Humpback whales and other large animals are most at risk in the ocean's modern mass extinction. | NOAA
The current extinction crisis threatening ocean animals is different from the five major past extinctions, according to a new study: the larger the animal, the greater its risk of extinction.
Because large animals play an important role in the ocean food web, "a threat profile focused on the largest species is particularly concerning from an ecological perspective," said lead author Jonathan Payne, an associate professor in the school of Earth, Energy and Environmental Sciences at Stanford. "It could trigger trophic cascades, causing restructuring of ecosystems and secondary losses of species through networks of interactions among species."
Payne and colleagues note that humans' preference for fishing and hunting larger species is probably producing the unusual, size-biased extinction pattern seen today. Fortunately, the results of their study, published in Science on 14 September, could equip scientists and policymakers to protect the oceans going forward.
"Knowing which traits are currently most strongly associated with threatened status allows policymakers to design management strategies specifically targeted to help conserve species with those traits," Payne said. "Also, the more we know about what general characteristics are associated with extinction threat, the more effectively we can anticipate the likely threat levels of species and genera that have yet to be assessed."
The research was published the same week that heads of state, scientists, business leaders, and others experts travel to Washington, D.C., to attend the third Our Ocean conference, which will be hosted by U.S. Secretary of State John Kerry. As in years past, Our Ocean 2016 will tackle key issues including marine protected areas, sustainable fisheries, marine pollution, and climate-related impacts on the ocean.
Scientists contend Earth is in the midst of a sixth mass extinction, one that could have characteristics as devastating as two of the major mass extinction events of the past 550 million years — the end-Permian extinction, which wiped out more than 80% of all genera 252 million years ago, and the end-Cretaceous extinction, which eliminated all non-avian dinosaurs 66 million years in the past.
While researchers have derived a number of calculations about the intensity of this crisis — speculating it is likely to surpass earlier major extinction events in terms of both the number of extinctions and the rate at which species will disappear — major questions remain. Among them is which species are likely to go extinct? Will biodiversity be affected evenly across the board, or will some species with particular traits be more likely to vanish?
To better understand the emerging biodiversity crisis in the modern oceans, Payne and his team turned to the fossil record as an archive of past planetary experiments in extinction.
A blue whale surfaces to breathe. Whales and other large marine animals have been targeted by human hunting. | NOAA
The researchers looked at past associations between the threat of extinction and the ecological traits like species habitat zone, and examined the same associations in modern marine animals. The analysis included 2,497 extinct and modern marine animal species, a broader range than in previous studies, Payne said.
Payne and his colleagues found that modern ocean species with larger body sizes are significantly more likely to face the threat of extinction. The opposite was true in the past — either smaller-bodied species were more likely to go extinct, or there was no association between body size and extinction threat.
In earlier extinction events, the ocean zone that animals inhabited — whether they lived in the open ocean compared to the sea bottom, for instance — also helped to predict which species would disappear. This factor does not appear to affect modern extinctions, the researchers found.
"The extreme selectivity of the modern extinction threat with respect to body size is best explained by the size bias in human hunting and fishing activities, which often preferentially target the largest animals in the oceans, or the largest animals within their respective taxonomic groupings," said Payne. "Combining the threat from human hunting and fishing activities with the stresses imposed by ocean warming, acidification, and de-oxygenation is likely to compound the stresses imposed by hunting and fishing activities in the future."
In the past, factors like ocean acidity, temperature, and oxygen concentration may have more greatly influenced a marine organism's likelihood of surviving, Payne explained, and tolerance to such factors is not strongly associated with body size.
The researchers warn that removing marine animals at the top of the food web could significantly disrupt the ocean's overall ecology for potentially millions of years, which is why effective management strategies will be critical.
"Without a dramatic shift in the business-as-usual course for marine management," Payne and his team write, "our analysis suggests that the oceans will endure a mass extinction of sufficient intensity and ecological selectivity to rank among the major extinctions of the past 500 million years."