El Nino frequency drives tipping point in coastal ecological communities

When strong El Niño events become more frequent – occurring 5 or more times per century – eastern Pacific coastal ecosystems undergo dramatic faunal turnover, according to a new study. The findings, which were gleaned from a 12,000-year record of bird and fish remains excavated from the Escorpiones bone deposit site in northwest Baja California, provide new insights into how the El Niño/Southern Oscillation (ENSO) shapes coastal ecological communities and reveals a critical tipping point that could have important implications for understanding changes to the region’s future ecosystems. ENSO is a major source of global climate variability. Its El Niño phase – characterized by a warming of eastern Pacific sea surface temperatures and intense winter storm activity and heavy precipitation along the America’s Pacific coast – is known to have profound effects on ecosystem dynamics. Warm water during these events prevents the usual upwelling of nutrient-rich cooler water along the coast, causing declines in phytoplankton and zooplankton, which has tremendous downstream effects on the population and distribution of many fish, seabird and marine mammal species. However, due to a general lack of long-term records, little is known about how ENSO variation influences coastal faunal community composition over centennial or millennial timescales. Since El Niño events are expected to become more frequent under climate change, understanding this relationship is crucial to forecasting long-term ecological changes in the future. Using a 12,000-year-long record of animal bones and artifacts recovered from the Escorpiones site and a high-resolution geological record of ENSO variability from Lake Pallacocha in Ecuador, Jack Broughton and colleagues evaluated the impact of El Niño variability on coastal biotic communities. Broughton et al. found that when El Niño was infrequent, particularly as it was from 5,000 to 7,000 years ago, coastal fauna (fish and birds) was highly variable and ancient human activity was high. However, when El Niño events became more frequent, specifically more than 5 per century, fish diversity decreased while bird diversity increased and human activity declined, indicating an ecological tipping point between communities. “El Niño is sometimes called ‘the naughty child’ because of the climate-driven disasters it often brings,” write Daniel Sandweiss and Kirk Maasch in a related Perspective. “If the past is the key to the future, studies such as that of Broughton et al. offer tools for better predicting what this naughty child may do in the coming centuries.”