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"It is funny that one is not allowed to destroy species so they become extinct, but still it is possible to introduce new species, although it results in the danger of another species disappearing. The matter has not been properly considered,", Petri Nummi says with regret.

An Alien tale

Henrikki Timgren


Aliens have secretly been taking over the world for thousands of years. With increasing globalisation, invaders in the animal and plant kingdoms have accelerated their invasion of new lands and continents. According to a recent report by the Ministry of the Environment, there are over 600 plant or animal species with foreign origins in Finland alone. The spread of invader species is regarded as the most serious threat to biodiversity behind the loss of habitats due to human actions.

Cercopagis pengoi is floating in the dark, wait- ing. The monotonous chug of the ship’s diesel engines is drowned out in the angry howl of water pumps. Not that the noise disturbs the Cercopagis. Maybe the tip of its primitive sensory organ sends a slightly dizzy sensation to it as it rushes towards long-awaited freedom in a vortex of discharge water from a Russian cargo ship. If the Cercopagis had a brain, it might wonder as it plunges into the cold, murky dock in Kotka, southeastern Finland, whether it was worth while leaving the Caspian Sea behind. Many biologists wonder the same. Cercopagis pengoi, a predatory cladoceran, is one of the hundreds of invader species in Finland, which give headaches to conservationists as well as the economy.

According to a recent report commissioned by the Ministry of the Environment, over six hundred foreign animal and plant species have made their home in Finland over the millennia. Cercopagis pengoi, which is a zooplankton, is one of the most recent invaders: it was first discovered in the Baltic Sea in 1992. In a few years, it succeeded in spreading throughout the Gulf of Finland and has also been spotted in the Gulf of Bothnia. The impact Cercopagis has on the ecosystem of the Baltic Sea is not yet fully known, but scientists think that Cercopagis might destroy plankton, which constitutes the main diet of certain species of fish. It is quite a nuisance to fishermen. Cercopagis forms large, slimy plankton rafts, which clog up nets and seines. It is feared that the floating slimy Cercopagi armies will soon invade Finland’s inland waters as well.

Invader species are divided into roughly three. The first group includes those that have arrived ages ago with agriculture. Few people even think of these “invaders”, such as the house mouse and numerous cultivated plants, as alien. The second group consists of aliens imported deliberately in historic times, such as the pheasant and the white-tailed or Virginia deer introduced to delight hunting barons and country squires. The third and most notorious group of aliens are the modern – or should one say post-modern – invaders, which travel from all corners of the globe in the ballast of mega-tankers, the cargo holds of supersonic aircraft, on the sides of nuclear-powered submarines and on the soles of tourists’ shoes. It is estimated that at this very moment three to four thousand different aquatic species swim around in the ballast of vessels cruising the world’s seas.

Only now do invaders make the headlines

Petri Nummi, docent, sits in his brightly lit office, holding a North American beaver skull. “It is almost impossible to distinguish the North American and the European beaver from one another. They can be distinguished only by examining the skull,” Nummi explains and points to the yellowed but strong chisel-like teeth of a Castor canadensis.

The difference was not spotted by conservationists in the 1930s, who, in good faith, shoved North American beavers into Finland’s inland waters. The purpose was to revive the population of the European beaver, which had become extinct in Finland in the 19th century. Owing to its better ability to reproduce and its slightly stronger teeth, the North American beaver began to take over habitats from the European beaver, which was just beginning to revive. Now there are approximately 1,500 European beaver and 12,000 North American beaver in Finland.

An assistant in the Department of Applied Biology, University of Helsinki, Petri Nummi’s relationship with Cercopagis and Castor canadensis resembles that of a policeman’s to criminals. In his work, Nummi studies the impact of invader species on ecosystems. He is the man behind the recent survey of alien plants and animals in Finland, published by the Ministry of the Environment.

According to Nummi, who has just returned from the Convention on Biological Diversity in Montreal, people have only recently woken up to the global dimension of the invader problem.

“Controlling invaders has been a central theme in the nature conservation of island nations for the past 50 to 100 years. In New Zealand, conservation means destroying and killing invaders by all means necessary, so that at least a fragment of the islands’ indigenous nature would be preserved. It has now been discovered that similar invasions by alien species also take place on the continents, it just isn’t perceived as easily.” Today, the invader species are regarded as the most serious threat to biodiversity behind the loss of habitats due tohuman actions. The spread of invaders has become easier with increased shipping and travelling, though the phenomenon as such is age-old.

“The invader effect of humans has been charted from the end of the Ice Age. According to a famous, although controversial theory, the Paleoindians, arriving in America across the Bering Strait 40,000 years ago, destroyed plants and animals in a ‘blitzkrieg’. That is, as the human front advanced from Alaska, it annihilated the largest animals, mastodons and mammoths, which had a great impact on the ecosystem. Indeed, there are dozens of large mammals which became extinct in America right about this time.”

