物种大灭绝对现代世界的启示

The Cretaceous Period（白垩纪） of Earth history ended with a mass extinction¹ that wiped out numerous species, most famously the dinosaurs². A new study now finds that the structure of North American ecosystems³ made the extinction worse than it might have been. Researchers at the University of Chicago, the California Academy of Sciences and the Field Museum of Natural History will publish their findings Oct. 29 online in the Proceedings⁴ of the National Academy of Sciences. The mountain-sized asteroid⁵（小行星） that left the now-buried Chicxulub impact crater⁶ on the coast of Mexico's Yucatan Peninsula is almost certainly the ultimate cause of the end-Cretaceous mass extinction, which occurred 65 million years ago. Nevertheless, "Our study suggests that the severity of the mass extinction in North America was greater because of the ecological⁷ structure of communities at the time," noted⁸ lead author Jonathan Mitchell, a Ph.D. student of UChicago's Committee on Evolutionary⁹ Biology.   Mitchell and his co-authors, 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 are based on a computer model showing how disturbances¹⁰ 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. "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."   The results of Mitchell and his colleagues 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 -- results in communities that were more fragile when faced with large disturbances.   "Besides shedding light on this ancient extinction, our findings imply that seemingly innocuous（无害的） changes to ecosystems caused by humans might reduce the ecosystems' abilities to withstand unexpected disturbances," Roopnarine said.   The team's computer model describes all plausible¹¹ diets for the animals under study. In one run, Tyrannosaurus might eat only Triceratops, while in another it eats only duck-billed dinosaurs, and in a third it might eat a more varied#p#分页标题#e#¹² diet. This stems from the uncertainty¹³ regarding exactly what Cretaceous animals ate, but this uncertainty actually worked to the study's benefit.