Over the vast plains of the open ocean, where wave lines may be the only markers, seabirds, including albatrosses, manage to find food.
They feed on swarms of krill — tiny crustaceans that swirl near the surface of the ocean — and their relatives. But over the last 50 years, they also feed on a bounty of plastic from trash, usually broken up into small pieces by exposure to waves, ultraviolet radiation and other factors, often called microplastics.
Many theories about why these birds and other marine animals eat plastic have been raised. Some people have speculated that they mistake it for food because of its appearance. To two scientists from the University of California, Davis, that explanation didn’t fully account for what they knew about birds. And in a study released Wednesday in Science Advances, their suspicions were confirmed.
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Matthew S. Savoca, a graduate student at the university and the paper’s lead author, said, “People say it’s because the animals don’t know any better, or they’re stupid, or it looks like other food they eat,” he said. “But that really doesn’t take into account that these animals have been honed evolutionarily over hundreds of thousands of years to find little patches of food in the open ocean.”
Gabrielle A. Nevitt, a professor of life sciences who specializes in animal behavior and sensory biology and an author of the paper, has been studying these types of seabirds for decades. In her previous research, she found that these types of birds (which the paper describes as “procellariiform species”) have a strong sense of smell, and respond to a certain chemical, dimethyl sulfide, as a cue to find their prey.
Dimethyl sulfide is released by phytoplankton as it gets eaten by a predator or breaks down in the ocean or on shore, signaling to these birds and others to come eat the phytoplankton’s predators (like krill).
Dr. Nevitt and Mr. Savoca found that the chemical is also released when tiny pieces of plastic are present in the ocean, often a result of “biofouling,” which describes the process when algae colonizes pieces of plastic, and then die or are eaten by other organisms.
In this study, the scientists used plastic beads of the type used in bottles, bags, textiles and hundreds of applications, ranging from four to six millimeters in diameter. After the microplastics had been in the ocean for about three weeks, dimethyl sulfide was found in the water and air around them in concentrations high enough that these types of birds may be able to smell, the scientists found, using tools that are otherwise meant for measuring sulfur in beer or wine.
The study suggests that the odor of dimethyl sulfide on or around marine plastic debris is “maladaptive foraging behavior” — that the birds are using their evolutionary traits to forage for food in ways that might be bad for them, causing problems like chemical toxicity or obstruction. According to the study, a recent projection model concluded that more than 99 percent of all seabird species will have eaten plastic debris by 2050.
Dr. Nevitt said that the study could have implications for other marine animals. Those that eat similar species to these birds — like baleen whales — or those that may also be attracted to dimethyl sulfide — like sea turtles — could be at risk. The researchers hope the study will help determine strategies for how to fight this growing environmental problem, as plastic pollution increases in the ocean.
“Fifty or 60 years ago, there was no plastic in the ocean,” Mr. Savoca said. Now, estimates hover in the hundreds of millions of tons. “We’re changing the world so rapidly that these animals can’t evolve rapidly enough to keep up.”