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Department of Biology News

Biologists publish study of biological consequences of stream acidification

UTA postdoctoral researcher Katrina Pound, left, and associate professor of biology Sophia Passy, hold a copy of the journal Diversity and Distributions with their story on the cover.

A National Science Foundation funded study by biologists from The University of Texas at Arlington about beta diversity and its response to environmental stress is featured as the cover article of a prestigious journal of conservation biogeography.

Sophia Passy, associate professor of biology, co-authored the paper, titled “Beta diversity response to stress severity and heterogeneity in sensitive versus tolerant stream diatoms,” which appears in the March issue of Diversity and Distributions. Co-authors were Katrina Pound, a postdoctoral researcher in Passy’s lab, and Gregory Lawrence, of the U.S. Geological Survey’s New York Water Science Center.

The study examined the biological consequences of stream acidification due to acid rain in the Adirondack Mountains of northeastern New York. The region is one of the most acid-impacted areas in the United States, where streams and lakes have unnaturally high acid concentrations.

“While acidification in itself is stressful, it also results in greater solubility of inorganic aluminum in waters, which is a toxin,” said Pound, lead author of the paper. “We examined streams across gradients of acidity and inorganic aluminum.”

Prior research has shown that acidification reduces biodiversity, but the research by Passy and her colleagues is the first to assess the effect of stream acidification on beta diversity of algal functional groups, comparing species that are tolerant of vs. sensitive to acidification.

“Traditionally, it has been considered that high beta diversity is a desirable quality of an ecosystem,” Passy said. “However, we demonstrate that there are circumstances where a beta diversity increase is an early warning of ecosystem degradation, i.e. when acid stress eliminates sensitive species, causing streams to vary in composition. We caution that continued acid stress in such ecosystems would result in regional biodiversity loss, primarily affecting species that are indicators of healthy streams.”

Beta diversity is the variation in species composition among localities. The term was introduced in 1960 by R.H. Whittaker, who defined it as “the extent of change in community composition, or degree of community differentiation, in relation to a complex-gradient of environment, or a pattern of environments”.

“Beta diversity is important to the resilience of ecosystems because if a species goes extinct at a certain locality, its population can be rescued by immigration from other localities,” Pound said.

The severity and heterogeneity, or non-uniformity, of stress are major factors in restricting beta diversity, but their relative influence is poorly understood by ecologists.

“We addressed this question by examining the patterns of beta diversity in stress‐sensitive versus stress‐tolerant stream diatoms and their response to local versus regional factors along gradients of stress severity and heterogeneity,” Passy said.

Beta diversity was measured as multivariate dispersion of communities across high stress, low stress, and high and low stress (heterogeneous) environments, encompassing 200 stream samples. Null models – which are pattern-generating models based on randomization of ecological data – were implemented to assess community similarity relative to randomly assembled communities, and the importance of local assembly processes versus the regional species pool, Pound explained.

The scientists found that beta diversity responded to both severity and heterogeneity of stress, but turnover along these gradients was mostly driven by sensitive species. The overall beta diversity and beta diversity of sensitive species became more constrained by the lack of variety in the regional species pool, as opposed to local assembly mechanisms.

“While heterogeneous stress contributed to beta diversity, severe stress suppressed beta diversity through elimination of sensitive species,” Passy said. “Therefore, an increase in beta diversity in an environmentally‐stressed region may serve as a forewarning for future loss of sensitive species, should the stress continue to intensify.”

Clay Clark, professor and chair of the UTA Department of Biology, said that the team’s research provides important information about the harmful effects of acidification and how different ecosystems handle the stress.

“This work by Dr. Passy and her colleagues is worthy of wide exposure, which is evidenced by its selection as the cover article in the March edition of Diversity and Distributions,” Clark said. “It is also another valuable contribution by our department to the study of global environmental impact, one of the main themes of UTA’s Strategic Plan 2020.”

Passy received an M.S. in Biology from Sofia University in Bulgaria in 1986. She earned a Ph.D. in Ecology from Bowling Green State University in Ohio in 1997 and held postdoctoral positions at the University of Minnesota Medical School and the Rensselaer Polytechnic Institute in New York before coming to UTA in 2001.