The mountain-sized asteroid that left the now-buried Chicxulub impact crater on the coast of Mexico's Yucatan Peninsula is almost certainly the ultimate cause of the end-Cretaceous mass extinction.
However, a new study by researchers at the University of Chicago, the California Academy of Sciences and the Field Museum of Natural History suggests the severity of the mass extinction in North America was greater because of the ecological structure of communities at the time.
Lead author Jonathan Mitchell and his colleagues, Peter Roopnarine of the California Academy of Sciences and Kenneth Angielczyk of the Field Museum, reconstructed terrestrial food webs for 17 Cretaceous ecological communities.
Seven of these food webs existed within two million years of the Chicxulub impact and 10 came from the preceding 13 million years.
The findings were based on a computer model showing how disturbances spread through the food web. Roopnarine developed the simulation to predict how many animal species would become extinct from a plant die-off, a likely consequence of the impact.
"Our analyses show that more species became extinct for a given plant die-off in the youngest communities," Mitchell said.
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"We can trace this difference in response to changes in a number of key ecological groups such as plant-eating dinosaurs like Triceratops and small mammals," Mitchell said in a statement.
The results paint a picture of late Cretaceous North America in which pre-extinction changes to food webs - likely driven by a combination of environmental and biological factors - resulted in communities that were more fragile when faced with large disturbances.
"Besides shedding light on this ancient extinction, our findings imply that seemingly innocuous changes to ecosystems caused by humans might reduce the ecosystems' abilities to withstand unexpected disturbances," Roopnarine said.
The computer models showed that if the asteroid hit during the 13 million years preceding the latest Cretaceous communities, there almost certainly would still have been a mass extinction, but one that likely would have been less severe in North America.
In their paper they suggest that the drying up of a shallow sea that covered part of North America may have been one of the main factors leading to the observed changes in diversity.
The study provides no evidence that the latest Cretaceous communities were on the verge of collapse before the asteroid hit.
The study was published in the journal Proceedings of the National Academy of Sciences.