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Effectiveness of rope bridge arboreal overpasses and faunal underpasses in providing connectivity for rainforest fauna

  • Miriam, Goosem
  • Nigel, Weston
  • Sally, Bushnell
Published Article
Publication Date
Aug 28, 2005
Road Ecology Center John Muir Institute of the Environment
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Rope bridge overpasses and faunal underpasses were effective in restoring rainforest habitat connectivity for many tropical rainforest species that suffer high levels of road mortality or that avoid large clearings, such as those for roads, and, therefore, suffer barrier effects. Faunal underpasses furnished with logs and rocks to provide cover were constructed in 2001 at a hotspot for tree-kangaroo mortality. The narrow road and 120-m-wide strip of abandoned pasture divided two blocks of rainforest severing an important highland wildlife corridor through an agricultural landscape. No rainforest small mammals were recorded crossing the gap in six months of trapping prior to the road upgrade. During the upgrade, corridors of rainforest trees were planted through the pasture to connect with underpass entrances. Underpass use was monitored weekly using sand tracking beds complemented by infrared-triggered digital cameras. Weekly road kill data were collected for 12 months prior to construction and continues on two 0.5-km road transects in the vicinity of the underpasses and two transects along a highway dividing similar rainforest habitat 5km to the north. In 2004, bird and small mammal use of the planted corridors was investigated. Many terrestrial rainforest species use the underpasses, including medium-sized and smaller mammals and terrestrial birds, together with two confirmed passages of the rare target species, Lumholtz’s tree-kangaroo. Road mortality near the underpasses has remained low, whereas road kill rates are much greater along the narrow rainforest highway without underpasses. Community composition of rainforest birds within the corridors is approaching that of edge rainforest nearby, demonstrating effectiveness at this early stage of growth. However, although rainforest small mammals reside in the corridors, feral and pasture species still dominate, emphasizing the need for longer growth periods to encourage greater use by rainforest specialist mammals of the connectivity afforded by corridors and underpasses. Several rope bridges erected 7m above narrow roads and designed for use by rare arboreal rainforest mammals have also proven effective and are regularly used by the obligate arboreal Lemuroid ringtail possum, which will not cross roads on the surface or via underpasses. Several other possums that rarely venture to ground level are also regular crossers. Structures also provide safe crossing routes for arboreal species that otherwise suffer road mortality. Monitoring using active infrared-triggered cameras, scat and hair collection, and spotlighting has shown all target rainforest ringtails and other possums using rope tunnel and cheaper rope ladder designs. Similar designs have since been installed elsewhere in Australia over four-lane highways. Subsequent rainforest studies will investigate use of longer rope bridges above a wide highway using mark-recapture and radio-tracking to determine home range and provide population information prior to construction, followed by systematic monitoring of the rope bridges.

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