The researchers attribute the rapid disappearance of these species to an increase in the frequency of strong El Niño events. Evidence in modern times shows that the first species has moved south as a result of recent episodes of El Niño. The second species is known to die more quickly when water temperatures rise.
"Looking at the mollusks, we are pretty certain that El Niño was absent or occurred very infrequently prior to 5,800 years ago, and that it was probably not more frequent than every 40 to 50 years between 5,800 and around 3,200 to 2,800 years ago," said Sandweiss.
Other researchers using lake cores and tree rings in the Galápagos Islands and the highlands of Ecuador have reached similar conclusions.
The temples and village sites in Peru were abandoned at about the same time that El Niño began occurring more frequently.
After building and growth continued for several thousand years, there was a hiatus several hundred years long, said Sandweiss. When settlements next arose, however, they had different characteristics than previous ones.
"When mounds are again constructed, not only are they larger, but they are part of a different political and religious system," he said.
Co-authors of the report include Kirk Maasch of the University of Maine; Richard L. Burger of Yale University; James B. Richardson III and Harold B. Rollins of the University of Pittsburgh; and Amy Clement of the University of Miami.
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