Exotic plants which are on the increase in temperate zones have better immune systems than related indigenous species. This is why global warming leads to invasions by plants from the south, according to conclusions published in this week’s Nature by researchers from the NIOO and Wageningen UR.
Very invasive plants are less bothered by hungry insects or disease-carrying soil organisms, and can therefore spread more rapidly than indigenous species, driving them out. This emerged from tests carried out on nine indigenous plants from the Millingerwaard and six successful exotics, including species such as the late goldenrod, the biennial wormwood and the narrow-leafed ragwort. There were exotics from North America and South Africa as well as from southern Europe. ‘Species which have spread exponentially in the past ten years in particular, and are therefore potentially invasive’, explains Van der Putten. ‘They are not yet pests like the American black cherry, but it might only be a question of time.’
The researchers used the desert locust and the peach leaf louse as above-ground enemies. In theory, all the plants should react to them in the same way: they’ve never encountered the locust before, while they are familiar with the leaf louse. But that’s not what happens. The exotics are less bothered by the insects and their growth is less inhibited by the soil organisms from the Millingerwaard.
‘They seem to have characteristics that make them more resistant to enemies’, says Van der Putten. ‘They must have encountered quite a few on their way from southern Europe. So they’ve been through a kind of sieve, and that’s what makes them so resistant and so different.’
Plant species which spread successfully during climate change seem to have the same characteristics as invasive species from other continents. Researchers therefore believe that there is a real danger of bio-invasions.
Yet not every exotic species is potentially invasive. Van der Putten estimates that about one in every thousand exotics becomes a pest. ‘Compare them with pickpockets. You can establish a typical profile for a pickpocket, and a thousand people might fit it. But only one of them actually becomes a pickpocket. We have now proved that these exotics do fit the profile.’