The dangerous white-tailed deer

In addition to the North American beaver, the most notorious invaders in Finland include the American mink, the signal crayfish and the Aphanomyces astaci fungus, which causes a serious crayfish disease called the crayfish plague. Each of these has caused considerable damage. The American mink, which originally escaped from fur farms, has already destroyed Finland’s populationof European mink, which is all but extinct. The crayfish plague, which arrived from America to Finnish inland waters in the 1890s, spelled an economic catastrophe to the Finns who depended on crayfish fishing for their livelihood. Funnily enough, the Finnish crayfish population, weakened by the plague, was supplemented in 1967 by introducing American signal crayfish, which are resistant to the fungal disease. In other words, in some areas the original culprit for the plague had the honour of taking over the last habitats from their Finnish cousins, which were weakened by the disease.

Nummi says that the focus in invader control has showed too much bias toward economic factors. The possible impact of signal crayfish on the indigenous population was not discussed in the 1960s.

“It is funny that one is not allowed to destroy species so they become extinct, but still it is possible to introduce new species, although it results in the danger of another species disappearing. The matter has not been properly considered,” Nummi says with regret. “Controlling invaders is done better when it concerns creatures that threaten economic interests, such as agriculture or forestry. The principles are there, but in previous years creatures which only threatened wildlife were ignored.”

Short-term economic thinking can, in the end, harm the money-makers themselves. A small change in the ecosystem can cause an unpredictable chain reaction, even an ecocatastrophe.

That nearly happened in 1934, when Finnish immigrants in Minnesota got the bright idea to donate a few white-tailed deer to delight the inhabitants of their native country. The Minnesota Finns did not know that many white-tailed deer carry a parasite, which is only a nuisance to deer, but fatal to elk. Had we been less lucky, we would now have to go to Helsinki Zoo or to the northern wilderness, where there are no white-tailed deer droppings, to see elk with their magnificent antlers.

The catastrophic Australian opossum

Finland has so far been spared ecocatastrophes, but Australia and New Zealand, in particular, are in big trouble.

“Nature has different relationships resembling food webs, which are far more complex than what humans can discover. It is clear that invaders pose the threat of ecocatastrophes, which could also have an effect on humans.”

A good example of the unpredictable nature of food webs is the brush-tailed possum or the Australian opossum, whose favourite hobbies include destroying New Zealand ecosystems. “The brush-tailed possum is one of these leaf-eaters, a seemingly harmless little creature, but because there has never been herbivorous mammals in New Zealand before, there is no coevolution of plants and herbivores,” Nummi explains.

Coevolution refers to a situation where species compete in developing means of outdoing each other: herbivores develop a more efficient digestive system and plants then retaliate by developing thorns, for example.

“The lack of coevolution is evident, when herbivores are taken to an island – the plants are defenceless. The brush-tailed possum eats flowers and fruit, which decreases the numbers of birds which also eat them, which in turn leads to seeds being spread less. That is why, for instance, the mistletoe has declined. In addition, the brush-tailed possum itself eats mistletoe. After the plants became scarcer, the possum then has unexpectedly begun to prey on bird nests, which once again reduces pollination. Nobody could foresee consequences like these.”

Animal rights activists – the worst enemies of nature conservation?

Protecting living organisms in one’s own area is best done if one can act quickly. The Montreal Convention sketched general outlines for controlling invader species. The programme is epitomised in the slogans: prevention, early detection, eradication and control.

Of these, prevention is not only the most effective, but also the least expensive way to ward off unwanted invaders. Prevention consists of, for example, changing the ballast of ships in the open sea or more carefully controlling the import or export of living organisms and creatures. “In New Zealand, every seed is dug out of the soles of travellers’ shoes. If you have an apple in your suitcase, it’s a crime almost worse than if you had drugs ,” Nummi says with a laugh.

After a foreign species has taken root in a new country, uprooting it is extremely difficult or, at least, very expensive. After the harm is done, it is necessary to resort to quite drastic measures, such as eradication, to protect indigenous living organisms. The attitude of island nations to invader species seems quite brutal to us starry-eyed Europeans. Who would want to butcher a cute brush-tailed possum?

“Some nature reserves in New Zealand and the United States have large populations of feral horses. They are quite big grazers, so there have been calls to reduce their numbers. The project has experienced strong opposition from young girls. The opposition is easily explained. I too remember when I was a child, hunting seemed wrong. In child-like thinking, hunting is confused with cruelty to animals,” Nummi muses.

The thought of eradicating a species as part of nature conservation is a difficult one for many who care about animals. Nummi sees it, in the end, as a choice between the right to life of an individual animal and a whole species. Four years ago, Italian authorities discovered that the grey squirrel, originally an American species, had gained a foothold in the country. The population then numbered only a couple of thousand, and against all the laws of bureaucracy, the authorities drew up an emergency plan against the colonising squirrels. People involved in the prevention of cruelty to animals, however, sued the government, and as judges pored over codes of law, the grey squirrel multiplied and jumped from one olive tree to the next. When the courts finally decided, last year, that the grey squirrel must go, it was too late to evict them.

“In this case, animal protectors clearly acted against nature conservation. People should realise that nobody is going to kill a cute rabbit or a furry beaver just out of mere malice. It is a case of nature conservation, in which the method is the eradication of a species. There are many people who confuse nature conservation and the prevention of cruelty to animals,” Nummi sighs and reveals that he has been thinking of writing a controversial piece called “Animal rights activists – the worst enemies of nature conservation”.