Roads, railways and the rapid rate of land development are breaking up the landscape and fragmenting habitats, causing big problems for many species by isolating populations and creating barriers to movement.
Among the many species of small mammal affected are dormice, specifically the hazel or common dormouse (Muscardinus avellanarius). They’re our only native species of dormouse, with a population that is declining in both number and range.
The fragmentation of our countryside by railways and roads can potentially have a serious detrimental effect on dormouse populations being able to mix and breed
They are a rare species that is vulnerable to extinction in the UK if something isn’t done.
One important and successful method employed by the People’s Trust for Endangered Species (PTES) is the managed release of breeding pairs into woodlands in certain parts of the UK since 1993.
The dormouse, however, is an arboreal species that lives in the tree and shrub canopy, and usually at very low densities, explains Ian White, the PTES dormouse officer.
“The fragmentation of our countryside by railways and roads can potentially have a serious detrimental effect on dormouse populations being able to mix and breed.”
Connected woodland canopies are also vital for dormice as they search for food, and as the juveniles move out the area they were born in to seek out their own territories in autumn.
And that’s where wildlife bridges offer a potential lifeline.
The UK already has a number of wildlife bridges to help dormice and other wildlife cross dangerous open spaces safely. They include simple arboreal structures like ropes and poles, to more complex tunnels and tubes that connect tree branches; as well as purposely built "green" land structures that resemble more conventional bridges covered in vegetation over busy roads.
But there is still much debate as to their cost and just how effective they really are.
For example three aerial bridges of wire mesh tubes suspended between trees and tall poles were specially assembled over a new bypass near Pontypridd, Wales at a reported cost of £190,000. They divided opinion on whether it was money well spent and how many dormice were actually using it.
While there is little proof that dormice use the arboreal style of bridge, in 2012 dormice were found to be using Britain’s first wildlife land bridge over the A21 at Scotney Castle.
“Apart from this green bridge at Lamberhurst in Kent, where dormice were found to have bred on the bridge, there has been no evidence to date, of wild dormice using a smaller wildlife bridge,” says White.
Better by design
But that situation might be about to change.
A pilot study by the PTES in Briddlesford Woods on the Isle of Wight, a perfect location due to the pristine ancient woodland and very healthy population of dormice, tested a new design of wildlife bridge inspired by a successful conservation project in Japan, and featured on BBC Two’s Autumnwatch.
It's a very different style to other arboreal dormouse bridges in the UK, with a more tubular and triangular structure. The modular units stretch over an existing bridge that spans a railway track running through the woods; these areas don’t need linking – dormice are already on both sides – it’s purely to find out if this design works.
“The bridge is based on a Japanese design that had been shown to work for the similar sized Japanese dormouse,” says Mr White.
This bridge is only a prototype and will be taken down over winter and re-erected next year, when conservationists will look at the frequencies of species crossing the bridge, compared with those crossing on the ground.
“If it is shown to work the aim will be to produce a commercial design that could be used to link areas where dormice are known to exist,” he says.
It's early days but the preliminary signs are encouraging as a dormouse and red squirrel have been caught on camera using the bridge, the dormouse can be seen crossing the bridge in the short clip below (credit PTES).
Crucially, this design is a fraction of the cost of some other dormice bridges.
What value are you able to put on a structure that could reduce roadkill and enhance the genetic diversity of a range of species to enable populations to better survive in the future?
As a country, explains Mr White, we spend millions on roads and rails to try and make our journeys easier; these slowly fragment our countryside into smaller and smaller areas.
“What value are you able to put on a structure that could reduce roadkill and enhance the genetic diversity of a range of species to enable populations to better survive in the future? They don’t need to be expensive and we may be able to adapt existing structures.”
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