Plastic debris floating in the ocean has become a powerful new passport to far-away destinations for a wide variety of invasive species, according to new study published today in the journal Science by biologists from the Smithsonian Environmental Research Center, Williams College and other institutions.
Following the tsunami of March 11, 2011, in Japan, millions of objects were swept out to sea, from small pieces of plastic to fishing boats and docks. In years since, scientists have recorded entire communities of coastal species—mussels, worms, hydroids (sea anemone and jellyfish relatives), crustaceans and bryozoans—attached to these rafts and washing up on beaches in Hawaii and western North America. Nearly 300 raft-riding invasive species have appeared on the shores of Hawaii and the U.S. West Coast, two-thirds of which have never been seen previously on the U.S. West Coast. The scientists suspect many invasive castaways escaped notice.
Much of the debris rafts are made of fiberglass and other plastic materials that do not decompose and can easily survive six or more years at sea, the scientists say. Manufacture of such materials became common in the mid-20th century, and their dominance is only expected to rise.
“I didn’t think that most of these coastal organisms could survive at sea for long periods,” Greg Ruiz, study co-author and marine biologist at the Smithsonian Environmental Research Center says. “But they haven’t had much opportunity in the past. Now, plastic can combine with tsunami and storm events to create that opportunity on a large scale.”
Mollusks such as mussels occurred most frequently of all invertebrate groups. Worms, hydroids, crustaceans and bryozoans that form branch-like underwater colonies were not far behind. While arrivals have slowed, they have not stopped. The team was still finding new species when the study ended in 2017.
None of the rafting species discovered were considered capable of surviving a long transoceanic voyage between continents, mainly because the open ocean is a harsh place for creatures accustomed to hospitable environments of coastal waters. Researchers suspect the slower speed of ocean rafts—which float at 1-2.3 miles per hour—may have allowed their passengers to gradually adjust to their new environments. The sluggish pace also may have made it easier for some species to reproduce and for their larvae to attach to the debris.
“There is huge potential for the amount of marine debris in the oceans to increase,” says James Carlton, an invasive species expert with the Maritime Studies Program of Williams College and Mystic Seaport in Connecticut. Carlton is lead author of the study.
According to a 2015 report in Science, over 10 million tons of plastic waste enter the world’s oceans each year. That figure may increase 10 times by 2025, in part as hurricanes and typhoons become more frequent due to climate change. To date, no rafting species from Japan is known to have colonized the West Coast of North America due to the 2011 Japanese tsunami. However, it can take years after a nonnative species first arrives to detect a newly established population. This provides a window of time for action, yet the full consequence of the tsunami-driven rafting event remains unknown.
“This has turned out to be one of the biggest, unplanned, natural experiments in marine biology, perhaps in history,” says co-author John Chapman of Oregon State University.
Scientists largely agree that prevention is the most effective way to combat invasive species. As preventing tsunamis is not an option, the Smithsonian’s Greg Ruiz suggests the main focus should be managing plastic.