The problems began when the Duke of Bedford released about a dozen Reeves muntjac, Muntiacus reevesi, on his Woburn estate in 1901. The small deer (about the size and shape of a stocky whippet) originate in subtropical south-east China, but had no problems adapting to the British climate and vegetation.
This is largely thanks to a truly awe-inspiring breeding cycle. Once the does become sexually mature at seven months, they are virtually permanently pregnant and average 1.6 young each year. Although foxes probably take a few fawns, generally the muntjac has no natural enemies, and being so small they are capable of hiding in cover no more substantial than a nettle patch.
As a result they have made themselves at home even in the most crowded parts of the South-east, and numbers in some areas are reaching plague proportions. In the early Nineties, the Joint Nature Conservancy Council estimated that there were more than 50,000 muntjac, concentrated mainly in the Home Counties and the Midlands, but with sightings reported from virtually every English and Welsh county. Today there are certainly far more.
Muntjac are primarily browsers, shunning grass for the leaves and buds of shrubs, trees and flowers. Like all deer they can cause serious damage in young commercial plantations by nipping off the shoots and soft bark of young trees.
Ironically, the worst problems so far are in nature reserves. "They can heavily over-graze the ground cover and are altering the floral structure of some of our best ancient woodlands," says John Hall, of the Essex Wildlife Trust. "In some places we've had no bluebells flowering for almost a decade."
Although Britain's native small deer, the roe (which is considerably bigger than the muntjac), also damages young trees, this is at a higher level and generally causes fewer problems: "At least roe deer have been part of the ecosystem since the last Ice Age," points out Mr Lyster. "Our native plants seem more or less adapted to cope with them, but muntjac are devastating many delicate habitats."
The ancient woodland of the Warburg Reserve, near Henley in the Chilterns, is a good example of this. Managed by BBONT, the Wildlife Trust for Berkshire, Buckinghamshire and Oxfordshire, this mainly comprises mature beech woods and coppices. It is particularly rich in plants, with more than 450 species recorded on the site, including 15 orchids. Unfortunately, the wide variety of flowering and fruiting plants, coupled with the thick undergrowth, also makes it ideal muntjac habitat.
When BBONT bought the 270-acre reserve in 1968 there were only a handful of fallow deer and no muntjac in the area, so damage from deer browsing was not a problem. That is far from the case now: "Fallow numbers have trebled and it's anyone's guess how many muntjac there are," says Nigel Phillips, BBONT's senior reserves officer. "They've occupied a vacant niche, and the flora's suffered dramatically because they nibble at grass level - bluebells, Solomon's seal and herb-Paris all seem shell-shocked."
So what can be done about this menace? "We're seriously thinking about fencing off entire woodland reserves," says Iain Corbyn, BBONT's senior conservation officer. "But this has problems - most obviously because it also excludes other, desirable species and makes it difficult for the public to gain access; and then there's the cost." At pounds 5-pounds l0 per metre, depending on the terrain, this presents a serious drain on valuable resources. Besides, as he points out, the deer are sufficiently small to be regular Houdinis; constructing a genuinely muntjac-proof fence is next to impossible.
Culling is the only sensible answer, according to Hugh Rose, Scottish secretary of the Deer Society and a Suffolk-based farmer, but he says the deer's phenomenal reproductive capabilities make this difficult to do humanely: "Muntjac can be shot all year because they don't have a specific breeding season, but shooting does can be tricky; you don't want to orphan an unweaned fawn," he points out.
As a result, marksmen are advised to follow a strict protocol when culling does - shooting only big, healthy specimens (probably pregnant, without milk-dependent young) and sparing rangy females and those accompanied by males (these have probably just given birth).
Putting this into practice can be awkward. "Our reserves are open to the public so we have to be extremely careful when culling with high-powered rifles," says Mr Corbyn. "In fact we only cull on one reserve [Warburg]."
A bigger potential problem comes from the howls of protest from animal rights activists and wildlife lovers. There is a silver lining to every cloud, however. And certainly Nigel Phillips has a sense of poetic justice. "These deer are an excellent source of meat," he says. "Why worry about BSE when muntjac are available?"
A booklet, `Muntjac', price pounds 3, is available from The British Deer Society. Send an A5 sae to Burgate Manor, Fordingbridge, Hampshire SP6 I EF.