Passy lab publishes study on threat posed to U.S. stream ecosystems by climate change

Friday, May 07, 2021 • Greg Pederson :

Biologists at The University of Texas at Arlington have published a study showing that if humans continue with business as usual regarding actions that cause climate change, many economically important species could be driven to the brink of extinction within 50 years.

The study was conducted by researchers in the lab of Sophia Passy, UTA associate professor of biology. Katrina Pound, postdoctoral researcher, was lead author and was joined by co-authors Chad Larson, a stream ecologist with the Environmental Assessment Program at Washington State Department of Ecology in Lacey, Washington, and Passy. Pound and Larson earned doctoral degrees from UTA with Passy as their faculty advisor.

The article, titled “Current distributions and future climate‐driven changes in diatoms, insects and fish in U.S. streams,” was published in the January edition of the journal Global Ecology and Biogeography. Its message is clear: The future of stream ecosystems depends on efforts to address climate change today.

The study assesses the potential impacts of climate change on the diversity and distribution of 336 freshwater species across 1,227 streams in the continental United States. The species include fish, aquatic insects, and diatoms, which are single-celled algae. The team developed species distribution models for using water chemistry, watershed, and climatic variables.

Four climate change scenarios were examined: a mitigated scenario where carbon dioxide emissions decline to zero; two stabilizing scenarios where emissions peak and then decline; and a worst-case scenario where emissions continue to increase through the end of the century, referred to as “business as usual.”

“Our results indicate a widespread decrease in cold-water taxa, projected to decline most drastically in localities where they are most abundant today. Given that such localities serve as important sources for colonists, the loss of cold-water species there is likely to have a negative impact at a regional scale,” Passy said.

“Fish, including several varieties of trout, were the most impacted, with projections showing a loss of more than 80 percent of their current distribution under the business as usual scenario,” Pound said.

Added Larson, “We observed an increase in a number of warm-water and environmentally tolerant taxa. On average, warm-water taxa were projected to nearly double their distribution in the most extreme climate change scenario, in other words, if we continue business as usual.”

This is highly problematic because many of these species have characteristics of environmental concern, such as invasiveness, hybridization, and threat to water quality, the team noted. Several minnow species, including red shiners, blacktail shiners, bullhead minnows, and blackspotted topminnows, are expected to expand their range.

“Minnows are common bait bucket releases and hold great potential for introduction throughout their full forecasted range,” Pound said. “These seemingly harmless feeder fish are threats because they often hybridize with closely related minnows, diluting the gene pool of native species.”

Other species of concern include predacious longnose gar and flathead catfish, and high fecundity fish such as gizzard shad which dominate fish assemblages and outgrow predators.

The study’s results also suggest diminished distributions of functionally important taxa within diatom and insect communities.

“Results for diatoms suggest a decline in both understory and overstory taxa, which could result in lower algal biomass, affecting stream food chains,” Pound said. “In addition, all aquatic insects classified as environmentally sensitive were cold-water. These insects are indicators of healthy streams but may be eliminated by climate change.”

Algae and insects are important food sources for fish, so dramatic restructuring of their communities could diminish overall stream biomass.

Widespread increase of warm-water taxa and eradication of cold-water taxa may also lead to biotic homogenization, a phenomenon in which biological communities become more similar to each other. Biotic homogenization reduces the integrity of ecosystems making them more vulnerable to both natural and human disturbance.

Given that the magnitude of these biotic shifts depends on the severity of climate change, appropriate current policy decisions are necessary to preserve freshwater ecosystems, the researchers wrote.

The research was funded by a grant from the National Science Foundation’s Division of Environmental Biology.