Can field trials improve the design of road-crossing structures for gliding mammals?

Gliding mammals are sensitive to habitat fragmentation that produces canopy gaps beyond their gliding capability. Specific structures (canopy-bridges and glide poles) are now commonly installed in large road construction projects to enable road crossing by threatened gliding mammals. However, these structures are being installed with limited understanding of how their design features influence their use. We conducted field testing of several design features (horizontal glide launch-beams at the top of poles; rope size and complexity in rope-bridges) using free-ranging gliding mammals, and scaled-down structures at two locations. Our aim was to identify preferred features to optimise structure use. This may confirm current designs or identify the need for refinement. We found that squirrel gliders (Petaurus norfolcensis) and sugar gliders (Petaurus breviceps) preferred a forward-pointing over a sideways-pointing glide beam. A single-rope rope-bridge was favoured over a mesh rope-bridge by sugar gliders but not squirrel gliders. No preference was shown by either species between mesh or ladder-style rope-bridges that differed in rope strand spacing. Large, permanently installed ladder or mesh rope-bridges commonly have single ropes connecting them to the adjacent forest. We investigated the use of a permanently installed 50-m long single-rope rope-bridge. Infra-red camera monitoring over 366 nights detected squirrel gliders on this bridge on 172 nights, common ringtail possums (Pseudocheirus peregrinus) on 144 nights and common brushtail possums (Trichosurus vulpecula) on 120 nights. This confirms acceptance of the single rope by a range of species and provides confidence in installing rope-bridges that may vary in rope size and complexity.