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Nature Studies by Michael McCarthy: Why are some alien invasions welcomed?

Stick insects are some of the world's most curious and recognisable creatures. They provoke fascination not only for their startling resemblance to sticks or leaves, a perfect piece of evolved camouflage, but also for the sheer anorexic skinniness of many of them; perhaps there is an ancient gene in us that triggers alarm at the sight of anything preternaturally thin.

Officially, stick insects are phasmids; there are more than 3,000 species, mostly in the tropics, and in fact it is a phasmid, the Lord Howe Island stick insect, which is believed to be the rarest invertebrate in the world. It was endemic to Lord Howe Island, isolated in the Pacific nearly 400 miles east of the Australian mainland, until it died out about 80 years ago; but in 2001 a tiny group of the creatures, no more than 30, was found on Ball's Pyramid, an uninhabited and largely lifeless volcanic cone 13 miles away; they are now being captive-bred in Melbourne Zoo.

You would doubtless be surprised to learn that there are British stick insects; but there are. Three New Zealand species have become naturalised in the UK over the past 100 years, all in the South-west: the prickly stick insect, first found in Paignton in 1909; the unarmed stick insect, found in Truro in 1910; and the smooth stick insect, first found in 1949 in Tresco Abbey Gardens on the Isles of Scilly. All are thought to have been brought in by Cornish nurseries importing New Zealand plants; they have survived thanks to the warmth of the South-west winters, and this autumn Buglife, the invertebrate conservation charity, is organising a survey entitled the Great Cornish Stick Insect Hunt to see how they are spreading and how they survived last winter's unusually fierce cold.

Jolly good. No one appears to take exception to the presence of these fascinating aliens from across the world; it seems to be the case that our stick insects are entirely benign, and to discover one in your Cornish privet hedge is merely charming. But what if they were venomous spiders? How would we regard them then? The fact that insects from Down Under can end up flourishing in British gardens only emphasises, in our increasingly globalised society, that the phenomenon of alien or invasive species is likely to become one of increasing concern, as many aliens can have devastating impacts on the ecosystems which they invade.

Take birds. Nearly 200 bird species have gone extinct in the past 500 years; some were lost through over-hunting, like the great auk or the passenger pigeon, but the vast majority were birds of islands, especially flightless ones, which were devastated by the introduction into their ecosystems, by human actions, of foreign species such as rats or cats or snakes. Hawaii has lost 30 per cent of its original bird species, while Guam, the island in the western Pacific, has lost more than 60 per cent of its birds in the past 50 years because of one species, the brown tree snake, accidentally introduced from mainly Asia some time after 1945.

The International Union for Conservation of Nature now regards the introduction of non-native species as the second greatest threat to the world's biodiversity after habitat destruction. But the situation presents us with difficult choices, for often the only way to combat the dire effects of an alien is to extirpate it, assuming that is possible. (There would be little profit in attempting to persuade the brown tree snake to change its behaviour).

And in recent years there has been a growing resistance, at least at the more radical end of society, to humans playing God, as is sometimes said, and taking the decision to wipe out a creature, that other creatures may survive. Now this resistance has gone further, and is morphing into the argument that being prejudiced against creatures which are foreigners is no different to being prejudiced against people who are foreigners and have come to share our society.

In Britain the argument is becoming particularly polarised with regard to the grey squirrel, the American animal introduced in the 19th century which has now driven the more attractive native red squirrel to extinction in most of England (largely through being a carrier of the parapox virus, which is fatal to reds, but harmless to the greys). You can actually see right-left political battle lines being drawn up about Sciurus carolinensis; the fervent wish to extinguish it on the one hand, often with country people, and an angry resistance to the idea on the other, which perhaps has more urban origins.

Countering the effects of invasive species on the natural world is going to be difficult enough in the years to come without politicising it. It is an issue over which we need to be practical rather than judgmental: let's not demonise grey squirrels, which are only doing what comes naturally. Yet there is no denying that their expanding presence in Britain has been of major deleterious consequence for our wildlife, and will continue to be so; ultimately, their presence will entail the red squirrel's entire destruction. Whatever we decide to do about it, we need to be guided by common sense; although that, as Dr Johnson said, is the rarest human virtue.

Fuelling an unwelcome growth in monster nettles

In our front garden in suburban south-west London a stinging nettle is growing which is now about nine feet tall. I didn't notice it at first as it was growing under the beech hedge, until it burst out of the top; it's still growing and doubtless will only be stopped by the frost. Ask me how and why and I will reply: cars. The deposition of nitrogen compounds from millions of car exhausts is increasingly an unwanted fertiliser of the soil, everywhere.