The world's oceans are awash with space invaders, marine life that rafts onfloating rubbish to colonise new beachheads and displace existing species.Dick Ahlstrom reports
The plastic drinks bottles and floating rubbish casually tossed into the world's oceans have taken on a new role, serving as rafts for seagoing organisms. Alien species are washing up on the world's most remote beaches, colonising places that were almost impossible to reach before plastic litter.
This invasion is highlighted in a report, published in the journal, Nature, by Dr David Barnes of the British Antarctic Survey in Cambridge and University College Cork. He has spent the past 10 years trekking to the world's most remote regions looking for washed up flotsam and the presence of marine hitchhikers.
For 30 million years organisms such as insects, crabs, shellfish, molluscs, marine worms and corals could not readily stray from their local habitats. While floating tree trunks, coconut seeds and volcanic pumice stone afforded some opportunity to reach distant shores, these means of dispersal were severely limited.
Floating plastic rubbish has changed all that, explains Dr Barnes who likens the new travel opportunities for these organisms to the introduction of the commercial airliner for us. His work shows that rubbish of human origin in the sea has roughly doubled the propagation of alien species in the subtropics and more than tripled it closer to the poles.
It is difficult to quantify the newcomers, he says, because we have no baseline list of species for many of these remote locations, chosen for study because they had no local source for the floating debris. He is in no doubt, however, that the number is substantial.
He found that 50 per cent of the rubbish washed up on these farflung shores was colonised by organisms. Each piece might carry hundreds of individual creatures across a range of species.
"God knows where they come from," he says. Many die off immediately in unfamiliar surroundings and vulnerable offspring also often die. In some cases, however, colonisation is successful and, Dr Barnes adds, "Once they are there, they are there forever".
Opportunistic creatures have no pretensions about the craft that carry them about. "The most common material would be commercial plastic drinks bottles and plastic rings that hold six-packs of beer," he says. They also raft happily on plastic rope, nets, parts of broken plastic chairs as well as the more traditional timber and wood.
He cites three reasons why plastic has worked so well for this form of species dispersal. First, plastic is now ubiquitous in our daily lives and as a result is in our refuse, "getting everywhere" carried on the oceans. Second, it breaks down very slowly and remains buoyant.
The third reason is perhaps the most powerful. "Organisms like settling on plastic," preferring it to either metal surfaces or wood, he says. While some are transported on the hulls of ships, the long slow journey by plastic bottle, with none of the fast moving water, means more immature organisms are likely to survive the trip.
Sometimes the newcomer can coexist with local species, but in other cases it is a matter of dog eat dog. Naturally occurring species have had millions of years to adjust to one another, leading to an equilibrium. This balance is upset when an alien species arrives. The newcomer can sometimes out-compete its rivals, becoming dominant and taking over a habitat. With carnivorous species, the local inhabitants simply get eaten into oblivion. The arrival and rapid dispersal in our inland waterways of the persistent zebra mussel is a good example of the former option.
The associated species loss is no small matter as it further reduces biological diversity. It also leaves finely balanced isolated habitats such as the Galapagos Islands or the Southern Ocean at genuine risk, Dr Barnes says.
WHILE Antarctica receives less floating rubbish than say the tropics, the impact on local organisms has been disproportionately higher, he believes. "We have had the biggest effect there even though the amount of rubbish is low. You only need a one in a thousand rate of colonisation if you have millions of pieces of rubbish."
Climate change has enhanced the risks, he adds. "If freezing temperatures are the main barrier to invasion by marine-borne organisms, then polar warming could alleviate this constraint." The circumpolar current has kept Antarctica isolated from rafting organisms for 25 million years, he says, but warming might allow more newcomers to survive and start a new life in a new home.