El Niño/Southern Oscillation (ENSO) is a global climate phenomenon best known for the El Niño phase characterized by warm sea surface temperatures in the eastern Pacific Ocean, intense winter storms and heavy rainfall. El Niño is impacting plants, animals and people around the world by devastating Pacific fisheries, triggering droughts in some parts of the world and causing massive flooding in others.
Many models predict that climate change will fuel stronger and more frequent El Niño events. However, our knowledge of ENSO and its influence on ecosystems from historical or instrumental records only goes back about 200 years, making it difficult to understand what the long-term future will hold for a dominated world. by El Niño.
A team of researchers led by the University of Utah has created a fine analysis of the impact of El Niño on animal communities over the past 12,000 years. The study was published online in the journal Science September 8, 2022.
In the new study, the authors mined a coastal rock shelter site called Abrigo de los Escorpiones (Escorpiones), one of the largest and best-dated collections of vertebrate bones deposited by humans and raptors. on the Pacific coast of North America. The site was excavated by Ruth Gruhn and Alan Bryan of the University of Alberta between 2000 and 2004. For this study, the authors focused on the fish and bird fauna, identifying species from small fragments of ‘bone. They then compared the faunal assemblages to sediment layers from Lake Pallcacocha in Ecuador, one of the most widely applied continuous records of prehistoric El Niño events.
Their analysis revealed a striking pattern: when five or more major El Niño events occurred per century, marine and terrestrial ecosystems dramatically restructured toward a phase of low marine productivity and high terrestrial productivity. The five-per-century tipping point first occurred about 7,000 years ago and continued for several millennia, resulting in a period of stable but low marine productivity.
If strong El Niño events increase in the future, as predicted by many climate models, this ecological threshold suggests that ENSO will play an increasingly important role in controlling the structure of future terrestrial and marine ecosystems in the eastern Pacific. , and more broadly of the terrestrial biosphere.
“Our limited window of experience with El Niño leads us to believe that more El Niño equates to more variability, more change, more change. But our perceptions are based only on a very narrow window of time at the modern era which, on the grand scheme of things, is a time when El Niño is very rare,” said Jack Broughton, professor of anthropology at the University of Utah and lead author of the study. “We’ve shown that overall, a lot of El Niño equates to a lot of stability. For eastern Pacific coastal communities, that means a stable stretch of unproductive marine ecosystems. And that has huge implications for so many people. different aspects of life on Earth.
In years without El Niño, the eastern Pacific coast teems with life thanks to the cold, nutrient-rich water that rises to the surface and supports the plankton communities that marine life relies on. During El Niño, the sea surface becomes much warmer, causing plankton densities to drop. This trickles down to the higher trophic levels – small fish that eat plankton, larger fish that eat small fish, birds that eat fish, marine mammals that eat birds and fish. In contrast, El Niño brings heavy rainfall to terrestrial ecosystems across the region, producing a productivity boon for terrestrial resources. This is consistent with a 2015 study in which Broughton and collaborators found that rabbit populations were strongly controlled by El Niño variation, based on data from Escorpiones and Lake Pallcacocha.
Along with implications for the future, the findings shed light on important moments in our past, including human migration to the Americas, variable human use of coastal and inland habitats, and the extinction of the flightless duck. Chendytes.
A rock shelter and a lake bed: extraordinary archives
Rain and water from the surrounding highlands have flowed into Lake Pallcacocha over the past 12,000 years. During the large thunderstorms that are the signature of El Niño events, a huge volume of material pours into the lake and settles on the bottom in a thick, light-colored layer. In years with fewer El Niño events, sediments settle in much thinner and darker layers. Previous work has radiocarbon dated each layer and divided them into hundred-year increments. The thick bands corresponded precisely to instrumental and historical records of strong El Niño events over the past ~200 years.
