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When species compete, physical structures and ecological relationships matter

January 23, 2019

EUGENE, Ore. - Jan. 23, 2019 - When competing for limited resources, structures in an environment can be the difference between species coexisting or one species eliminating another. Relationships between species are important, too, according to new research.

Scientists have suspected that there is a deep relationship between biodiversity and physical structure of the environment, but nailing down that relationship has been elusive.

Now, two University of Oregon researchers have revealed part of that relationship by crunching mathematically rich formulas in thousands of supercomputer simulations across multiple scenarios. They focused on the influences of physical structures, such as packed particles in soil and epithelial cells in the mammalian gut, on the survival of organisms living in those environments.

Their findings in a paper published in the Jan. 8 issue of the Proceedings of the National Academy of Sciences.

The accomplishment puts on firm footing physical conditions that contribute to the wide array of biodiversity seen in nature and shows a possible route around competitive exclusion. Down the road, these findings could help in the development of devices, including medical implants, said co-author Tristan Ursell, a professor of physics and member of the UO's Institute of Molecular Biology and Materials Science Institute.

Competitive exclusion argues that two species battling for the same resources cannot stably coexist in an ecosystem unless they adopt different characteristics or strategies to reduce competition. The concept was coined in 1932 by Soviet biologist Georgii Gause and is often called Gause's law.

"This research addresses a longstanding question," Ursell said. "If on the one hand competitive exclusion pushes a system to have a single dominant species, why do some environments have thousands of species co-existing in a limited environment? We offer a possible explanation that says it is the structure of the environment that allows that to be true."

The findings and ongoing research could lead to new design principles, he said.

"Let's say you want to build a device that houses groups of different microbial species together. Our work suggests that one way to do that is to design variations in the physical structure that stabilize coexistence of those species," Ursell said. "For instance, you might want evenly matched competitors. Structural modifications of the environment can help you do that. With such a device, you could push the ecology of the system in a desired direction."

One possibility might be an implant that is structurally designed to interact with a person's gut microbiome and ensure a healthy and stable balance of microbes, said Ursell, who is a member of the UO's META Center for Systems Biology, where scientists are seeking to understand host-microbe systems and their role in human health.

The computer modeling was based on so-called Lotka-Volterra simulations, long used in ecological research for studying predator-prey relationships. In this case, however, Ursell and co-author Nick Lowery, a postdoctoral researcher in the Institute of Molecular Biology, ran thousands of simulations modifying the structure of the physical environment in each simulation and then measured ecosystem stability through time.

That let them explore relationships between population dynamics and spatial structures that occur in natural environments, like soils or the mammalian gut. In both two- and three-species systems, structures in the environment altered the competitive dynamics, but in different ways. First, they looked at two competing species, and found that structure localized interactions so that coexistence was stable over time.

That type of change in the competitive landscape, he said, is not unlike how military forts in olden times effectively altered the interface of a conflict and allowed two competing groups to coexist, as long as a fort remained standing.

"There is an interplay between the competitive strength of the species and the physical structures in a given environment," Ursell said. "When competition between species is unbalanced, the structure matters a lot for providing stability. Structure matters, and that has not been characterized."

In another scenario, they looked at three species competing by the rules of the children's game rock-paper-scissors. There, structure had a destabilizing effect, leading to a single dominant species. Consider, he said, a scenario where species A can attack and kill species B, but species B cannot attack A. Species B can kill species C, which, in turn, can attack and kill species A. In the absence of spatial structure, a cycle ensues that allows for survival of all species.

However, altering that system by introducing spatial structure, he said, surprisingly disrupted that cyclic pattern and destabilized the entire system.

"Together, these findings strongly suggest that the physical structure of the environment can interact significantly with the specific nature of interspecies interactions within resident communities to affect stability and dynamics, and more generally indicate that physical attributes of the environment must be considered when assessing the stability of resident communities," Ursell and Lowery concluded in their paper.

The research, they added, helps to unravel the varied forces that act on microbial communities in complex environments.
-end-
The National Institutes of Health, through the National Institute of General Medical Sciences, supported the research. A portion of the project, done at the Aspen Center for Physics, was supported by the National Science Foundation.

Media Contact: Jim Barlow, director of science and research communications, 541-346-3481, jebarlow@uoregon.edu

Source: Tristan Ursell, assistant professor, Department of Physics, 541-346-5231, tsu@uoregon.edu

Note: The UO is equipped with an on-campus television studio with a point-of-origin Vyvx connection, which provides broadcast-quality video to networks worldwide via fiber optic network. There also is video access to satellite uplink and audio access to an ISDN codec for broadcast-quality radio interviews.

Links:

Ursell faculty page: https://physicalbiology.tumblr.com

Department of Physics: https://physics.uoregon.edu/

META (Microbial Ecology and Theory of Animals) Center for Systems Biology: http://meta.uoregon.edu/

Institute of Molecular Biology: http://molbio.uoregon.edu/

Materials Science Institute: https://materialscience.uoregon.edu/

University of Oregon

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