Researchers want to find out if decreasing numbers of snakes caused by urbanisation has led to inbreeding among adders

With a quick dart of the arm, snake catcher Nigel Hand snares his prey and holds the wriggling adder aloft.

The bronze snake, hissing and flicking out its black forked tongue, has been snatched from under its gorse-bush home as part of the first ever genetic survey of the UK's only venomous snake, amid fears that dwindling populations are behind worrying signs of inbreeding, such as missing eyes and deformed spines.

But the young female caught by Hand looks healthy, barring a 3cm-long scar on her back left by a buzzard or a kestrel. She squirms friskily, despite having just emerged from a five-month hibernation, as a bright spring sun shines through the alders and birches that fringe the riverside meadow.

"Adders are living on the edge in more senses than one," says Jim Foster, national reptile specialist for Natural England. They like to live on the margins of open ground and in the shelter of woods, basking in the sun then diving for cover. But habitat loss of heaths and meadows means they are also on the edge in population terms, with at least a third of the 1,000 known populations diminishing. Those smaller groups have fewer than 10 adults and even the bigger clans have just a few dozen.

"We also still have a few problems from people going out and killing them, even though it is illegal," says Foster. He says the dangers of adders are small – the last death from an adder bite was in 1975 – and that attitudes to the snake, also known as the viper, have improved vastly over the years.

Taking a DNA sample from the 50cm-long adder is harmless if undignified, with a small swab inserted into its vent, an opening that doubles as the excretory and genital orifice. "Swabbing the mouth is quite dangerous, so we thought we'd try the other end," explains Foster. Samples are being taken from both small and large populations at 16 sites across the country. Scientists at Oxford University will then compare the samples to see if the smaller clan groups are indeed genetically impoverished.

"These abnormalities are very worrying, but we don't know yet if it is to do with a lack of genetic diversity. That's why this project is essential," says Foster. Studies of rattlesnakes in the US have shown clear links between inbreeding and deformities. Malnutrition and disease are other possible, if still worrying, causes.

The adder project highlights a looming issue for wildlife in the UK: maintaining genetic diversity in isolated populations. "Genetic management is a new field and we have to think about it now in the UK, as our landscape becomes ever more fragmented," says Foster.

The preferred solution is to provide wildlife corridors to allow populations to mix, for example by converting strips of farmland to meadow, or creating a series of glades through overly dense woodland. But this will be impossible at adder sites surrounded by built-up areas. In those cases, individuals will be translocated across the country to reinvigorate the gene pool. It is a last resort, says Foster, but has been done successfully with adders in Sweden and with natterjack toads in the UK.

A small population of the toads in Lincolnshire had become dangerously inbred, Foster explains, so two years ago Natural England introduced toads brought in from Bedfordshire. "The alternative was to let the group go extinct," says Foster.

But translocation does have risks, as the infusion of new DNA can obliterate special genetic adaptations to local conditions.

This happened in the Tatra mountains in Slovakia where mountain ibex from Turkey and Egypt were used as an influx of new blood, but changed the breeding pattern so much the herd became extinct.

Back in the Surrey meadow, the conservationists combing the undergrowth for the next snake to swab are determined to ensure modern DNA technology can play its part in halting the decline of the adder.

"They are a sentinel species – high up in the food chain. So a good healthy adder population will support all the species below them, giving a good, healthy ecosystem overall," says Jamel Guenioui, who manages the Surrey Wildlife Trust site. Adders feed on field voles and lizards, which in turn eat grasses and insects respectively.

Hand points out the extraordinary display known as "dancing adders", which will be seen in the coming weeks as males battle to win mating rights. "It's a wrestling match of strength and stamina, highly ritualised. It's one of the great spectacles of nature in the UK."

"We have lost the wolf, the lynx and lots of other key species," he says. "But, although there are very few of them, we still have this small, beautiful and enigmatic snake."

Adder facts

• Adders are the only snakes that lives within the Arctic circle and this adaptability to cold may be why it is the UK's only venomous snake

• Most reptiles lay eggs, but adders give birth to live young, which are about as long as a pencil. This is probably another adaptation to cold climates

• Females do not breed every year, as they need enormous stores of energy to see them through hibernation and birth

• Some adders have yellow tips to their tails, which may be worm-like lures for prey, or mating signals

• Adders eat about six to 10 animals a year, killing them with venom delivered through 7mm-long fangs

• Although rare, some individuals can be pigmented entirely black, the "blackadder"