The construction of roads obstructs dispersal by small mammals, especially arboreal species that avoid activity on the ground, but the effectiveness of bridges that enable animals to cross roads is often questioned. Experiments in Japan, however, suggest that the barrier effect of roads may be mitigated by the construction of arboreal animal pathways. At least four species of arboreal mammals will make special use of these artificial structures rather than cross at ground level, even across a narrow and little-used road.
The Japanese dormouse Glirulus japonicus, known as yamane, is a rare and Near Threatened species that is only found in the mountain forests of Japan. It was designated a protected species in 1975, committing the national and local government to conservation measures. It is active at night in trees, feeding on flowers, fruits and insects, and rarely comes to the ground except to hibernate. The construction of roads and railways through forests could pose a significant threat by fragmenting the species' habitat and obstructing free movement of individuals, compromising metapopulation functionality and isolating vital feeding areas. Dormice and other arboreal animals play a vital role in pollinating the forest trees of Japan and dispersing their seeds. These species are an essential part of the forest community but are vulnerable to habitat change and negatively affected by barriers to their dispersal and free movement.
In 1998 a special bridge was constructed to help dormice to cross a new road being built through a key area of forest in Yamanashi Prefecture. It consists of a large gantry of the type normally used to support overhead road signboards, with space allowed for branches, nest boxes and ropes to be installed in a passageway 8.9 m above the road. This is enclosed in steel mesh to prevent debris falling on the traffic below and also to protect small mammals from attack by owls. Ropes and creepers link the bridge to roadside trees and shrubs. Within 4 weeks dormice built breeding nests on the bridge and Japanese wood mice Apodemus argentatus were living up there the following year. Both species produced young in the nest boxes, with traffic flowing underneath. Camera surveillance suggests that these small mammals also regularly used the bridge as an alternative to making a risky dash across the road at ground level.
Such bridges are expensive but arboreal species are small and therefore bridges for their benefit can be small too, substantially reducing costs. Subsequent experiments focused on smaller suspended bridges to enable arboreal animals to cross above roads without descending to the ground. In 2007 the first arboreal animal pathway was built, effectively creating an animal footbridge for species living in the treetops. It consists of three thin cables linked by a series of triangular supports along their length. The base of the triangle forms the floor of the pathway, made of metal mesh that does not trap snow or form icicles in the winter (which could fall and damage cars below). The apex of the triangle has a rope along which dormice can travel, hanging below it in the manner characteristic of yamane. At intervals along the bridge small metal shelters offer protection from owls and bad weather. Ropes and creepers link the pathway to the forest canopy either side of the road.
Despite its highly artificial nature, being all-metal and nothing like the normal surfaces to which arboreal animals are accustomed, video cameras showed that the pathway was used over 800 times in 3 months by four mammal species (dormouse, wood mouse, marten and squirrel). They could easily have run across the road (it is 13 m wide) yet chose to travel some distance to the bridge and use it rather than chance coming to the ground. In October 2011 another animal pathway was constructed. The first dormouse used it within 7 hours.
Clearly these arboreal animal pathways are welcomed and used by the animals they are intended to benefit. Their construction was also a major success in terms of public involvement, with cooperation also from local governments, a communications company and construction company engineers. The bridges also highlighted important issues regarding forest animals, wildlife conservation and the need for a sensitive approach to road building. The project has won several major awards from the Japanese Government and the construction industry.
