A remarkably quick habituation and high use of a rope bridge by an endangered marsupial, the western ringtail possum

Kaori Yokochi,Roberta Bencini

doi:10.3897/natureconservation.11.4385

Abstract

Rope bridges are being increasingly installed worldwide to mitigate the negative impacts of roads on arboreal animals. However, monitoring of these structures is still limited and an assessment of factors influencing the crossing behaviours is lacking. We monitored the use of a rope bridge near Busselton, Western Australia by the endangered western ringtail possums (Pseudocheirus occidentalis) in order to identify the patterns of use and factors influencing the crossings. We installed motion sensor cameras and microchip readers on the bridge to record the crossings made by individual animals, and analysed these crossing data using generalised linear models that included factors such as days since the installation of the bridge, breeding season, wind speed, minimum temperature and moonlight. Possums started investigating the bridge even before the installation was completed, and the first complete crossing was recorded only 36 days after the installation, which is remarkably sooner than arboreal species studied in other parts of Australia. The possums crossed the bridge increasingly over 270 days of monitoring at a much higher rate than we expected (8.87 ± 0.59 complete crossings per night). Possums crossed the bridge less on windy nights and warm nights probably due to the risk of being blown away and heat stress on warmer days. Crossings also decreased slightly on brighter nights probably due to the higher risk of predation. Breeding season did not influence the crossings. Pseudocheirus occidentalis habituated to the bridge very quickly, and our results demonstrate that rope bridges have a potential as an effective mitigation measure against the negative impacts of roads on this species. More studies and longer monitoring, as well as investigating whether crossing results in the restoration of gene flow are then needed in order to further assess the true conservation value of these crossing structures.

Highlights

Roads can act as a barrier to movement and gene flow in wildlife populations and cause genetic isolation and fragmentation
In another study conducted in the same study area in Busselton, we found that an artificial waterway nearby was causing greater genetic divergence among P. occidentalis than Caves Road (Yokochi 2015); installation and monitoring of a rope bridge across this waterway is strongly recommended given the willingness of the possums to utilise these crossing structures in this area
Roads pose negative impacts on wildlife and their impacts need to be mitigated by providing safe passages especially for threatened arboreal species

Summary

Introduction

Roads can act as a barrier to movement and gene flow in wildlife populations and cause genetic isolation and fragmentation This can results in lowered fitness and adaptability, which increases the risk of population extinction (Forman and Alexander 1998). To mitigate against these impacts, an increasing number of wildlife crossing structures are being installed worldwide because they have the potential to prevent road mortality and habitat fragmentation by providing animals with safe passages across roads (Clevenger and Wierzchowski 2006). Canopy bridges, have been installed worldwide to mitigate negative impacts on arboreal species, including several opossum, monkey, dormouse and squirrel species (Norwood 1999, Teixeira et al 2013, Sonoda 2014). In the eastern parts of Australia, rope bridges have been built for gliders, possums, and koalas (Phascolarctos cinereus); monitoring of the use of these structures by the target species is still limited to a handful of cases (Weston et al 2011, Goldingay et al 2013, Soanes et al 2013), and assessment of factors influencing the use of these structures is lacking

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Discussion

Conclusion