The Escorpiones Rock Shelter, located on the northwest coast of Baja California, Mexico, contains a 26-foot (8-meter) deep deposit of shells, animal bones, and artifacts archaeological records, with 97 radiocarbon dates revealing a history of 12,000 years. chronological history of the deposition of humans and raptors. This study identified 18,623 specimens of marine fish and sea and land birds representing 132 species, painstaking work that has been carried out over the past 14 years. The researchers organized the specimens in 100-year increments to align with the paleo-ENSO record.
Pallcacocha revealed a virtual absence of ENSO between 11,000 and 7,000 years ago, with the frequency of El Niño increasing dramatically between 7,000 and 5,000 years ago. In the Escorpiones record, bird and fish species were extremely variable before the emergence of ENSO. When El Niño entered the high-frequency phase, animal communities restructured strongly and then remained relatively stable. Seabirds, such as common guillemots and shearwaters, and fish species associated with cold waters, such as rockfish and kelp bass, were in high abundance when El Niño events were rare, but declined dramatically. spectacular after the start of the high frequency phase. Additionally, land bird species have exploded during strong El Niño years.
“That magic number is five strong El Niño events per century – we didn’t just look at it. It’s based on statistical tests that show us that entire faunas are changing this way, not just a few isolated species.” , Broughton said.
Human migration, land use and the extinction of flightless ducks
This ecological tipping point has broad implications, including for our understanding of human colonization of the Americas. The results support the “kelp route hypothesis” that early settlers followed healthy kelp forests from northeast Asia, across the Bering Strait and into the Americas. According to Escorpiones records, the eastern Pacific coast had nutrient-rich, El Niño-free kelp forests about 12,000 years ago when humans would have made the journey.
The study also shows that peoples of the past were sensitive to these changes, spending much more time on the coast – at Escorpiones – during those times when El Niño was infrequent and the marine environment was highly productive. They moved away from the site, presumably inland, during El Niño years when terrestrial habitats flourished.
“This dossier provides insight into how past populations have adapted to the specific challenges caused by climate change, in this case migrating to more productive environments when coastal ecosystems have declined,” said Brian Codding, professor of anthropology at U and co-author on the study. Recording the frequency of use of human sites was based on trends in frequencies of recovered artifacts by century.
The results also suggest that El Niño played a role in the extinction of Chendytes, a flightless duck about the size of a goose that is said to have been over-hunted by humans. Escorpiones provided the most detailed account of the duck’s history and suggests that while people had been eating it for at least 6,000 years, it only disappeared around the time when El Niño frequencies were increasing. dramatically and where there were many other changes in the marine ecology, including depressions of other seabirds.
“Escorpiones’ detailed record illustrates the importance of environmental change in driving the extinction of Chendytesthat endured millennia of human predation only to disappear after El Niño events became more frequent,” said Tyler Faith, associate professor of anthropology at U, curator at the Utah Museum of Natural History and co – author of the study.
There are some limitations to the study. The lake only records moderate or strong El Niño events, so the authors could not assess whether weak El Niño events impacted faunas. Further work assessing the patterns between weak and strong El Niño events could clarify this issue. To better understand these patterns, researchers are currently analyzing how mammals such as seals, sea otters, and sea lions were affected by El Niño and whether additional millennial-scale El Niño threshold effects may be present. .
Many other factors influence the marine and terrestrial ecosystems of the study area, including other climate systems and processes. The authors point this out — but it seems that El Niño controls marine/terrestrial ecosystems when it happens often. During phases when ENSO is quiet, other oceanographic or climate systems (such as the Pacific Decadal Oscillation, North American Monsoon) have strong effects.
“El Niño events have the greatest effect on these ecosystems when they occur above the critical threshold of five times per century,” Broughton said. “If strong El Niño events increase in the future, our analysis suggests that land productivity will increase, but eastern Pacific marine ecosystems will be forced to return to a more stable but less productive state.”
Other study co-authors are Joan Coltrain and Isaac Hart of the University of Utah, Kathryn Mohlenhoff of Paleowest, and Ruth Gruhn of the University of Alberta.