A space craft returning to Earth is unknowingly carrying an alien life form. The space craft lands, and the alien is set free. Earth turns out to be a perfect living place. The alien establishes itself, eating the plants and animals in its surroundings and reproducing rapidly. The local wildlife can neither kill nor compete with the newcomer. It's not long before the locals start to die out.
 
Science fiction? No, reality. But the aliens are invading not from space, but from different parts of our own planet. And they move around not by space crafts, but by ships.
 
Every day, on nearly every ship cruising our oceans and lakes, millions of stowaways are hitching a lift. They get onboard when ships load ballast water, which is stored in the hull to give the ship stability. In this way, over 4,000 species are estimated to travel around the world daily, including plankton, algae, fish, jellyfish, and other invertebrates.
 
"The scale of this is enormous," says Andreas Tveteraas, a shipping expert at WWF. "Every hour, around 7.5 million litres of ballast water are released into US waters alone. Overall, some 10,000 billion litres of ballast water are transferred around the world each year. And this water is full of living organisms and pathogens."
 
When the ships reach their destination, which may be thousands of kilometres away, the freeriders are let out with the ballast water into the surrounding sea or lake.
 
Some of the stowaways are not able to survive in their new environment. But some can — and only too well. These aliens can become invasive, rapidly out-competing local fauna or flora. The effects on the local ecology, economy, and even on human health are devastating.
 
"Invasive species are a major threat to global biodiversity, and have turned into a huge concern for marine areas," says Andreas Tveteraas.
 
One of the best-documented examples is in the Black Sea. A voracious comb jellyfish from North America (Mnemiopsis leidyi) has wiped out fish stocks, destroying commercial fisheries and affecting the entire marine ecosystem.
 
First arriving on ships from the American Atlantic coast in 1982, the comb jelly eats both zooplankton, the food of commercially important fish in the Black Sea, and the eggs and larvae of the same fish species. With no enemies in their new home, the jellies propagated at an alarming rate. By the mid-1990s, they accounted for 90 per cent of the total biomass in the Black Sea — a biomass more than the total annual fish catch around the world. And the species quickly spread into the neighbouring Azov Sea too.
 
The invasion contributed to the near collapse of Black Sea commercial fisheries within a few years. The once quite prosperous seafood industry has lost about US$1 billion since the jellies were let out. And anchovy fisheries in the Azov Sea, already under stress from pollution and overfishing, have collapsed completely.
 
Dolphin numbers in the Black and Azov Seas also dropped dramatically, as the fish they used to feed on disappeared. The entire ecosystem has been disrupted — the jellies have even reduced the amount of oxygen in the Black Sea. They've now entered the Caspian Sea, where they are similarly wreaking havoc.
 
"This catastrophe should stand as a warning to the international shipping community on the dangers of ballast discharge in foreign waters," says Dr Erkki Leppäkoski from Åbo Akademi University, Finland, a world expert on alien introductions.
 
Alien plants can be as damaging as alien animals. In Australia, the Asian kelp (Undaria pinnatifida), a large-leaf brown seaweed, is rapidly invading new areas and replacing native seagrass communities which are essential nursing and feeding grounds for many commercially important fish and crustaceans.
 
The involuntary introduction of alien species can also affect human health. In the Philippines, alien algae have bloomed several times, causing so-called red tides. The algae, and the highly toxic substances they produce when blooming, are absorbed by filter-feeding oysters and other shellfish. When eaten by people, the toxins in the contaminated shellfish can cause paralysis and even death.
 
The effects of introducing alien marine species into new ecosystems are usually irreversible. This makes transfer of ballast water and invasive marine species perhaps the biggest environmental challenge facing the global shipping industry.
 
"Shipping must join the strive for a sustainable future, and in order to do so, the industry has to quickly eliminate the threat of environmental disasters caused by the inadequate handling of ballast water," says Andreas Tveteraas.
 
So what can the shipping industry do?
 
"The ultimate solution would be to ban the discharge of ballast water that has not first been ’disinfected’," says Andreas Tveteraas. "But as yet there's no perfect solution."
 
A variety of methods can be used to kill organisms in ballast water, such as UV irradiation, the removal of vital oxygen, and filtration. Equipment is already available for treating ballast water on smaller ships, such as cruise ships. But so far nothing can deal with the huge amounts of ballast water on large cargo ships within an acceptable time frame.
 
Another option is to empty ballast water in the open sea, and reload with this sea water which contains less organisms than coastal water. A number of countries, including the US, Canada, Australia, and New Zealand, have already adopted measures like this to try and minimize the introduction of alien species in their ports. These countries also recommend that ballast water is not taken up in areas likely to contain a high number of organisms, such as shallow water, and that water is not let out in sensitive areas.
 
However, the global nature of the problem calls for a global convention to regulate ballast water intake and discharge. This week, the International Maritime Organization — the UN body responsible for international shipping —finally adopted such a convention.
 
Once ratified and enforced, the convention will require all ships to implement a Ballast Water Management Plan, which will demand that new vessels be fitted with equipment for treating ballast water after 2009 and that all ships be fitted from 2016. In the meantime, ships will be required to exchange ballast water 200 nautical miles from land before entering a port.
 
"This new convention is a step in the right direction," says Andreas Tveteraas. "But there are worrying loopholes that unscrupulous states and captains could take advantage of. WWF will work to ensure that the convention is put into practice quickly and that it is strictly enforced to properly address the problem. We will also work to persuade responsible ship owners and charters to treat ballast water as soon as possible."
 
But in the meantime, we can expect that aliens will continue to hitch their daily rides around the world. And it's anyone's guess when and where one will find a new habitat to invade. 
 
*Jessica Lindström Battle is an environmental writer based in Switzerland.  
 
Further information: 
 
WWF's work with the IMO on ballast water 
WWF has been working with the IMO for a number of years on the adoption of an internationally binding ballast water convention. This issue has been on the IMO agenda for 15 years, but was only adopted at the IMO's Ballast Water for Ships International Conference, which took place from 9–13 February 2004 in London, UK.
 
WWF will work to ensure that the convention is ratified quickly and that it enters into force as soon as possible. In addition, WWF will work to ensure that the shipping industry takes responsibility for the following:

• ballast water treatment equipment be fitted on existing ships as soon as adequate technology exists

• optimal standards for ballast water treatment equipment so that advances in technology are mirrored in increasingly higher levels of ’disinfection’ of ballast water

• protection for vulnerable parts of the high seas (the 70 per cent of ocean outside national jurisdiction), as the high seas today are seen as an open dumping ground for ballast water of coastal origin

• strict rules on special permissions for non-treatment of ballast water that can become loopholes in the convention, making it ineffective

• special requirements in sensitive areas, imposed by the concerned coastal state