Alien invaders

Invasive fauna and flora are an immense problem for conservationists, foresters and farmers alike. Now researchers have developed a method for appraising which species are especially dangerous. By Simon Koechlin

(From "Horizons" no. 106, September)Picture: Konrad Lauber, Flora Helvetica @ 2007 Haupt Bern

Their names sound exotic, even auspicious. There’s the red-eared slider, the Asian long-horned beetle, the tree of heaven and the giant hogweed. But these species of fauna and flora all belong to the most ‘persecuted’ in Switzerland. Customs officials, municipal gardeners and conservation societies all over the country are trying to ferret them out, trap them or dig them up, because they’re on the list of species that are known as ‘invasive’. And yet they are animals or plants that at one time were introduced here by us (in some cases intentionally, in others not) and that spread naturally. They are displacing native species, damaging land and forests, and even leading to human health problems.

12 billion euros per year

These problematic species are so great in number that the ecological and societal consequences are literally unimaginable. The Swiss Federal Office for the Environment (FOEN) has so far counted more than 800 alien species that have established themselves in Switzerland. More than 100 are regarded as being invasive. In Europe as a whole, the number of non-native species is reckoned to be more than 12,000, of which 10% are invasive. Earlier this year, the EU enacted a new regulation against them. The EU already spends more than EUR 12 billion a year, both in combatting invasive species and in repairing the damage they do. And the costs are rising.

Given this order of magnitude, one has to ask what priorities our politicians should be setting. Should our money and resources be spent combatting the zebra mussel that’s displacing native mussels and blocking our water pipelines and sluice systems? Or would it be better spent in the fight against the corn rootworm that can destroy whole fields of maize? What’s more important: that a nature reserve isn’t completely overgrown with giant hogweed, or that a city park isn’t littered with the droppings of Canadian geese?

Comparing mammals and plants

These are questions that the authorities everywhere find difficult to answer, including in Switzerland. In a position paper published in late 2013, the conservation association ‘Pro Natura’ wrote, “current approaches often lack clear priorities about combatting invasive, alien species, just as they fail to specify the habitats where the scarce financial means available might be deployed on a priority basis”. Wolfgang Nentwig shares their opinion. He’s an ecologist at the University of Bern and one of the best-known researchers in the field. “Regrettably, too little is being done”, he says.

However, it’s been difficult to set priorities until now, because there’s been no means of appraising the impact of invasive species accurately or of allowing us to compare the influence of different groups of organisms, such as mammals and plants. A consortium of scientists from across the world has been developing such methods, and Nentwig and his colleagues at the University of Bern are involved in it. They have developed a kind of damage rating that functions as follows. They hunt for existing studies on the impact of alien species, which can range from studies on quantifiable effects to estimates made by experts. Then, on the basis of this quantitative and qualitative data, the impact of every individual species is assessed and allocated to one of twelve different categories, such as their impact on animals, vegetation, agriculture, forest management and human health.

Fish less harmful than birds

In a recently published study, Nentwig and his colleagues have used this method to examine and compare 300 alien species that have taken root in Europe – mammals, birds, fish, arthropods and plants. It transpires that, on average, alien mammals have the most serious impact – on the environment, on the economy and on society. The least damage, they suggest, is done by fish. Mammals are often of particular importance in an ecosystem because of their size and their adaptability, and also because they have a broad diet, says Sabrina Kumschick, the lead author of the study. She wrote her doctoral dissertation at the University of Bern and today works at the Centre of Excellence for Invasion Biology at the University of Stellenbosch in South Africa. “For this reason, we weren’t really surprised by the position that mammals assumed in our assessment”.

This assessment tool will in future allow authorities to carry out a better comparison of the consequences of invasive species. This in turn can help them to focus their resources better when trying to protect native species. Gian-Reto Walther is responsible for alien species at FOEN, and he is grateful for such practically oriented research. What’s especially valuable is that this new method allows for comparisons between different groups of organisms. Previous systems allowed you to compare plants with plants, he says, but not plants with mammals. Walther doesn’t think that Switzerland is doing too little, however. “There are already lots of measures underway to combat invasive species”, he says. Often, however, they’re not well enough coordinated. But that is going to change, he says. FOEN is currently developing “a strategy to combat alien species”.

Someone has to decide

Fighting uninvited intruders is going to be a Herculean task, but all the experts are aware of this. Not least because no one quite knows how to proceed. “According to the information we have at present, it’s already too late to exterminate many species”, says Nentwig. One example is Japanese knotweed, which comes from East Asia and has spread rapidly across Europe since the mid-20th century. In Switzerland, only the Upper Engadine is still free from this robust plant that is so quick to proliferate. And because Japanese knotweed can sprout from the tiniest piece of shoot left in the soil, it’s almost impossible to cope with it.

Damage ratings are at least a further piece of the jigsaw helping us to bundle our strengths and prioritise properly in the battle against invasive species. That’s why Nentwig and Kumschick are planning to develop their method further. But these assessment lists will never be more than an aid to decision-making. Because whether it’s more important to society to keep a park free of goose droppings or a nature reserve free of hogweed is something that will still have to be decided by someone on the spot.