This might be why her team got contrasting results in their analysis: The more individual microplastics in the gut, the greater the microbial diversity, but the higher mass of microplastics, the lower the diversity. The more particles a bird eats, the greater the chance that those hitchhiking microbes take hold in its gut. But if the bird has just eaten a higher mass of microplastics—fewer, but heavier pieces—it may have consumed fewer microbes from the outside world.
Meanwhile, particularly jagged microplastics might be scraping up the birds’ digestive systems, causing trauma that affects the microbiome. Indeed, the authors of the plasticosis paper found extensive trauma in the guts of wild flesh-footed shearwaters, birds that live along the coasts of Australia and New Zealand, that had eaten microplastics and macroplastics. (They also looked at plastic particles as small as 1 millimeter.) “When you ingest plastics, even small amounts of plastics, it alters the structure of the stomach, often very, very significantly,” says study coauthor Jennifer Lavers, a pollution ecologist at Adrift Lab, which researches the effects of plastic on sea life.
Specifically, they found catastrophic damage to the birds’ tubular glands, which produce mucus to provide a protective barrier for the inside of the stomach, as well as hydrochloric acid, which digests food. Without these key secretions, Lavers says, birds “also can’t digest and absorb proteins and other nutrients that keep you healthy and fit. So you’re really prone and susceptible to exposure to other bacteria, viruses, and pathogens.”
Scientists call this a “sublethal effect.” Even if the ingested pieces of plastic don’t immediately kill a bird, they can severely harm it. Lavers refers to it as the “one-two punch of plastics” because eating the material harms the birds outright, then potentially makes them more vulnerable to the pathogens they carry.
A major caveat to today’s paper—and the vast majority of microplastics research—is that most scientists haven’t been analyzing the smallest of plastic particles. But researchers using special equipment have recently been able to detect and quantify nanoplastics, on the scale of millionths of a meter. These are much, much more numerous in the environment. (This is also why the finding that there are 11 billion pounds of plastic floating on the ocean’s surface was probably a major underestimate, as that team was only considering particles down to a third of a millimeter.) But the process of observing nanoplastics remains difficult and expensive, so Fackelmann’s group can’t say how many might have been in the seabirds’ digestive systems, and how they too might influence the microbiome.
It’s not likely to be good news. Nanoplastics are so small that they can penetrate and harm individual cells. Experiments on fish show that if you feed them nanoplastics, the particles end up in their brains, causing damage. Other animal studies have also found that nanoplastics can pass through the gut barrier and migrate to other organs. Indeed, another paper Lavers published in January found even microplastics in the flesh-footed shearwaters’ kidneys and spleens, where they had caused significant damage. “The harm that we demonstrated in the plasticosis paper is likely conservative because we didn’t deal with particles in the nanoplastic spectrum,” says Lavers. “I personally think that’s quite terrifying because the harm in the plasticosis paper is quite overwhelming.”
Now scientists are racing to figure out whether ingested plastics can endanger not only individual animals, but whole populations. “Is this harm at the individual level—all of these different sublethal effects, exposure to chemicals, exposure to microbiome changes, plasticosis—is it sufficient to drive population decline?” asks Lavers.
The jury is still out on that, as scientists don’t have enough evidence to form a consensus. But Lavers believes in the precautionary principle. “A lot of the evidence that we have now is deeply concerning,” she says. “I think we need to let logic prevail and make a fairly safe, conservative assumption that plastics are currently driving population decline in some species.”