Across the world’s drylands, streams are lifelines. They nourish ecosystems, sustain communities of fish, and connect watersheds. Yet these fragile waterways often face scorching heat and long droughts.

For the fish that live in dryland regions, known as xeric fish, survival already means enduring extremes. Now, climate change is testing their limits even further.

Troubling fish trends in drylands

In recent study co-led by the University at Buffalo, researchers analyzed four decades of data from nearly 1,500 streams in the United States and Australia.

The findings revealed a troubling trend: the number of fish species has declined in many arid regions, where water is becoming scarcer and temperatures continue to rise.

Dr. Corey Krabbenhoft, the study’s lead author, is an assistant professor in UB’s Department of Biological Sciences.

“If highly adapted fish can’t hack it in warm and dry climates, it doesn’t bode well for fish that are used to much milder climates,” said Dr. Krabbenhoft. “Fish living in dry climates are a canary in the coal mine when it comes to climate change.”

Why streamflow matters

Streamflow is more than water moving downhill. It provides oxygen, disperses sediments, and connects habitats, allowing fish to feed and reproduce. Without steady flow, ecosystems unravel.

Some freshwater fish manage to survive in intermittent streams, which only flow part of the year. Over half of U.S. rivers fall into this category, with the proportion climbing to 80 percent in the southwest.

It was in this region that Dr. Krabbenhoft and her collaborator, USGS biologist Jane Rogosch, first began studying fish in xeric climates. They later joined forces with USGS ecologist Freya Rowland to examine long-term data on fish diversity and streamflow from 1980 to 2022.

Declining rainfall and fish habitats

Modeling showed rising temperatures coupled with declining rainfall. Annual precipitation dropped in both the U.S. and Australia, while the number of rainless days increased steadily. Not only did streams dry more often, but dry spells lasted longer.

“This may not sound like a lot, but it’s actually a significant increase, especially when added up over the course of 42 years, and can have significant impact on an ecosystem,” noted Dr. Krabbenhoft.

The researchers examined 191 species of xeric fish and found U.S. streams now average two fewer species than in 1980. Smaller species tied closely to plants and algae were most vulnerable, particularly those with limited ranges.

Threats to fish beyond climate

Surprisingly, species decline was not directly linked to changes in streamflow, despite clear evidence of hotter and drier conditions.

Instead, the study points toward multiple overlapping stressors. Habitat loss, invasive species, water diversion, and pollution add layers of pressure that weaken fish resilience.

“This doesn’t mean that climate change isn’t an important piece of the puzzle, but it does mean it’s not the only piece of the puzzle,” said Dr. Krabbenhoft.

The researchers found that nearly all imperiled xeric fishes face more than one threat.

Habitat modification was the leading driver, but predation and competition from non-native species were also common. Endemic fish with small ranges were especially at risk.

Fish survival in drylands

The study explored which traits made species more vulnerable. Fish of conservation concern tended to be smaller-bodied and occupied lower trophic levels. Such traits limit their ability to disperse, compete, or withstand environmental fluctuations.

Many xeric fishes have evolved to tolerate intense heat and hydrologic extremes, but those same adaptations can become disadvantages when stressors compound.

For example, reliance on specific spawning cues linked to rainfall may break down as precipitation patterns shift. Endemic species, often confined to a single basin, have little room to retreat when habitats degrade.

Climate change shifts fish

The researchers warn that prolonged aridity could accelerate biotic homogenization. As native species decline, invasive generalists expand, making fish communities across regions more alike. This erodes unique biodiversity and destabilizes food webs.

Warming waters could also give invasive species an advantage. Non-native piscivores are often more adaptable and may prey on smaller native fishes already weakened by habitat loss and low flows.

Such dynamics suggest that climate change will not act alone but will interact with human pressures in complex ways.

A fragile future

The story of xeric fish highlights an urgent warning. If species already adapted to extreme environments are struggling, those in milder regions may soon follow.

Protecting streams from further harm requires not just confronting climate change but also reducing local pressures that weaken resilience.

What happens in these drylands today could foreshadow the fate of freshwater ecosystems worldwide. Continued monitoring, conservation of refuges, and careful management of human impacts may decide whether these unique fish persist in a hotter, drier future.

The study is published in the journal Ecology and Evolution.

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