations in the famously fragile population: flash floods.

“Devils Hole is a really small ecosystem that responds very quickly to climate stresses.”

Sudden pulses of rainfall, projected to grow more intense with climate change, stir the species’ cavernous habitat, researchers reported at AGU’s Annual Meeting 2023 in San Francisco. The fish have a tangled relationship with the sediment that floods add to their natural fishbowl. The finding offers state and federal agencies deeper insight into how the fish adapts to its harsh environment.

“Devils Hole is a really small ecosystem that responds very quickly to climate stresses,” said the study’s lead author, hydrologist Mark Hausner of the Desert Research Institute in Nevada. Floods disturb the ecosystem in the short term, he said, but could generate a greater food source for the pupfish in the long run.

Fish on the Brink

Devils Hole is a watery window into a limestone cave network and aquifer beneath the parched desert, fenced off in a detached portion of Death Valley National Park in Nevada. Geologists believe shaking from an earthquake collapsed the geothermal cavern at least 500,000 years ago and estimate it reaches more than 137 meters (450 feet) deep. The rectangular hole, 22 meters (72 feet) long and 3.5 meters (11 feet) wide, features a rock ledge submerged in shallow water on one side. The pupfish depend on this shallow shelf to lay their eggs and forage for algae, insects, and diatoms.

Devils Hole pupfish (Cyprinodon diabolis) are shimmers of blue the length of a paper clip. Their numbers exceeded 400 before the 1990s, then plummeted to a low of 35 in 2013. For the past 10 years, however, the population has been on the rise. Today, 263 live in the wild.

Scuba divers have plunged 30 meters (100 feet) into the balmy 92°F water twice per year since 1972 to count the fish. But scientists did not consistently track benchmarks such as sunshine, air temperature, and food abundance until the population started crashing. When that happened, the U.S. National Park Service, the U.S. Fish and Wildlife Service, and the Nevada Department of Wildlife initiated a long-term, rigorous ecosystem monitoring plan for Devils Hole in 2011.

A Rush of Sand and Stone

Hausner and his colleagues used the monitoring data to trace how 120 different environmental factors may have influenced the plight of the pupfish between 2010 and 2019. The team used a machine learning algorithm that works like a decision tree to point to potential relationships. This analysis identified which factors best predicted the population’s ups and downs, unveiling a complex relationship between the fish census and overland floods.

Occasional torrents, which took place in 2015 and 2016, washed through the desert, carrying water laden with sediment and organic matter into Devils Hole. First, the flow unloaded rocks and sand onto the shallow shelf, smothering fish eggs, algae, and small insects. That influx also decreased the shelf’s water depth, making it warmer, and temporarily depleted oxygen.

“I think they’re definitely onto something.”

But after several months, the pupfish rebounded when nutrients delivered by the pulse of water worked their way up the food chain. Algae and diatoms flourished, feeding the pupfish population.

The monitoring data and analysis could help answer many questions that remain about the pupfish’s resilience, said Christopher Martin, a biologist at the University of California, Berkeley who was not involved in the research. “This is long overdue,” he said. “I think they’re definitely onto something.”

Scientists expect that major floods in the region will grow in magnitude and frequency in response to climate change. It’s too early to say how that will affect population trends for the pupfish, Hausner said, because desert rainfall events are so erratic.

Fans of the pupfish are glad to have any additional information that might predict the species’ fate. “When you don’t have a clear picture of what’s going on, dealing with a crisis is very difficult,” said Kevin Brown, author of a book on Devils Hole pupfish and a past collaborator of Hausner’s.

—Alix Soliman (@alixoutdoors), Science Writer

Citation: Soliman, A. (2024), Flash floods may support one of the world’s rarest fish, Eos, 105, https://doi.org/10.1029/2024EO240002. Published on 5 January 2024.
Text © 2024. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.

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