Why nature should be left to take its course

Once rarer than hen's teeth after a ruthless extermination by gamekeepers and farmers, they are now as common as crows, circling the motorway, perching in groups on fences and dead trees, gathering each morning to gorge on the spoiled meat laid out for them by the local landowner. Residents don't even look up any more when that high call sounds over the sprawling gardens. The story isn't quite the same yet in rural Wales, which saw the first successful reintroductions, or in Scotland, where a glimpse of a red kite is still as exciting as an osprey sighting once was, but it seems only a matter of time before one of them reaches for the green ink to demand a cull.

Planned (re)introductions are a delicate ecological balancing act that call for detailed understanding of potential sustainability and impact on other species. Unplanned or uncontrolled introductions are another matter entirely, and almost invariably disastrous. For the moment, the Oxfordshire red kites have much the same status as bred-and-fed pheasants on a shooting estate, and likely soon to become a similar target for anyone with a gun and a grudge.

There are more positive avian examples around: the osprey most obviously, and the avocet at Minsmere which became the symbol of the Royal Society for the Protection of Birds and for responsible encouragement of marginal species, but recently a white-tailed sea eagle (tagged and unmistakable) was seen on the shores of Loch Striven in Argyll, many, many miles from the species' original point of reintroduction on the Small Isles. A positive sign, perhaps, that a once extinct species is now prospecting for new territories.

There are those who would like to reintroduce the wolf, last seen in the aftermath of Culloden, to Scotland, and to introduce bears and beavers, the latter I assume to help soften up the populace on the prospect of further dam-building in the Highlands. These are theme-park ideas, virtually unworkable and a needless provocation to exotic poaching.

Almost as controversial are animal culls. Like parts of Scotland, Oxfordshire is overrun with deer, many of them introduced Sika deer. As well as presenting a problem to forestry and late-night motorists, there is a growing risk of cross-breeding and consequent degeneration of native strains. Short of mass contraception, rifles seem to be the only answer, albeit one that guarantees a dam-burst of green ink, pro and anti.

The Small Isles were back in the news last week, but this time on the subject of brown rats and woodmice. The latter, larger than usual and of special scientific interest, are indigenous. The former arrived on boats, liked the place and took over. The dilemma: how to eliminate the bad guys without doing harm to the Beatrix Potters. The solution: round up the woodmice - I assume they don't use Border collies for this - and ship them off to Edinburgh Zoo and the Highland Wildlife Park in Kingussie to form sustainable populations while the brown rats are poisoned. There are tiny ethical questions here and some interesting evolutionary questions - like: will the mice that return to Canna be different in any regard from their time in internment? - but no passionate outcry.

Not so when Scottish Natural Heritage proposed a cull of hedgehogs on the Uists and Benbecula. For a time, Mrs Tiggywinkle became a cause clbre, a lady called Tina Swindle formed Uist Hedgehog Rescue and was thrilled when an attempt to use dogs to hunt down the islands' gypsy cutlets (I have the recipe) failed, presumably both practically and legally. Meanwhile in Edinburgh, bureaucrats stroked their designer beards and muttered about spiny problems.

Irony - and the lack of it - always stalks these issues. Probably the vast majority of introductions are accidental and unplanned. Every schoolchild knows what a ship's rats can do to the eggs and chicks of flightless birds. But what of deliberate introductions that go wrong? I live on a cat-free shooting estate. This spring, a single tabby moved into a house not controlled by the estate and systematically wiped out a generation of common gull chicks on the beach, together with uncounted oystercatchers, ringed plovers, willow warblers and a goldcrest family.

None of these are exactly endangered species. Think, though, what happened on Stewart Island in New Zealand where the kakapo - the world's heaviest parrot, nocturnal, and really half-Polly, half-owl - has been brought to the brink by feral cats, the descendants of household pets brought there to minimise the loneliness. There's an ironic symbolism in the discovery that snakes introduced into Cuba have done so well on a diet of rats and frogs that they've now exploded in numbers.

Power at Camp X-Ray and Guantanamo Bay is regularly shorted out by serpents slithering along the lines. If you want the most vivid illustration of what uncontrolled introduction can do, take a look at the cover of REM's Murmur. That bizarre, smothering plant, the perfect emblem for Michael Stipe's muffled, opportunistically allusive vocals, is a legume called kudzu, now better known as the "mile-a-minute vine". Introduced to the United States as a novelty at the Japanese Pavilion in 1876, it found it liked the place as much as brown rats liked Canna, and it has pretty much taken over the south.

We're not immune ourselves. Parts of the west are infested with rhododendron ponticum, and there is now a crisis line in Cornwall devoted to the eradication of Japanese knotweed. How strange that the Japanese, who never managed to invade continental America, should have done more damage with a creeping vine than with kamikaze.

Too much intervention, too slackly policed, is a form of environmental suicide. Small-scale rescues like the Canna Woodmouse Extraction are viable precisely because they are small. The sea-eagles are coming back slowly and Scottish red kites are establishing at a sensible rate. But you don't have to be Enoch Powell to recognise there are dangers as well.

Extinctions have happened through history, with or without human intervention. The best, fatalistic advice is probably: leave well alone.