Interloper?

Here is the Afon (River) Gwaun and on its bank a patch of Japanese knotweed Fallopia japonica. Of all invasive, non-native plants this is the one that has attracted most revulsion because it proliferates so rapidly via underground rhizomes which are very deep-rooted, persistent, and almost impossible to destroy. The plant has no natural predators in Europe (so nothing eats it) and can regenerate from a small pieces of root which is why it is so often seen along a watercourse where fragments have washed up or along railway sidings where garden refuse has been tipped. Once established it can blight the land because it has to be eradicated before building can take place.

Japanese knotweed was introduced into British gardens in the late 19th century; it's a handsome plant that quickly colonizes damp ditches and neglected corners. To its credit it is not toxic, it doesn't sting or stink, nor does it have thorns. Its unobtrusive flowers are attractive to bees and the young shoots are eaten in Japan. But it's been designated one of the world's 100 most invasive species and in the UK it's an offence to plant or to unintentionally disperse it - it has to be burnt or buried according to strict measures. Huge sums of money have been spent attempting to control it and recently permission was granted to release a tiny Japanese insect that feeds upon it in the hope that this will check its spread.

In my recent blip about rewilding I mentioned the idea promoted by some environmentalists of allowing underpopulated areas of the country to revert to nature. Without human intervention this would mean that native species would have to compete with non-native/introduced plant (and animal) species, in many cases to the advantage of the latter. Would it matter? Do we need to control invasive species so actively? It's easy to forget that many of the plants and trees we regard as a natural part of our flora were originally introduced, albeit as far back as the Roman invasion (that word again!)
Are these species invaders or are they migrants, doing what human have done since humans evolved by moving into new and favourable locations? Are they, like us, colonists that spread into different parts of the world regardless of the indigenous populations? Are they a threat to our way of (plant) life?

I've always been chary of the often frankly xenophobic language used in relation to Japanese Knotweed ("malevolent intruder" - Daily Telegraph), along with Himalayan Balsam, Rhododendron and other aliens: usually described as 'rampant' 'prolific' and in need of destruction and exclusion for fear they will 'crowd out' native species and 'take over' our countryside. So it was not entirely a surprise when I recently discovered that in Nazi Germany landscape architects explicitly promoted the planting of native species only, and botanists followed suit by declaring war upon a harmless small woodland plant, Impatiens parviflora.
"As with the fight against Bolshevism, our entire Occidental culture is at stake, so with the fight against this Mongolian invader, an essential element of this culture, namely, the beauty of our home forest, is at stake."

OK, Japanese knotweed isn't harmless but nor is it a malevolent intruder: it's simply doing what we all do, finding somewhere to put down roots.

[A good short piece on the dangers of nativism and the demonization of aliens: http://michaelpollan.com/articles-archive/against-nativism/ Other articles if you're interested: http://arnoldia.arboretum.harvard.edu/pdf/articles/483.pdf http://arnoldia.arboretum.harvard.edu/pdf/articles/646.pdf]