A study from the Missouri Ozarks highlights the importance of spatial aspects of biodiversity for the healthy functioning of naturally occurring forests.
Tree beta diversity, a measure of site-to-site variation in the composition of species present within a particular region, is more important for ecosystem functioning than other components of biodiversity at broader scales, according to biologists at Washington University in St. Louis.
The study also found that when the spatial scale (the size of a region) increases, the association between beta diversity and tree biomass grows, which has implications for conservation planning. Ecology was the journal that published the research.
As part of her honors thesis in biology, Jacqueline Reu, who graduated from Washington University in 2019 with a double major in environmental biology and physics in Arts & Sciences, led the investigation.
Christopher P. Catano, a Washington University PhD graduate who is currently a postdoctoral research associate at Michigan State University, and Jonathan A. Myers, an associate professor of biology at Washington University’s Arts & Sciences department, were Reu’s mentors.
The data for Reu’s thesis was obtained at Tyson Research Center, Washington University’s environmental field station, as part of a large-scale forest ecology project directed by Myers’ research team. For the experiment, over 60 undergraduate students, high school students, and research technicians surveyed over 30,000 trees.
“A lot of studies have focused only on small scales when they look at biodiversity and ecosystem functioning,” said Reu, first author of the study. “Our study is one of the first that looks at multiple different measures of biodiversity, as well as direct and indirect effects of the environment, on ecosystem functioning as you increase scale in a natural system.”
“Our results back the theory that beta diversity, or the variation in species composition across space, is the best biodiversity measure at larger scales,” she said. “It’s stronger than the other diversity measures that we considered, like local and regional diversity. And its importance increases as you increase spatial scale.”
It’s not just the gain or the loss and the number of species that matters, it’s also changes in the distribution of these species and where you might find them in an ecosystem. This study and a few others recently have been pretty consistent, in the sense that they’re showing it’s (species) turnover, or variation across space, that seems to be really critical.
Christopher P. Catano
Studying landscapes in the Ozarks
Researchers identified 14 oak-hickory forest landscapes for this study, each of which contained at least three major habitat types common in Ozarks forests, including west or south-facing slopes, which are sunnier, drier, and nutrient poor; valleys and bottom lands, which are often shady, nutrient-rich, and have small streams running through them; and east and north-facing slopes, which are the most productive in terms of tree cover.
Each landscape had a nutrient and moisture availability gradient, as well as light availability and a diversity of topographic circumstances. The Tyson Research Center at Washington University, located 20 miles southwest of the Danforth Campus, offers rich, collaborative field activities for both scientists and students.
The researchers calculated the strength of the relationship between each diversity component and aboveground tree biomass, a property of forests related to ecosystem functioning, by quantifying the direct effects of three different diversity components: beta diversity, local diversity (the average number of species present in a small area), and regional diversity (the total number of species present in the larger landscape).
Finally, they looked at the strength of these correlations on 11 different spatial scales within each landscape, ranging from 20×20m to 120×120m. (For comparison, that’s a difference in size ranging from about the size of an end zone in American football to about 2.6 football fields.)
“It’s been a thorny thing in ecology and most sciences, really to try to identify the scale that we should use to study a system,” said Catano, who co-led the new study. “It leads to a lot of controversy and a lot of confusion.”
Other research has found comparable links between beta diversity and ecosystem functioning, but those previous studies tended to be based on small-scale comparisons.
Understanding how and why the relationship between beta diversity and ecosystem health scales up is a high-stakes investigation, in part because scientists are still grappling with the various ramifications of recent biodiversity loss.
“It’s not just the gain or the loss and the number of species that matters, it’s also changes in the distribution of these species and where you might find them in an ecosystem,” Catano said. “This study and a few others recently have been pretty consistent, in the sense that they’re showing it’s (species) turnover, or variation across space, that seems to be really critical.”
“(Beta diversity) is driving a variety of different functions, as well as the stability of these functions over time, as you scale up from small, local plot-based ecosystems to large, heterogeneous landscapes,” he said.
“This project highlights the utility of large-scale, long-term field research projects,” Myers said. “The study was largely fortuitous, because we had set up the 14 large forest plots across the Tyson landscape for a different project. But the way that we set them up was perfect for addressing questions about how environmental variation and species composition across space contribute to ecosystem functioning.”
“One implication is that if you homogenize the environmental conditions, that might also negatively impact ecosystem functioning,” Myers said. “Habitat loss or homogenizing the amount of nutrients and other limiting resources for organisms in an ecosystem could have cascading effects on ecosystem functioning and services. These are indirect effects that are challenging to predict if you don’t consider both environment and diversity together.”
Montioring ecosystem functioning
Reu, the study’s first author, has worked as an intern at the Smithsonian Marine Station in Fort Pierce, Fla., and at the Smithsonian Environmental Research Center’s Marine Invasions Lab in Tiburon, Calif., since graduating from Washington University. Her work at Tyson Research Center, however, continues to inspire her.
“Forests are my favorite ecosystem to be in,” Reu said. “Without plants, nothing else in the food chain would work at all: it wouldn’t exist. And that’s why I find studying them especially fascinating.”
Reu tramped into the forest with a small group of Tyson research fellows during her undergraduate summers, identifying trees, measuring their diameters, tagging and mapping them as part of a long-term forest monitoring program.
Seeds gathered from seed traps were recognized, and seedlings were cleaned, weighed, and cataloged by Reu. She learned herself a statistical programming language at night to assist her in sorting through the data she was collecting.
“I’ve always liked the math side of things,” Reu said. “For this project, I mostly taught myself to program in R, using YouTube videos.”
“It wasn’t the first computer language I’ve learned, but I think it’s the most thorough one I’ve learned so far,” she said. “Then I made some graphics using GIS as well.”
While preparing to go to graduate school, Reu will be working on a project with rare butterflies in New Hampshire this summer.
“I’ve always had a passion for ecology,” Reu said. “So when I was just looking at different options, Jonathan’s lab really stood out to me. He’s so inventive and they do really thorough work.”
“And I love trees, so that definitely helped,” she said. “The work at Tyson was very interesting, partly because it’s part of the Smithsonian Forest Global Earth Observatory (ForestGEO) network. That just opens a lot of doors.”
