October 30, 2012
Mass Extinction In The Cretaceous Period Was Worsened By Ecosystems
April Flowers for redOrbit.com - Your Universe Online
A mass extinction, wiping out numerous species including the dinosaurs, marked the end of the Cretaceous Period. A new study, published in Proceedings of the National Academy of Sciences (PNAS), reveals that the structure of North American ecosystems made the extinction worse than it might have been.
Mexico's Yucatan Peninsula is home to the now-buried Chicxulub impact crater, caused by a mountain-sized asteroid. This impact is almost certainly the ultimate cause of the Cretaceous mass extinction, which occurred 65 million years ago.
Jonathan Mitchell, Ph.D. student at the University of Chicago's Committee on Evolutionary Biology, said, "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."
The research team, which included Peter Roopnarine of the California Academy of Sciences and Kenneth Angielczyk of the Field Museum, reconstructed the terrestrial food webs for 17 Cretaceous ecological communities, seven of which existed within two million years of the Chicxulub impact. The ten remaining food webs came from the preceding 13 million years.
The analysis comes from a computer model, developed by Roopnarine, showing how disturbances spread through the food web. The simulation's purpose was to predict how many animal species would become extinct from a plant die-off since that was 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 study's results paint a picture of the late Cretaceous period in North America in which pre-extinction changes to food webs were likely driven by a combination of environmental and biological factors. These changes 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 model describes all plausible diets for the animals in the study. For example, one time a Tyrannosaurus rex might only eat Triceratops, a second run might have the T. rex eating only duck-billed dinosaurs, while in a third it might eat a more varied diet. This variability stems from the uncertainty around what exactly Cretaceous animals ate. This uncertainty, however, actually worked to the study's benefit.
"Using modern food webs as guides, what we have discovered is that this uncertainty is far less important to understanding ecosystem functioning than is our general knowledge of the diets and the number of different species that would have had a particular diet," Angielczyk said.
Modern food web data helped the simulations account for phenomena such as how specialized animals tend to be, or how body size relates to population size and their probability of extinction.
A large number of specific food webs from all the possible food webs in their general framework were selected. The team then evaluated how this sample of webs respond to a perturbation like the death of plants. The same relationships and assumptions were used to create food webs across all of the different sites. This means that the differences between sites stems only from differences in the data rather than from the simulation itself, making the simulation a fundamentally comparative method.
"We aren't trying to say that a given ecosystem was fragile, but instead that a given ecosystem was more or less fragile than another," Roopnarine said.
If the asteroid hit during the 13 million years preceding the latest Cretaceous communities, according to the model there would almost certainly still have been a mass extinction, but one that would have been less severe than what actually happened in North America. The team concluded that it was most likely a combination of changing climate and other environmental factors that caused some types of animals to become more or less diverse in the Cretaceous. They suggest that a shallow sea in North America drying up may have been one of the main factors leading to the observed changes in diversity. However, the study provides no evidence that the latest Cretaceous communities were on the verge of collapse before the asteroid hit.
"The ecosystems collapsed because of the asteroid impact, and nothing in our study suggests that they would not have otherwise continued on successfully," Mitchell said. "Unusual circumstances, such as the after-effects of the asteroid impact, were needed for the vulnerability of the communities to become important."
The findings of this study have implications for current day conservation efforts.
"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."
Image 2 (below): This illustration depicts the food web for ecological groups in the late Cretaceous Period as reported in a new paper published in the Proceedings of the National Academy of Sciences. Each ecological group includes a set of species that share the same set of potential predators and prey. Silhouettes show iconic members of each group. Arrows show who eats whom. Credit: Courtesy of Jonathan Mitchell, Peter Roopnarine and Kenneth Angielczyk