The 40 walled acres in the heart of the capital are so cloistered and protected from disturbance that they harbour 325 wild-plant species and 30 species of breeding birds.
They also contain 112 types of spider, 56 types of moss and 16 types of land snail, as well as a multitude of other species.
The garden's riches are revealed in its first wildlife survey in 35 years, published by the London Natural History Society with the encouragement of the palace's head gardener, Mark Lane.
"Buckingham Palace garden represents something unique in Britain," the report reads. "Here, in the heart of of one of the most urban areas of Europe, surrounded by traffic and a very large number of people, lie [40 acres] of greenery which remain largely secluded, other than briefly each summer during the garden parties."
The garden is adjacent to three royal parks - Hyde Park, Green Park and St James's park - but is much denser in its plant cover than they are.
The survey goes into detail on the types of small life forms it contains, such as slime moulds (the strange crosses between fungi and animals that live on the bark of trees), of which the garden contains 68 species. It also holds 60 species of bryophytes - mosses and liverworts - and 39 species of lichen.
But it is in the higher plants - wild ones, rather than those put there by the gardeners - that the garden is really rich. The 325 species recorded in the survey include bluebell and primrose, meadowsweet and foxglove, dog violet and ladies' bedstraw, scarlet pimpernel and lords-and-ladies. There are real surprises, too, such as a colony of spotted orchids which produced 30 flower spikes this year.
"To find wild orchids growing in the heart of central London is quite amazing," said Elinor Wiltshire, a botanist who surveyed the garden's plant life with David McLintock, another wild-flower expert who carried out the first survey in 1963.
"The garden has a very special character. You might expect it to be just lawns, flowerbeds and shrubs, but it is much more than that and its wild flora is absolutely wonderful."
The garden's bird life is also very rich, the report reveals, with 58 species seen during the survey, of which 30 bred. Breeding species include jays, barn owls, long-tailed tits and great crested grebes (on the lake), while visitors include green and great-spotted woodpeckers, tree creepers, sedge warblers and goosanders. The lake even plays host to kingfishers. "The density of wild birds [in the garden] is probably higher than anywhere else in the capital," the report says.
Doubtless because of the high wall, the survey found only six wild mammals: fox, grey squirrel, brown rat, wood mouse, house mouse and pipistrelle bat. The four-acre lake is poor in fish, with only three species recorded: roach, perch and gudgeon. The dace, recorded in the 1963 survey, have disappeared, and the occasional common frog is the only amphibian or reptile in evidence.
However, the garden in general is "a wonderful haven for wildlife", the report declares.
"If you go for a walk in one of the parks nearby, you just have a billiard table to walk on, some lollipop trees and dog muck everywhere," said Colin Plant, who edited the report. "But the Palace garden is a wildlife oasis. That there is so much, in one small place in the centre of London, is quite remarkable."
A second volume of the survey will be published in two years' time, dealing mainly with the garden's insects, such as butterflies and dragonflies.
The Natural History of Buckingham Palace Garden, London. Part 1. Available from 4, Falkland Avenue, London N3 1QR, price pounds 5 plus pounds 1 p&p.Reuse content