Ecological and economic impact

Whether they are called Japanese knotweed, giant hogweed (Heracleum mantegazzianum), Himalayan Balsam (Impatiens glandulifera), rugose rose (Rosa rugosa), garden lupin (Lupinus polyphylus), water primrose (Ludwigia grandiflora) or something else, these invasive plants are exotic species which have for the most part been deliberately introduced into our regions following the first gardening crazes. ‘In Belgium,’ continues Mathieu Halford, ‘there are around sixty species, which is certainly a very low proportion – less than 1% – out of the whole of the ornamental plants available on the market (tens of thousands). But,’ he insists, ‘the impacts can be considerable.’

Whilst we’re on the subject, what are these impacts and how does the plant manage to lay low its local competitors? ‘First of all there are impacts on indigenous species, bringing about losses of biodiversity,’ as was explained earlier. ‘There are also impacts on ecosystems. For example, certain invasive plants can modify the physico-chemical properties of the soil. The knotweed, to take its example, has a network of rhizomes – underground stems – which vary between 3 and 7 metres in depth; that enables it to pump up mineral elements deep down in the soil and to distribute them within the surface layers of the soil (the humus). The content in mineral elements is changed within them, which affects the nutrient cycle and the composition of the humus in comparison with what is found under ‘normal’ vegetation. Other invasive plants, aquatic ones, are for their part capable of asphyxiating water surfaces in forming dense carpets which cover the surface, thus preventing light from penetrating it and slowing the circulation of oxygen in the aquatic milieu.’ To be even more competitive, certain of these little plant invaders adopt the strategy of releasing, at the level of their root system, toxic substances into the soil and quite simply preventing the development of other plants. ‘And then there are finally invasive species from the Fabaceae family (the legumes, such as for example Robinia pseudoacacia and Lupinus polyphylus) whose specificity lies in capturing atmospheric nitrogen and freeing it into the soil. They thus enrich the habitat in terms of nitrogen, which causes problems in semi-natural environments: numerous environments of high ecological value are in effect characterised by soils called oligotrophic, in other words low in nitrogen; maintaining these conditions is vital in terms of the habitat and its vegetation characteristics. If a leguminous species invades this type of environment, if it enriches the soil in nitrogen, it will modify the botanic composition and the soil conditions and the vegetation typical of this habitat will be affected.’ The ecological impact of certain invasive plants is significant because not only is the flora affected, but the fauna which is connected to it will also be disturbed – pollinating insects, the flora of the soil and the bird populations, amongst others. It is important to specify that not all invasive plants have the same environmental impact. Certain are harmful to the biodiversity, whilst others are less problematical in Belgium in comparison to other European countries. Next to the ecological impact there is also an economic impact as it is very difficult to curb the advance of these species, hardy to say the least, once they have established themselves in natural environments. The methods which can be employed are extremely costly. The costs incurred by nature reserve managers are thus sometimes astronomical. Two examples: in Germany the annual costs for managing giant hogweed are estimated to be 12 million Euros; in Flanders a million Euros is required each year to manage water pennywort (Hydrocotyle ranunculoides).