Réserve naturelle régionale de la galerie du Pont-des-Pierres Protected Area and Landscape

The gallery at the heart of the reserve has an unusual industrial origin. In 1929, as part of a hydroelectric project on the Valserine river, engineers began excavating a diversion tunnel nearly one kilometer long to obtain a sufficient head of water. The planned power station was never completed, but the tunnel itself was finished and later came into the possession of Électricité de France in 1947. Abandoned by industry, the gallery was progressively taken over by nature.

Speleologists from the nearby town of Bellegarde-sur-Valserine were the first to document bats using the gallery as early as 1969, and systematic winter inventories began in 1972. By the late twentieth century the site had become recognized as a significant hibernaculum for cave-dwelling bats, and in 1998 grilles were installed at the entrance. These grilles allow bats to pass freely while excluding human visitors, preserving the security of the underground colony.

Formal protection followed relatively quickly. On 9 December 1997, the site was classified as a réserve naturelle volontaire, and the designation was inaugurated in June 2001 by Dominique Voynet. The protection regime was later modernized and on 8 July 2009 the site was reclassified as a réserve naturelle régionale, aligning it with the broader French framework of regional nature reserves. The management role has been carried successively by the Centre ornithologique Rhône-Alpes (CORA) Faune Sauvage and, after restructuring, by the Ligue pour la protection des oiseaux Auvergne-Rhône-Alpes.

The landscape of the reserve is defined by the deep, narrow gorges of the Valserine river in the southern Jura. The Pont des Pierres itself crosses the river at a striking point where cliffs rise sharply on either side, and the site commands open views into these gorges. The entrance of the gallery sits at roughly 445 meters in elevation while the river below flows at about 386 meters, illustrating the steepness of the terrain.

The artificial gallery runs through the rock below these cliffs, and although it is not a natural landform, it has become an integral part of the reserve's landscape character. Its 931-meter length, four openings to the exterior, and a ventilation shaft give the tunnel its particular microclimatic structure, which in turn defines how bats and humidity-sensitive organisms distribute themselves within it.

The natural value of the reserve is closely tied to the geology and microclimate of the Valserine gorges. The cliffs and rocky slopes support a mosaic of habitats, from exposed rock faces and thin cliff soils to riverside vegetation along the Valserine. The protected plant Aster amellus, the European Michaelmas daisy, is documented on the site and is one of the indicators used to assess the botanical interest of the reserve.

The gallery itself is the core of the site's ecological identity. Its length, partial openings, and stable internal conditions create gradients of temperature and humidity that allow different bat species to occupy the sections best suited to them. The result is a stratified underground habitat in which the entrance zone, the middle sections, and the deeper areas each support different assemblages of cave-dwelling wildlife. Long-term monitoring here provides one of the longest continuous records of bat population change in the region, and the data show a generally declining trend across the species present.

The reserve is distinguished primarily for its bat fauna. At least fifteen species of bat use the gallery, and four are considered especially noteworthy for conservation: Schreibers' long-fingered bat (Miniopterus schreibersii), the greater horseshoe bat (Rhinolophus ferrumequinum), the western barbastelle (Barbastella barbastellus), and the lesser horseshoe bat (Rhinolophus hipposideros). Their distribution inside the tunnel reflects the gradient of microclimatic conditions, with barbastelles favoring the entrance zone, Schreibers' bats the middle sections, and greater horseshoe bats the deepest parts.

Mammals other than bats also use the broader reserve. The European lynx, chamois, and European wildcat are recorded in the area, reflecting the wider Haut-Jura fauna that benefits from the regional park context. Cliff habitats host breeding birds such as the common raven, the wallcreeper, the peregrine falcon, and the common kestrel, while the fast-flowing Valserine supports the white-throated dipper. In addition, the reserve shelters protected insects of conservation interest, including the Dryad butterfly (Lopinga achine, 'bacchante' in French sources) and the golden-ringed dragonfly relative Cordulegaster bidentata.

The reserve's conservation significance rests on its role in protecting both an exceptional underground bat refuge and an underlying landscape of cliffs and river gorges. The gallery is one of the most important bat hibernacula in the southern Jura, and the more than forty years of continuous population monitoring make the site a key reference point for understanding regional trends in cave-dwelling chiroptera, which have shown a general decline during the monitoring period.

The area has accumulated multiple overlapping layers of protection. It is a ZNIEFF of type 1, originally designated as 'PONT DES PIERRES LA NAMPHEE' and later renamed 'Gorges de la Valserine en amont de Montanges'. It is covered by a prefectural biotope protection order for the cliffs of the Semine-Valserine confluence, and it is part of the Natura 2000 network as the site 'GALERIE A CHAUVE-SOURIS DU PONT DES PIERRES' (FR8201648). Public access to the gallery has been restricted, and protective grilles installed in 1998 ensure that bats can continue to use the tunnel while human disturbance is minimized.

Although the reserve itself is small and ecologically focused, its setting touches on a layer of regional industrial history. The Valserine river was studied in the early twentieth century as a potential source of hydroelectric power, and the gallery at Pont des Pierres is a tangible trace of those ambitions: an unfinished infrastructure project from 1929 that was overtaken by the energy policies of later decades and ultimately became a refuge for wildlife rather than for water diversion.

The reserve also fits into the broader cultural identity of the Haut-Jura, a region where small-scale agriculture, forestry, and karst and river landscapes coexist with a long speleological tradition. Local caving clubs from Bellegarde-sur-Valserine were the first to recognize the biological importance of the gallery in the late 1960s, anchoring the site's protection in a community with continued technical and scientific engagement with underground environments.

A nearly one-kilometer former hydroelectric tunnel repurposed as a bat sanctuary, monitoring ongoing since 1969. At least fifteen bat species, including the notable Schreibers' long-fingered bat, greater horseshoe bat, western barbastelle, and lesser horseshoe bat. A compact 9.27-hectare footprint layered atop the Valserine gorges with ZNIEFF, biotope protection, and Natura 2000 status. Cliff and riverside habitats hosting lynx, chamois, wildcat, peregrine falcon, wallcreeper, and protected insects such as the Dryad butterfly.

Because the reserve's main interest is the bat colony inside the gallery, and public access to the tunnel itself is forbidden, any visit is really a visit to the surrounding landscape of the Pont des Pierres and the Valserine gorges, with the bat presence being a feature of the protected underground environment rather than a directly observable attraction. The site is most pleasant in late spring, summer, and early autumn, when walking conditions along the hiking trail through the reserve are at their best and the cliff and riverside flora, including Aster amellus, is in active growth. Winter is the season when the bat population is most ecologically significant, because the gallery functions as a hibernation site, but this is also when any disturbance is most damaging, which is why access is restricted.