Larger marine animals at higher risk of extinction, and humans are to blame

Stanford Earth professor Jon Payne puts modern extinction in context by comparing them with Earth's five previous mass extinctions.

"We've found that extinction threat in the modern oceans is very strongly associated with larger body size," said Jonathan Payne, a paleobiologist at Stanford's School of Earth, Energy & Environmental Sciences. "This is most likely due to people targeting larger species for consumption first."

In a new study, to be published in the Sept. 16 issue of the journal Science, Payne and his colleagues examined the association between extinction threat level and ecological traits such as body size for two major groups of marine animals -- mollusks and vertebrates -- over the past 500 years and compared it with the ancient past, stretching as far back as 445 million years ago and with a particular emphasis on the most recent 66 million years.

"We used the fossil record to show, in a concrete, convincing way, that what is happening in the modern oceans is really different from what has happened in the past," said study co-author Noel Heim, a postdoctoral researcher in Payne's lab. 

Specifically, the authors found that the modern era is unique in the extent to which creatures with larger body sizes are being preferentially targeted for extinction. "What our analysis shows is that for every factor of 10 increase in body mass, the odds of being threatened by extinction go up by a factor of 13 or so," Payne said. "The bigger you are, the more likely you are to be facing extinction."

The selective extinction of large-bodied animals could have serious consequences for the health of marine ecosystems, the scientists say, because they tend to be at the tops of food webs and their movements through the water column and the seafloor help cycle nutrients through the oceans.

Judy Skog, program director in the National Science Foundation's Division of Earth Sciences, which funded the research, said the findings should be incorporated into decisions about how we manage ocean resources like fisheries. "These results show that larger marine animals are poised to disappear from the seas faster than smaller ones," Skog said. "Studies of the fossil record indicate that this trend didn't exist in the past -- it's a new development in today's world."

While Payne and his colleagues did not directly examine why large modern marine animals are at higher risk of extinction, their findings are consistent with a growing body of scientific literature that point to humans as the main culprits. "It is consistent with the tendency for fisheries to first exploit larger species and subsequently move down the food web and target smaller species," said study co-author Matthew Knope, a former postdoc in Payne's lab who is now an assistant professor of biology at the University of Hawai'i at Hilo.

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It's a pattern that scientists have seen before. On land, for example, there is evidence that ancient humans were responsible for the massacre of mammoths and other megafauna across the globe. "We see this over and over again," Heim said. "Humans enter into a new ecosystem, and the largest animals are killed off first. Marine systems have been spared up to now, because until relatively recently, humans were restricted to coastal areas and didn't have the technology to fish in the deep ocean on an industrial scale."

If there is one silver lining in the troubling new findings, it's that there is still time for humans to change their behavior, Payne said. "We can't do much to quickly reverse the trends of ocean warming or ocean acidification, which are both real threats that must be addressed. But we can change treaties related to how we hunt and fish. Fish populations also have the potential to recover much more quickly than climate or ocean chemistry," Payne said. "We can turn this situation around relatively quickly with appropriate management decisions at the national and international level."

Other co-authors on the study, titled "Ecological selectivity of the emerging mass extinction in the oceans," include Andrew Bush of the University of Connecticut and Doug McCauley of the University of California, Santa Barbara.