Non-native species may hitch ride here via ships' ballast, but few establish themselves
Grace Chua Straits Times 12 May 13;
Scientists are countering a new claim by an international group that Singapore has the world's highest risk of alien invaders: marine species like fish and plankton that hitch a ride across the ocean in ships' ballast water tanks.
Researchers from Germany and Britain gave Singapore the dubious distinction after modelling invasion-risk "hot spots and highways" worldwide.
Their work, published in the journal Ecology Letters on May 4, has drawn international media attention.
Global shipping has carried exotic animals and plants such as zebra mussels and killer algae around the world on ship's hulls or in their ballast water, which ships add and flush for balance.
The hitchhikers can damage local wildlife, take over waterways and clog drainage systems if they take residence.
But scientists here say the presence of aliens does not have to mean invasions. And so far, non-native species have not had a great impact here, unlike in places such as the United States.
In the Ecology Letters work, Dr Michael Gastner of the University of Bristol and his colleagues devised a model that took into account the amount of ship traffic, routes, types of ships, and whether the local climate was hospitable to foreign species.
The more ship traffic there is and the more ballast water released, the more likely organisms like molluscs and plankton are to hitch a ride to foreign waters.
They also found a sweet spot for the risk of invasion: If ships come from too far away, creatures in ballast water may not survive the long trip, whereas those from nearby ports might be indigenous to their destination anyway.
Besides Singapore, other hot spots for invasion are another busy port, Hong Kong, as well as the Panama and Suez canals.
But researchers here say the theoretical model may not match the reality, at least in Singapore.
While zebra mussels from Asia clog North American pipes and toxic seaweeds smother California ecosystems, the same does not appear to have happened here.
Singapore has 250 hard-coral species, 100 species of sponges, and hundreds more marine species. How many non-native marine or brackish-water species have established themselves here? Seventeen.
Why? "We don't know," said Assistant Professor Darren Yeo, who studies invasion biology at the National University of Singapore and wrote a paper on the 17 species with colleagues last year. Some are from shipping, others are escapees from fish farms.
One theory is a concept called biotic resistance.
In tropical marine environments with a wide range of species, all the jobs are taken, so to speak: All the ecological niches are filled and there is no room for outsiders. Also, such environments may be resilient because native species prey on or out-compete non-natives, Prof Yeo said.
What's more, no one knows for sure what is actually native, since little is known about the actual distribution of many marine organisms, added Dr Tan Koh Siang, who heads the Tropical Marine Science Institute's (TMSI) marine biology and ecology group.
Researchers are still finding new species or new records here.
A five-year Comprehensive Marine Biodiversity Survey of Singapore's marine life will help deliver a baseline count.
The survey started in late 2010, and a major expedition to the Southern Islands is scheduled for the end of this month.
Singapore's marine ecosystems, are likely to have already been shaped by urbanisation and hundreds of years of shipping in South-east Asia, said Prof Yeo.
Dr Gastner and his colleagues acknowledged that there may not be enough field data, especially in less-studied Asia.
Prof Yeo and Dr Tan pointed out that the line between friend and foe, native and non-native, is not clear. Local species can also cause biofouling or become pests; foreign ones like the Caribbean bivalve might even help to filter water as they feed.
Still, they said, it is better to be safe than sorry, and precautionary measures to lower the risk of alien invasions should still be strengthened.
For instance, some large ships swop their ballast water mid-voyage with a fresh intake, though this has to be done with care or it can destabilise and damage ships. Others treat their ballast water to kill stowaway creatures before releasing it.
Singapore is a member of the International Maritime Organisation, which has a convention that by 2016 will require all ships to have ballast water management plans.
"The best method is to prevent them from coming in in the first place," Prof Yeo said.
Some exotic hitchhikers
Caribbean bivalve
The Caribbean bivalve or black-striped mussel, Mytilopsis sallei, is a non-native species here. In Singapore, it has established itself on concrete walls and monsoon drains.
Caribbean serpulid worm
The Caribbean serpulid worm, Hydroides sanctaecrucis, is found on buoys and other structures here, side by side with native species of marine worms.
Caulerpa taxifolia seaweed or ‘killer algae’
The Caulerpa taxifolia seaweed is known as “killer algae” as it both grows out of control and pushes other seaweed species out. In turn, it drives fish and other sea creatures away as it is toxic to them. The seaweed, widely used in aquariums, is native to the Indian Ocean, present in Singapore and has been known to smother ecosystems in California and the Mediterranean.
Zebra mussel
This bivalve, native to the Caspian Sea in Central Asia, lives in fresh and brackish water. It spread through Europe in the 19th century, reached North America in recent decades, and is notorious for clogging power and water treatment plants’ pipes and littering beaches with razor-sharp shell fragments. It can also attach to other living things with hard shells, such as crayfish and turtles. It is not present in Singapore.
