Journal Club

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Journal Club: Protozoan predators help pinpoint how evolution and ecology shape predator-prey dynamics

By studying protozoan predators and bacterial prey with varying genetic diversity, researchers shed light on evolution’s role in predator-prey dynamics. Image credit: M. Jalasvuori

By studying protozoan predators and bacterial prey with varying genetic diversity, researchers shed light on evolution’s role in predator-prey dynamics. Image credit: M. Jalasvuori

A hungry lynx bounds after a scampering hare. Occasionally the lynx secures its catch. Often, it doesn’t. For ecology students, this back and forth battle is the textbook example of an ecological process driving predator–prey population dynamics. The predator population size tracks prey population size, growing when food is abundant and shrinking when it’s scarce. Ecological mechanisms, like predation, cause predator and prey populations to fluctuate over time like two oscillating waves.

But the reality is more complicated. New findings published in Proceedings of the Royal Society B add to a growing body of research suggesting ecology isn’t the whole story. The ability of predator and prey to adapt may also shape the dance of their population cycles over time—and ultimately, which populations stick around, and which die out.

“One of our ideas here was to show people that when they think about diversity and coexistence, that they shouldn’t only think about ecology,” says coauthor and evolutionary ecologist Teppo Hiltunen of the University of Helsinki and University of Turku, Finland. “Evolution is also important.”

To explore these interactions in a controlled setting, Hiltunen’s team turned to microorganisms. The team created hundreds of tiny worlds: glass vial communities filled with Escherichia coli and Pseudomonas fluorescens, bacteria living together in the presence or absence of an oblong protozoan predator. One strain of E. coli wriggled in all the vials, but the genetic diversity of Pseudomonas, and of the predator, varied in different treatments.

In the simplest of the communities studied, one strain of E. coli and one strain of Pseudomonas vied for resources in the absence of any predator. E. coli was the better competitor, and Pseudomonas dropped to near extinction in just a few days. In other vials, one strain of E. coli competed with many genetically diverse strains of Pseudomonas. Now the formerly poor competitor had the genetic diversity to adapt faster. After 15 days, Pseudomonas composed more than half of the bacterial community.

Still other vials held E. coli, Pseudomonas, and a protozoan predator. Predation kept both bacterial species in check. Neither took over or went extinct, regardless of Pseudomonas’ genetic diversity. “As you can nicely see, the predator helps the coexistence of the two,” says community ecologist Ursula Gaedke of the University of Potsdam, Germany, who did not work on the new study, but has previously collaborated with the authors. “Without the predator, and adaptation,” she says, “E.coli wins over Pseudomonas and that’s it.”­­

Finally, in a fourth category of experimental treatment, the protozoan predator coevolved with both bacterial prey for 20 months before the 15-day trial, meaning it had high genetic diversity. Then in the experiment, Pseudomonas easily outcompeted E. coli and drove it extinct. The researchers still don’t know exactly why, but speculate that coevolution influenced the outcome, perhaps because “Pseudomonas benefits from the presence of the predator in general,” Hiltunen says. “Coevolved predators are more efficient and in this case   Pseudomonas benefits even more.”

Taken together, the results show that adaptation and coevolution of both predator and prey influence which species can coexist in a community. Rather than competing, ecological and evolutionary processes likely influence one another, says study coauthor Lutz Becks, an evolutionary ecologist at the University of Konstanz, Germany.

The implication is that, even for longer living species such as mammals, ecological mechanisms may not be the only factors driving their oscillating population cycles. Evolutionary considerations, including genetic diversity, could factor into species models of coexistence, even on relatively short timescales. Previous research had already suggested that classic ecology-centric theories are insufficient to explain population dynamics in nature. “In searching for the reasons why,” Gaedke explains, “it was discovered that the prey and the predator can adjust or adapt to each other.” Now, she adds, the researchers recognize that “evolutionary processes and ecological processes are really interacting.” The latest study offers a more realistic approximation of real-world communities, she says, and considers variable levels of adaptation in both predator and prey.

Ecologist Simon Hart, a postdoctoral researcher at the Swiss Federal Institute of Technology in Zurich, views the paper “as one of the early salvos in the next generation of eco-evolutionary studies” that examine interactions between ecology and evolution happening at the same time. “We still have quite some way to go before we truly understand how the effects of eco-evolutionary processes on community dynamics are mediated,” he says.

Hiltunen hopes to make future experiments more realistic by including external stressors, such as antibiotics, that could strike real-world bacterial populations. Such work could have a practical end: Better approximations of natural communities could, in principle, inform wildlife population management and predictions of food web responses to global change.

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