title: Use and effectiveness of engineered road crossing-structures for wildlife in eastern Australia
Creators: name: Taylor, Brendan Dominic
Publisher: name: Griffith University
Year: 2010
Subjects: subject: road
subject: wildlife
subject: habitat
subject: bridges
subject: landscapes
subject: gliders
subject: squirrel
subject: marsupial
subject: bandicoot

description: Roads are primary agents of habitat fragmentation and transform landscapes on a global scale. Roads remove habitat and create barriers or filters to the movement of many wildlife species. Moreover, roads disrupt population processes by breaking up continuous populations into smaller, isolated subpopulations or metapopulations, which can suffer genetic drift and become more vulnerable to stochastic processes. Consequently, understanding and mitigating the impact of roads on wildlife has attracted increasing international interest in recent decades. Foremost in efforts to re-connect wildlife populations severed by roads has been the installation of crossing-structures. These are engineered structures designed to provide the opportunity for animals to safely cross either over the road (e.g. land-bridges, rope or wooden canopy-bridges) or under the road (e.g. underpasses, culverts, ecopipes). Crossing-structures enhance the permeability of roads to animal movement thereby improving the ‘functional connectivity’ of the surrounding landscape, or the degree to which the inter-fragment landscape (matrix) facilitates movement. Crossing-structures have thus become a common feature of new road projects in developed countries. Research into the efficacy of crossing-structures is very much in its infancy and has largely focused on assessing whether structures are being used by different wildlife species. As a consequence, we know very little about higher order questions pertaining to population processes, particularly whether the viability of local populations is enhanced by the deployment of crossing-structures. Furthermore, the deployment of crossing-structures has largely focused on terrestrial vertebrates and scant attention has been directed at mitigating road impacts on arboreal mammals, particularly gliding mammals (gliders). This thesis endeavours to address some of these information gaps by presenting the results of a number of investigations into wildlife use of road crossing-structures located in subtropical, eastern Australia. The topics of investigation include: i) glider use of wooden poles on a road land-bridge; ii) home-ranges of gliders residing near road crossing-structures; iii) modelling the effect of connecting habitat patches across road-gaps on glider metapopulation viability; and, vi) determining the effect of road widening on bandicoot use of highway underpasses. They represent the first studies of their kind to investigate such topics. The Australian squirrel glider Petaurus norfolcensis (Marsupialia: Diprotodontia) is a gliding marsupial distributed along the eastern seaboard of Australia. It is listed as threatened in the southern part of its geographic range due to habitat fragmentation. I investigated whether 7-m high wooden poles installed on a land-bridge could enable squirrel gliders to cross a road canopy-gap at two locations in Brisbane, the capital city of Queensland. This represented the first attempt in the world to facilitate road crossing by a gliding mammal in this way. At one location no road crossing was revealed from trapping over a 3-year period. Radio-tracking over 8 months revealed 3 occasions when animals whose home-ranges were aligned along the road had crossed the road. Hair-sampling devices revealed use of the poles at one location during 15 of 20 sampling sessions over a 3-year period, and at the second location during 6 of 6 sampling sessions over a 6-month period. Detection of hair on all poles during some sessions at both locations suggests complete crossings could occur. These observations suggest that wooden poles can be used to facilitate road-crossing by gliding mammals. Obtaining information on the use of novel road-crossing technologies such as gliding poles is a necessary first step to understanding their efficacy. However, the ultimate test of a crossing-structure’s effectiveness is in establishing to what extent has it alleviated the barrier effect of the road and prevented the extirpation of local populations. Accordingly, I utilised the population modelling program VORTEX to assess the viability of a metapopulation of squirrel gliders and Australia’s largest gliding marsupial, the greater glider (Petauroides volans), occurring in forest remnants in the fastest urbanising region of Australia. Population studies were conducted over two years within two of these forest remnants (one small, one large) connected across a major road by a land-bridge with gliding poles and rope canopy-bridges. Whereas I obtained direct evidence of squirrel glider use of the land-bridge poles, it is currently unknown whether greater gliders will use the crossing structures but available tree height and spacing do not allow a glide crossing and fences with metal flashing prevent access to the road by terrestrial and arboreal mammals. My modelling revealed that even a relatively low rate of dispersal facilitated by these structures would substantially reduce the probability of extirpation of the smaller subpopulation for both glider species. This rate of dispersal is plausible given the small distance involved (~50 m). The inclusion of wildfire as a catastrophe in my model suggests that these two remnants may encounter an undesirable level of extinction risk. This can be reduced to an acceptable level by including inter-patch movement via dispersal among other forest remnants. However, this requires connection to a very large remnant 8 km away, through a set of smaller remnants that straddle two motorways. These motorways create discontinuities in forest cover that are beyond the gliding ability of both these species and will require the deployment of crossing-structures to allow inter-patch movement. Whereas our knowledge of wildlife use of road crossing-structures has grown considerably in recent years, little is known about the effect that road widening may have on patterns of use by target fauna. To address this, I investigated the effect that duplicating a highway had on bandicoot (Marsupialia: Peramelemorphia) use of underpasses at Brunswick Heads in north east New South Wales (NSW). This study represented one of the first of its kind in Australia and spanned the period of pre and post-construction over an eight year period. Evidence from trapping data showed that individuals of the two bandicoot species present at the study site (i.e. Isoodon macrourus and Perameles nasuta) incorporated both sides of the road corridor into their home ranges and that the majority of movements were likely associated with foraging. The trapping data also demonstrated spatial overlap between and within sexes and congeners suggesting that underpasses were functionally available to resident animals and were each being used by multiple individuals. Despite this, gaining clear insights on the effect that road-widening had on underpass use by bandicoots was confounded by drought, loss of high quality habitat, a decline in bandicoot abundance, the presence of foxes and different design characteristics of new underpasses. Further surveys are required at Brunswick Heads to elucidate this issue. The results of my investigations on bandicoot and glider use of road crossing-structures provide strong support for the need to manage road impacts on wildlife at multiple scales. At the patch or site-level, a number of enhancements are required to improve the functionality of the investigated crossing-structures and to make the study sites more amenable to the target species. At the landscape-level, management needs to focus on improving functional connectivity in the surrounding landscape to strengthen the viability of local metapopulations. This will require the combination of habitat restoration works, wildfire control and the deployment of crossing structures where large roads and motorways breach forest cover. In the case of gliders, the deployment of crossing-structures can be achieved rapidly and cost-effectively by installing gliding poles along motorway edges and in median strips at locations where forest cover is breached. Future research at these study sites needs to continue for much longer periods (i.e. 5-10 years) and incorporate genetic techniques. This will assist in improving our understanding of the isolation effects of road barriers and whether or not gene flow is restored by the installation of crossing-structures. This should be routinely combined with viability assessments (e.g. PVA) to determine how populations are affected by roads and whether they might benefit from crossing structures. Research is also required to provide insights to dispersal behaviour, particularly for gliders, and the likely use of road-crossing structures. Loss of habitat and habitat connections is continuing in my study landscapes and this is likely to have dire consequences for wildlife if land managers are unable to retain appropriate habitat cover with corridors and install effective wildlife road crossing-structures where large roads bisect wildlife habitat.