Seabirds mistakenly eat plastic debris in the ocean not just because it looks like food, but because it smells like it.
When mealtime comes around, many species of ocean-faring birds, including albatrosses and petrels, follow their noses to the smell of dying algae, a sure sign that tasty krill are eating up the microscopic plants.
In this tiny feeding frenzy, the algal cells burst and release a distinct-smelling sulfurous compound. The algae use this chemical as a kind of distress call, signaling their bird allies to come eat their predators.
But it's possible for this process to be hijacked, according to a UC Davis study published this week in Science Advances.
In the world's increasingly plastic-strewn oceans, marine creatures — especially birds — are ending up with stomachs full of non-nutritious trash.
Plastic pollution presents a growing threat to marine life. According to a 2014 analysis, 5 trillion pieces of plastic (weighing a combined 250,000 tons) are floating in the oceans. More than 200 species of marine mammals, turtles, birds and fish have been found to mistakenly eat plastic, and Australian researchers recently projected that by 2050, 99% of all seabird species will have ingested at least a little of the man-made material.
But why do so many birds mistake plastic for food?
The researchers behind the new study suggest plastic pollution may set a kind of "olfactory trap" for the seabirds as they sniff out prey.
As plastic floats in the ocean, it accumulates a host of organic matter, including that stinky sulfur compound known as dimethyl sulfide, or DMS. When the birds get a whiff of the plastic, they eat it up as if it were their natural prey.
"If your nose has told you that you can find food here, it's much more likely that you'll let your guard down and eat garbage by accident," said study leader Matthew Savoca, a UC Davis doctoral student in ecology.
Savoca and colleagues made the connection by floating the three most common types of plastic in Bodega Bay and Monterey Bay. The plastic was enveloped in special mesh bags to avoid littering and harnessed to buoys.
Three weeks later, the scientists returned to the buoys and retrieved the plastic, which was covered with algae.
"I sniffed some of it, and oh, man, did it reek of rotting seaweed," Savoca said. "We can smell [the sulfur], so that's a good indication that the birds can also smell it, because they're very sensitive to it."
To identify the chemical behind the smell, the scientists took the soggy plastic to UC Davis's Department of Viticulture and Enology. There, a food and wine chemist used a device called a gas chromatograph to confirm the source of the odor was dimethyl sulfide.
Many marine animals use DMS as a chemical cue to forage, including tiny zooplankton and giant baleen whales. Tube-nosed seabirds are particularly sensitive to the compound. They also happen to be severely affected by plastic consumption.
Drawing on plastic ingestion data from more than 1,000 tube-nosed birds representing 25 species, the researchers found a strong link between a species' sensitivity to the sulfur compound and how frequently it ate plastic.
Seabird species that followed the scent of the chemical to find food wound up ingesting plastic five times more often than species that did not, the study found.
Among this sample, almost all the birds that nest underground relied on DMS to hunt. Using this information, the researchers expanded their study to 62 species of burrow-nesting tubenoses — more than 20,000 individuals. These birds ate plastic three times more frequently than birds that nest on the surface.
Because these birds begin life underground in pitch darkness, their noses become highly fine-tuned, learning to rely more on olfactory cues than visual ones. So if something smells like food, the birds are likely to trust their noses.
The results suggest that the more secretive, less charismatic seabird species — namely, the ones that nest underground — may require closer monitoring for plastic ingestion, Savoca said.
"Plastic might not only be visually confusing for these birds, but chemically confusing," Savoca said. "It's a more insidious threat; [the birds] are not making dumb decisions. It's just that plastic can be very deceptive in this regard."
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