Chaos in natural populations appears to be much more common than previously believed, according to a new analysis by scientists at UC Santa Cruz and NOAA Fisheries.
Populations of organisms in natural ecosystems fluctuate widely, and an important question for ecologists is whether those fluctuations are regular (varying around a theoretical “stable” equilibrium), random (completely unpredictable), or chaotic. Chaotic systems, such as weather, can be predictable in the short term, but not in the long term, and they are very sensitive to small differences in 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 the UCSC’s Institute of Marine Sciences.
Rogers is the lead author of the new study, published June 27 in Nature Ecology & Evolution. Her co-authors are Bethany Johnson, a UCSC graduate student of applied mathematics, and Stephan Munch, a NOAA fishing ecologist and adjunct professor at UCSC in the Departments of Applied Mathematics and Ecology and Evolutionary Biology.
The researchers found evidence of chaotic dynamics in more than 30 percent of the populations they analyzed in an ecological database. Previous meta-analyses assessing the prevalence of chaos in natural field populations had shown that chaos was absent or rare. But that may be due to limited amounts of data and the use of inadequate methods, rather than the inherent stability of ecosystems, the authors said.
“There’s a lot more data now, and how long you have a time series makes a big difference in detecting chaotic dynamics,” Munch said. “We also showed that methodological assumptions in previous meta-analyses were biased against detecting chaos.”
For the new study, the researchers used new and updated chaos detection algorithms and had them run rigorous tests on simulated data sets. They then 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, longevity and body size. Chaos was most common among plankton and insects, least among birds and mammals, and intermediate among fish.
“Many short-lived species tend to have chaotic population dynamics, and these are also species with boom-and-bust dynamics,” Rogers said.
The results suggest that there may be intrinsic limits to ecological predictions and caution should be exercised against using equilibrium-based conservation and management approaches, particularly for short-lived species.
“From a fisheries management standpoint, we want to predict fish populations so we can set limits on fishery harvests,” Rogers explains. “If we don’t recognize the existence of chaos, we could miss out on short-term prediction capabilities using methods appropriate for chaotic systems, while over-confident in our ability to make long-term predictions.”
This work was supported by the NOAA Office of Science and Technology, NOAA Sea Grant and the Lenfest Oceans Program.
- Tanya L. Rogers, Bethany J. Johnson, Stephan B. Munch. Chaos is not rare in natural ecosystems. Natural Ecology & Evolution, 2022; DOI: 10.1038/s41559-022-01787-j
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