Student - September 20, 2007

Are exotics a threat?

The Coordinating Body for Invasive Exotics, part of the Plant Protection Service of The Netherlands (PD) in Wageningen, tracks down species that are not native to this country. The economic damage caused by these ‘intruders’ is estimated at 1.3 billion euros. But are alien species only a threat or do they also contribute to species diversity in the Netherlands?

The PD Annual Report 2006
‘An exotic is a species that does not occur naturally in our country. An invasive exotic is a non-native species that can settle here through rapid spread and has a negative effect on the biodiversity. An exotic can suppress or infect other indigenous species. Some exotics have been imported into the Netherlands as biological pest control agents. The PD carried out two surveys in 2006 on biological pest control agents in the environment. […] The survey showed that the American whitefly predatory bug Dicyphus Hesperus, used in indoor aubergine cultivation, escapes from greenhouses, meaning an increased risk to biodiversity. Other exotic species have been imported as water plants for aquariums, an example of an invasive plant being Cabomba caroliniana. The dyke and polder board for the Amstel and Vecht rivers has been fighting this invasive plant in the Loosdrechtse plassen as it is threatening a botanical reserve.’

Professor Marten Scheffer, chair of Aquatic Ecology and Water Quality Management
‘We seriously believe that on a world scale we are losing species at a rate comparable with that of the time of mass extinctions long ago. Is that bad? There are different opinions. The planet will survive, but it’s quite possible that we are losing species that are valuable to humanity. Invasions contribute to the loss of biodiversity and humans are doing a lot to encourage biological invasions. By linking the River Rhine to the Danube we now have all sorts of species in the Rhine that weren’t there originally. As a result of pollution in the seventies, 95 percent of the indigenous species in the Rhine had disappeared. Now that the Rhine is cleaner again you can see that invasive exotics take over more easily than if they had had to fight against all sorts of indigenous species.’

Leen Moraal, insect, tree and forest researcher at Alterra
‘Are exotics a threat? Depends on to what or who. In some cases we are talking about public health risks; there can be a shift in biodiversity and there is also cosmetic damage. The latest news is that an American gall fly has been found on Acacia trees in the Netherlands. Is that serious? No, not for the trees, and not for humans either. But there are cases where things have got out of hand. In America the Asian emerald ash borer has hit so hard that fifteen million ash trees have died in a short time. You can hardly regard a species like this as enriching the biodiversity. An oak hosts about 450 insect species that in turn provide ample food for birds in spring. But the plane, horse chestnut and acacia are sterile trees in the sense that they provide no food for birds. Many exotics arrive in wood used for packaging, but also in imported plants. It’s impossible to prevent this by restricting imports without damaging the economy.’

Hans Velderman, member of the public
‘I don’t think it’s such a problem that we have invasive exotics here. Originally all parts of the world were linked and plants developed in isolation once the continents drifted apart. If I eat a potato, or grain or maize, then I am aware that I wouldn’t be eating any of these if we had tried to prevent invasive species getting a hold. The same is true for cherries. I’m very happy with these exotics. If we were to fight all exotics, our forests would disappear and agriculture as we know it – with cereals, fruit trees, chickens and turkeys – would cease to exist. Seeds and insects make their way into the country with returning travellers, accidentally or on purpose. This is what has made the Netherlands more diverse, richer and more beautiful.’