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Alien invaders

发布时间:2019-03-08 06:16:02来源:未知点击:

By Debora MacKenzie THE fishermen of St Petersburg realised something was wrong when their nets started coming up clogged with stinking, grey sludge. At first they suspected pollution. But the true culprit was revealed only under the microscope. It was Cercopagis pengoi—a water flea that is native to the Black Sea. The crustacean arrived at St Petersburg in 1995. It found few predators and there was a population explosion. In some parts of the Baltic Sea, masses of the creatures—their tails hooked together to form clumps—can make it impossible to pull a fishing net through the water. The incident is the most visible example yet of a threat that is increasingly alarming marine scientists. The Baltic ecosystem, home to an important fishery and to a huge coastal population, is experiencing a massive invasion of foreign organisms. Alien invaders have already wrought havoc in the Black Sea. By 1990, an introduced American comb jelly, Mnemiopsis, had gobbled up so much of the zooplankton in the Black Sea that fish fry starved. Combined with pollution and overfishing, this has led to the collapse of Black Sea fisheries (“How the Soviet seas were lost”, New Scientist,11 November 1995, p 38). At a meeting in Llandudno in Wales last week, biologists warned that the Baltic could suffer the same devastation as the Black Sea. Jim Carlton, a marine biologist at Williams College in Massachusetts, who in the mid-1980s was among the first to sound the alarm over alien marine species, says: “So far, the Baltic has been lucky. We can’t predict which new species will cause major ecosystem changes.” Most of the invaders hitch a lift in the ballast water of ships. Rather than wait for disaster, he says, “we should be limiting the arrival of new species, by controlling the dumping of ballast water by cargo vessels”. While the US and Australia have now clamped down on dumping ballast water in their ports, dumping in the Baltic is largely unregulated. How much damage are these invasions likely to cause? The Baltic, the world’s largest brackish sea, is only 10 000 years old. It has shifted from salt to fresh to brackish several times as land masses have moved. This means the ecosystem has had to start again from scratch several times, says Sergej Olenin of the University of Klaipeda in Lithuania. “In many ways the Baltic is already a sea of invaders,” he says. This means there may be empty niches for new invaders to occupy. So far, around 70 alien non-microscopic species have set up house in the Baltic, alongside the 400 natives. Only 25 non-microscopic species successfully invaded the Black Sea before Mnemiopsis struck. But the impact on the Baltic so far seems small. No native species has gone extinct. And some aliens are longtime residents. Mya arenaria, a North American bivalve, arrived in the 11th century, clinging to the bottom of Viking longboats. Others were introduced deliberately. In the 1960s, Soviet planners brought in Pontogammarus, a Caspian shrimp, for local fish to eat. The shrimps have done well, and fish have changed their diet. Little else changed. If most of the invasions have so far been benign, their scale is still troubling. Piotr Czgruszka of the Agricultural University of Szczecin in Poland estimates that 97 per cent of the bottom dwellers in the Oder Estuary and 95 per cent in the Vistula estuary are now Marenzelleria. This bristly polychaete worm from Chesapeake Bay on the east coast of the US arrived in 1985, probably in ballast water. The worm seems to have displaced a few aquatic midge larvae. And it has certainly taken over. But has it done any real harm? The answer is still not clear. “Its burrows are several times longer than any native species make, which may liberate more nutrients from sediment,” suggests Olenin. This could add to eutrophication: excessive nutrients cause an explosion of algae, which die, rot and deplete the oxygen in the water. On the other hand, the worm’s larvae are a new food source for fish, says Stephan Gollasch of the Institute for Marine Research in Kiel, Germany. Other invaders also have good and bad sides. Besides fouling nets, Cercopagis feeds the economically important herring, says Gollasch. And although round gobies from the Black Sea (Neogobius melanostomus) are competing with native flounder off Gdansk, they taste just as good and might be controllable if people would start eating them, says Krzysztof Stóra of Gdansk University. But scientists are wary of suggestions that some introductions might be beneficial. “It’s a game of ecological roulette,” says Carlton. Some species have already surprised scientists. The shipworm Teredo navalis, a native of the tropics, started reaching the Baltic on ships in the 1730s. But it did not breed in the chilly northern waters. At least, not then: this year, says Gollasch, larvae have been found off Kiel. The species has adapted. No risk assessment model would have predicted that, he says. “We need a concerted, European approach to this problem. After these species are released, you can’t get them back.” Some newcomers are undeniably dangerous. In a study for the Nordic Council published last week, Gollasch and Erkki Leppäkoski of the University of Åbo Akademi in Turku, Finland, list the invaders that can travel in ballast, but have not hit the Baltic yet. They include toxic algal blooms; cholera, thought to have reached Peru in 1991 in ballast; Pfiesteria, a North American alga that kills fish and causes human disease; a Japanese seastar that has rampaged through shellfish beds in Australia and a Japanese snail that destroyed Black Sea mussel beds. In 1996, a study of German ports found that each incoming ship carries on average four million specimens of macrofauna—everything from zooplankton to fish—and as many as 110 million phytoplankton per cubic metre. Some 7000 ships a year dock in Klaipeda in the Baltic, Lithuania’s only port. “They dump 2 to 4 million cubic metres of ballast water here a year,” says Olenin. Yet there are almost no controls on dumping ballast water in European ports. In 1997, the UN’s International Maritime Organization (IMO) recommended that when ships reach mid-ocean they exchange the ballast water they took on in harbours—which is typically full of organisms—for relatively lifeless water. The recommendation is enforced in the US and Australia by inspectors who check the salinity of ballast water. Similar controls are planned for some British ports and all Swedish ones. The IMO’s guidelines are voluntary, and captains may ignore them if they feel that the weather makes ballast exchange at sea too dangerous. “There is tremendous resistance in the shipping industry to ballast controls,” says Gollasch. Captains may also be swayed by the knowledge that frequent exchanges wear out expensive ballast pumps faster. But foreign invasions are also expensive. The Australian government calculates that the accidental introduction of toxic dinoflagellate algae in its waters would cost as much as £80 million. The Caspian zebra mussels that now foul waterways throughout North America have so far cost £300 million to clean up. “I hope we won’t have to wait for a disaster like that before we start taking preventive measures in the Baltic,