Introduced and invasive species; new weeds?
If a weed may be defined (albeit losely) as 'a plant in the wrong place', how can this come about? Are we as a species responsible or to blame? In this country, 'old man's beard' or the wild species of Clematis vitalba is a plant often growing in country lanes or in woods, where it causes few, if any, problems. As the plant grows, it forms a thick, woody stem which entwines around any convenient supporting structure in its vicinity. It uses its leaf stalks (or petioles) to help it climb, these twist and twirl around nearby structures.
Hence, it will grow up the trees and shrubs in hedges and woodlands. We consider it to be a 'normal' plant of the hedgerow, but in places like New Zealand it is viewed with some concern. Clematis was introduced there as a garden plant, but its wind-blown seeds soon found their way to woods and forests. In these, it has grown rampantly up into the canopy. This has reduced the light available to the plants in the herb and shrub layers, reducing biodiversity. Also, the sheer weight of the Clematis can break the supporting trees.
Plants that are introduced to places or areas, where their natural checks and balances are absent, can become a problem or a weed. In some parts of the world, introduced species of Lupins or Lupines have become a problem. In Iceland, Lupinus nootkatensis is one such species. The natural environment of this species is gravel ridges or bars, dry slopes near rivers or the coast. It can, however, establish itself on artificial or disturbed environments such as the stony, sandy edges of roads and railways.
In the 1960’s, this Lupin was spread with active encouragement, both locally and nationally in Iceland especially. It spread particularly in places where ‘erosion control’ was an issue. It is now spreading, especially in areas where sheep no longer graze or exert any control over the spread of the lupin (sheep eat the small seedlings, so the plant cannot establish itself). The Lupin has moved into areas where there has been soil erosion and habitat destruction. The volcanic sands and lowland flood plains are ‘prime sites’ for colonisation by Lupinus nootkatensis, as are some heathlands. It is feared that the lupin will outcompete many native species, reducing the biodiversity of the systems.
In the UK, there are problems with various introduced species such as
- himalayan balsam
- giant hogweed
- japanese knotweed
- evening primrose
- canadian pondweed
- pigmy weed
- Rhododendron ponticum (plus some its pathogens!)
Most countries have problems with some introduced plant species, as their natural pathogens or herbivores are absent. Many plants have been moved around the world and now exist in places remote from their original home(s).
