News Release| The American Naturalist; Cal State L.A.

November 8, 2011

Dynamic predator-prey equilibrium of mussel beds

Cal State L.A.’s biological study featured in The American Naturalist


Picture of mussel bed boundary change.
Pictured: A side-by-side image of a portion of a mussel bed before and one year after the experimental addition of sea stars in one of the CSULA research sites in British Columbia. The red arrows and orange cones show how the lower boundary of the mussel bed has receded up the shore as a result of predator foraging as predicted by the mathematical model. In the image on the right, large numbers of sea stars can be seen attacking the mussels.


Los Angeles, CA -- “Lower mussel bed boundary is not an absolute refuge, but a dynamic equilibrium between sea star predation and mussel growth,” according to a recent Cal State L.A. study published in The American Naturalist November 2011 issue.

The American Naturalist ( is the oldest scientific journal in the world dedicated to the study of ecology, evolution, and behavior.

The article, entitled “Mussel Bed Boundaries as Dynamic Equilibria: Thresholds, Phase Shifts, and Alternative States,” features work conducted by Cal State L.A.’s professors of biological sciences, Robert Desharnais and Carlos Robles; postdoctoral associate and lead author Megan Donahue; and graduate student Patricia Arriola.

Funded by the National Science Foundation, the research provides ecologists vital information for the understanding of how physical disturbances shape the structure of populations and communities in near-shore marine ecosystems.

Using a mathematical model, CSULA’s scientists predict that the addition of predatory sea stars (Pisaster ochraceus) would raise the lower boundary of mussel beds, contrary to prior claims that mussel beds survive in a refuge from predation. The phenomenon was demonstrated in a related field study by Robles and his students on the rocky shores of British Columbia, where adding sea stars caused the mussel bed to retract up shore, thus concluding that “there’s no absolute refuge from hungry predators.”

Desharnais said, “The research alters long held beliefs regarding the ecological mechanisms that cause species zonation in the marine intertidal zone.”

For the research abstract, go to

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