Washington: A new study has revealed that the structure of North American ecosystems made the mass extinction of Cretaceous Period that wiped out numerous species, most famously the dinosaurs, worse than it might have been.
Researchers at the University of Chicago, the California Academy of Sciences and the Field Museum of Natural History were behind the study.
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, 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 ecological structure of communities at the time,” noted lead author Jonathan Mitchell, a Ph.D. student of UChicago’s Committee on Evolutionary 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 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. 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.
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 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.
Most likely a combination of changing climate and other environmental factors caused some types of animals to become more or less diverse in the Cretaceous, the researchers concluded.
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 has implications for modern conservation efforts, Angielczyk observed.
“Our study shows that the robustness or fragility of an ecosystem under duress depends very much on both the number of species present, as well as the types of species,” he said, referring to their ecological function.
The study also shows that more is not necessarily better, because simply having many species does not insure against ecosystem collapse.
“What you have is also important,” Angelczyk said. “It is therefore critical that conservation efforts pay attention to ecosystem functioning and the roles of species in their communities as we continue to degrade our modern ecosystems.”
The researchers are set to publish their findings online in the Proceedings of the National Academy of Sciences.