COUNTRY & GARDEN: Attack of the Ninja turtles

A STRANGE sight greeted me on a recent walk. In among a flock of grub-picking black-birds was one with white tail-feathers. Distrusting both eyesight and memory, I went back a few days later, and there it was again - indisputably a female blackbird with at least two snowy white tail-feathers. What strange mutation is this, I thought. A millennial portent? Proof of the havoc we have wreaked upon the natural world? Or the progeny of a blackbird and an interloper from another land?

That last idea is not as far-fetched as it sounds. The ruddy duck, an import from North America, has so successfully spread its genes via the female of the European white-headed duck that the latter species may be forever altered.

Import is the key word here. If creatures arrive naturally, all well and good, it seems. They come, they go, and die if they don't. In any case, there's not a lot you can do to keep out things with wings and fins. Be they beetles, butterflies, birds or fish, they are wont to cross geographical boundaries on wing, wind and tide. If they are helped along the way by man, either intentionally or accidentally, it's usually a disaster. Even pretty Psittacula krameri, it turns out, might be a threat to native species after all. Londoners are by now used to the un-English screeching brilliant- green flash of the ring-necked parakeet, a native of south-east Asian rainforests. Whether our indigenous bird population is used to either the sight or sound of them is another matter: where many parakeets lark about, such as in Richmond and Bushey Park, mob-winged jackdaws and crows can be seen harrying the visitors, as if to oust them by sheer force of numbers.

On the face of it, the locals should have it easy. There are only a few thousand parakeets in the whole country, their strongholds being Surrey and south-west London. So why worry? Why not welcome these colourful creatures which add to the gaiety of the skies? Ask the owls and woodpeckers, arguably just as weird and wonderful. When the first parakeets escaped from carelessly latched birdcages some 30 years ago, it was thought they would never survive. But our ever-warmer winters have ensured not just survival but hugely successful procreation - in the very tree-holes used by owls and woodpeckers.

We will be sorry if the night call of the one and the drumming of the other do not ring down the years of the third millennium. Or rather, our descendants will be. That is the trouble with introduced species: the damage done to native wildlife is often not realised until it is too late. Look at the plight of the red squirrel, which has all but lost the battle against its imported grey cousin. Look at the American mink, a minx that is now probably ineradicable.

That inflated symbol of the next 1,00 years (or is it of the last two?), the Dome, was at one point itself under threat from an incomer far more fearsome than a parakeet. Chinese mitten crabs - horrible-looking things with hairy claws believed to have arrived via ships' ballast, migrate up and down the Thames, burrowing into and destabilising the banks as they go.

Apparently, there are places at low tide where you could gather yourself a banquet in no time at all - if you happen to like crab gonads. These are prized delicacies in China, making the gourmets of Chinatown our only defence against the spread of this unwanted crustacean.

(You may be thinking that crab balls must make a very meagre morsel - not those of the mitten crab, which grows to the size of a dinner plate and whose testicles fill over half its shell during the autumn breeding season. The claw meat is good, too.)

Sixty or seventy other species are cited by the National Rivers Authority as proof of the success of its Thames clean-up. Bream, roach and salmon, fine; but butterfish, trigger fish, pine fish, West African elephant fish and sea scorpion? Perhaps these are no threat to the local species.

On the other hand, everything eats something, so it's quite likely that some of them are tough tourists who won't be fobbed off with a sub-standard meal - a feast of young, sea-going, Thames-spawned-and-reared salmon might be more to their taste. And the day will have come when mitten crabs and sea scorpions reign uncontested in an otherwise barren river.

Ponds in Regents Park and on Hampstead Heath were stripped of native species by hungry Louisiana Red Swamp Crawfish. Urban myth has it that the originals were released by the owner of a Cajun restaurant who had over-estimated his customers' appetite for the things, but they are also sold (as "Chinese Lobsters") by garden centres and aquarium suppliers. What they don't eat probably dies of the plague they carry. Greenwich Park wardens had no explanation for mysterious amputee ducks and a sudden dearth of ducklings.

The culprit? A large, extremely carnivorous American snapping turtle, caught trying to invade the park's police mess room. Probably imported during the Mutant-Ninja-Turtles craze in this country and released by a despairing owner who had no idea that the sweet little creature he'd bought would turn into a hissing, biting monster. Happily, there aren't too many of these around. Unhappily, another Ninja-inspired import is still causing havoc. The red-eared terrapin, thousands of which appear to have taken over ponds across London and the South, also eats anything up to the size of a small duck, so wherever it appears everything else disappears.

This awesome reptile might have some use, however. It might not only be partial to, but capable of killing the voracious North American bullfrog, another invasive importee. London's Evening Standard had some fun with this a few years back, running "silly- season" reports of giant "killer frogs". I seem to remember the story eventually making the national TV news.

There was a serious point behind the tall tales, however, which was that we should learn from past mistakes. It is just too much of a risk wilfully to import alien creatures, either for pleasure or profit, when we know nothing of their potential effect on the indigenous species. We live in interesting times, particularly climatically, and can't be too careful. Remember the ruddy duck.