A rare insect which has become extinct in Britain apart from on Dartmoor is being saved by a herd of native ponies.

The bog hoverfly has suffered a "catastrophic decrease" in range and was thought to be fully extinct until rediscovered in 1993.

It is found only at a number of sites in Devon and is classified as "vulnerable" in the UK.

Now, the Dartmoor National Park Authority is using a herd of ponies as a "habitat management tool" to prevent the wet heath from turning to woodland.

The sturdy creatures tend to eat bigger plants, which leaves the smaller ones favoured by the bog hoverfly to flourish.

Ecologists at the authority say the heath is full of typical moorland plants such as bell heather, bog mosses and bog asphodel.

As well as the bog hoverfly, it is home to a number of other insects, including fritillary butterflies.

If left unmanaged without the ponies help, the habitat would eventually scrub over and turn into woodland and the wildlife surviving there would be lost.

Naomi Barker, Ecologist for Dartmoor National Park Authority, said: "We are using native type ponies to graze the heath to maintain the existing community. The ponies are an excellent habitat management tool. They will nibble at the bigger tussocks of rush and grass and therefore give the smaller plants, such as the mosses and bog asphodel, a chance to persist. Their small hooves result in minimal damage to the ground." Ponies have lived on Dartmoor since the Bronze Age, with archaeological digs uncovering hoof prints from the period.

The Dartmoor Pony is now a rare breed and is distinct from some of the other ponies that live on the moor.