Chaos Is More Common in Populations Than Previously Thought

It’s possible that methodological and data restrictions, rather than ecosystems’ fundamental stability, are to blame for the notion that chaos is uncommon in natural populations.

Researchers from the University of California, Santa Cruz (UCSC) and National Oceanic and Atmospheric Administration (NOAA) Fisheries have conducted new research that suggests chaos in natural populations may occur far more often than previously thought.

Ecologists often debate whether population variations in natural ecosystems are regular (varying around a supposedly “stable” equilibrium), random (totally unpredictable), or chaotic. Like the weather, chaotic systems may be predicted in the short term but not in the long run, and they are very sensitive to minute changes in the initial conditions.

“Knowing whether these fluctuations are regular, chaotic, or random has major implications for how well, and how far into the future, we can predict population sizes and how they will respond to management interventions,” said Tanya Rogers, a NOAA Fisheries ecologist and research fellow at UCSC’s Institute of Marine Sciences.

The new study, which was just published in the journal Nature Ecology & Evolution, is led by Rogers. Her co-authors are Stephan Munch, an adjunct professor in the Departments of Applied Mathematics and Ecology and Evolutionary Biology at UCSC as well as an NOAA Fisheries ecologist, and Bethany Johnson, a doctoral student in applied mathematics at UCSC.

Over 30% of the populations they examined in an ecological database showed signs of chaotic dynamics, according to the researchers. Chaos was found to be either nonexistent or infrequent in natural field populations in previous meta-analyses that looked at its occurrence. However, the authors speculated that rather than being a result of ecosystems being inherently stable, this may have been the result of limited data and the use of inadequate methodology.

“There’s a lot more data now, and how long a time series you have makes a big difference for detecting chaotic dynamics,” Munch said. “We also showed that methodological assumptions made in prior meta-analyses were biased against detecting chaos.”

Prevalence of Chaos Among Species

For the new study, the researchers used new and updated chaos detection algorithms and put them through rigorous testing on simulated data sets. Then they applied the three best methods to a dataset of 172 population time series from the Global Population Dynamics Database.

Their analysis revealed interesting associations between chaotic dynamics, lifespan, and body size. Chaos was most prevalent among plankton and insects, least prevalent among birds and mammals, and intermediate among fishes.

“A lot of short-lived species tend to have chaotic population dynamics, and these are also species that tend to have boom-and-bust dynamics,” Rogers said.

Implications for Ecological Forecasting and Management

The results suggest there may be intrinsic limits to ecological forecasting and caution against the use of equilibrium-based approaches to conservation and management, particularly for short-lived species.

“From the fisheries management perspective, we want to predict fish populations so we can set limits for fishery harvests,” Rogers explained. “If we don’t recognize the existence of chaos, we could be losing out on short-term forecasting possibilities using methods appropriate for chaotic systems, while being overconfident about our ability to make long-term predictions.”

Reference: “Chaos is not rare in natural ecosystems” by Tanya L. Rogers, Bethany J. Johnson, and Stephan B. Munch, 27 June 2022, Nature Ecology & Evolution.
DOI: 10.1038/s41559-022-01787-y

The study was funded by the NOAA Office of Science and Technology, NOAA Sea Grant, and the Lenfest Oceans Program.

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