October 30th, 2012 by James Ayre
The period of time in the Earth’s history that modern researchers refer to as the Cretaceous ended with a mass extinction that eliminated very large numbers of species. The dinosaurs were among those that went extinct during this period of time. And now new research has found that the structures of the ecosystems present in North America at the time likely made the extinction far worse than it would have been otherwise.
The massive asteroid (the size of a mountain) that left the Chicxulub impact crater on the coast of Mexico’s Yucatan Peninsula is widely considered to be the fundamental reason for the end-Cretaceous mass extinction 65 million years ago. But it certainly wasn’t the only thing that decided which species went extinct and which survived. “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.
The research was done by reconstructing terrestrial food webs from 17 different Cretaceous ecological communities. Out of these ecosystems, seven of them existed within two million years of the Chicxulub impact and the other 10 of them were from the previous 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,” a University of Chicago news release stated.
“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 create 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,” the University of Chicago adds. Which should call to mind the massive damage that’s been done to ecosystems around the world in the past few hundred years. And the incredibly fast loss of biodiversity that has occurred throughout human-occupied areas during the last 10,000 years.
“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.
“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.
The University of Chicago adds:
“The team’s computer model describes all plausible 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 diet. This stems from the uncertainty regarding exactly what Cretaceous animals ate, but this uncertainty actually worked to the study’s benefit.”
The data that was derived “from modern food webs helped the simulations account for such phenomena as how specialized animals tend to be, or how body size relates to population size and thus their probability of extinction.”
“The researchers also selected for their study a large number of specific food webs from all the specific webs possible in their general framework and evaluated how this sample of webs respond to a perturbation, such as the death of plants. They used the same relationships and assumptions to create food webs across all of the different sites, which means the differences between sites just stem from differences in the data rather than from the simulation itself. This makes the simulation a fundamentally comparative method, Roopnarine noted.”
“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,” he said.
According to the computer models, if the asteroid hit anytime during the 13 million years previous to the last Cretaceous communities, there would have almost certainly still have been a mass extinction, but it would have likely been much less severe in North America.
From the news release: “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 research itself didn’t provide any evidence that the last Cretaceous communities were about to collapse before the asteroid impact. “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 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 new research was just published October 29th in the Proceedings of the National Academy of Sciences.
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