Ballast water from merchant ships has long been known to help animals and plants travel from one part of the world to the other and mess up aquatic ecosystems in their new environments. Now a global survey of ocean-going debris suggests that floating plastic--from soda bottles to fish nets--also gives noxious invaders free rides.
Over the past decade, ecologist David Barnes of the British Antarctic Survey sampled the debris that washed up on 30 remote island beaches around the globe, inspecting it for animals such as barnacles, polychaete worms, and bryozoans. Some of these hitchhikers were carried by natural life rafts, such as hunks of driftwood. But after analyzing roughly 200 pieces of flotsam from each island, Barnes estimates that human trash has more than doubled the number of pieces of beach litter on which animals can hitch rides to far-off-places. In tropical regions, Barnes counted one piece of human-generated trash for every piece of debris from natural sources; close to the poles, human junk outnumbered natural debris two to one, he reports in the 25 April issue of Nature. Barnes speculates that critters may travel further on plastic, because it degrades more slowly than wood and is less likely to sink than, say, metal or wood.
The most ominous trends, Barnes says, are in the Southern Ocean surrounding pristine Antarctica, where plastic has tripled the amount of flotsam. Although no fauna turned up in samples from Antarctica--probably because they can't take the icy temperatures--that may change, he warns, as Antarctic waters are expected to heat up 2°C over the next century. That would make the continent more hospitable to invaders. "It's the most prone point on the planet to marine invasions."
Invasives expert Jim Carlton of Williams College in Williamstown, Massachusetts, says the survey has helped highlight the "understudied" role of trash in marine invasions. "This is potentially a very important mechanism," Carlton says.