The conservation of mammals in Victoria's roadsides.
To put it into perspective, in Victoria we have 156 000 km of road travelled annually by more than 4 million registered vehicles (AustRoads 2003; Australian Bureau of Statistics). The resulting satellite imagery is striking (Fig. 1). Most of Victoria's land is within 1 km of the nearest road, meaning that in the vast majority of landscapes, wildlife is living in roadsides or roadaffected habitats. In fact, in many areas roadside vegetation may be the only suitable habitat that is left. For example, within highly-modified landscapes (e.g. agricultural or semi-urban landscapes), mature Eucalyptus woodland is almost entirely restricted to linear strips along roadsides, paddock edges and waterways (Fig. 2). Species that depend on woodland habitats are, therefore, restricted largely to living near roads (Suckling 1984; Bennett 1988; Gilbert 1998; van der Ree and Bennett 2003). Roadsides can represent high quality habitat for woodland mammals because they often host a high density of large, old, hollow-bearing trees--a critical resource for woodland fauna, including threatened species such as the Brush-tailed Phascogale Phascogale tapoatafa and Squirrel Glider Petaurus norfolcensis. Furthermore, just as the road network provides connectivity for people, the network of roadside habitats provides connectivity for woodland wildlife, allowing them to move through otherwise fragmented landscapes (e.g. Suckling 1984; Bennett 1990; Downes et al. 1997). The ability to manage the negative effects of roads on wildlife is, therefore, a critical component of conservation efforts for many threatened species.
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The danger zone
Living in roadsides presents several challenges to wildlife, and thus to the managers wishing to conserve it. There are many ecological impacts that can make roadside habitats unsuitable for some species (Forman et al. 2003; Donaldson and Bennett 2004; van der Ree et al. 2015). For example, edge effects, dust, noise and light pollution can displace the more sensitive species (e.g. Eigenbrod et al. 2009; Parris and Schneider 2009). However, here I focus on two main impacts of roads on species that live within roadside habitats: the mortality (roadkill) and barrier effects.
The most obvious, and distressing, impact is roadkill. Common mammal victims on Australian roads include kangaroos, wallabies, wombats, echidna, koalas, bandicoots and possums (e.g. Coulson 1982, 1997; Dique et al. 2003; Taylor and Goldingay 2004; Russell et al. 2009; Bond and Jones 2014). McCall et al. (2010) showed that the survival rate of Squirrel Gliders living adjacent to the Hume Freeway was approximately 60% lower than populations living more than 5 km away. Roadkill has also been implicated in the population decline of the Swamp Wallaby Wallabia bicolor in Royal National Park, NSW (Ramp and Ben-Ami 2006), and the Eastern Quoll Dasyurus viverrinus, and Tasmanian Devil Sarcophilus harrisii after a road upgrade in Tasmania (Jones 2000).
Roads also create barriers to movement, restricting access to resources and reducing dispersal and gene flow (Shepard et al. 2008; Holderegger and Di Giulio 2010). Arboreal mammals are particularly vulnerable to the barrier effect of roads, with larger roads such as the Hume Freeway and Goulburn Valley Highway proven to restrict the movement of Common Brushtail Possums Trichosurus vulpecula (Gulle 2006) and Squirrel Gliders (van der Ree 2006; van der Ree et al. 2010). Studies from Queensland show that even narrow roads can be barriers for smaller mammal species (e.g. Burnett 1992; Goosem 2001). Ultimately a road can be a tipping point in already vulnerable populations, where the loss of even a few individuals through roadkill, or the creation of barriers that restrict gene flow and access to patchy resources can lead to rapid population declines (Bennett 1991; Forman et al. 2003; Coffin 2007).
Managing the risks to wildlife
Given the extent of land already affected by roads, we need methods that effectively mitigate their negative effects on wildlife. Wildlife fencing can reduce rates of roadkill by keeping animals off the road. However, fences can make the 'barrier effect' of roads worse by further restricting animal movements, and are detrimental when species persistence depends on movement among multiple patches of habitat (Jaeger and Fahrig 2004). Wildlife crossing structures can allow safe passage across roads to improve habitat connectivity, and, when used in conjunction with fencing, they can simultaneously reduce roadkill. There are many different types of crossing structures, but they generally fall into two categories: those that allow animals to move over the road (e.g. rope bridges, land bridges, overpasses), and those that allow animals to move under the road (e.g. culverts, tunnels, creek-crossings). While crossing structures are more common in Europe and North America (van der Ree et al. 2007, van der Ree et al. 2008), road agencies in Australia are increasingly including crossing structures in road projects (e.g. Taylor and Goldingay 2003; Bond and Jones 2008; Goldingay et al. 2013). In Victoria, wildlife crossing structures have been constructed predominantly on major roads such as the Hume Freeway, Calder Freeway, Princes Highway and Peninsula Link (Abson and Lawrence 2003; van der Ree et al. 2008; Soanes et al. 2013; Soanes et al. 2015).
One of the most famous structures in Victoria is the aptly-named 'Tunnel of Love', at the Mount Hotham Ski Resort. A long-term population study of the critically endangered Mountain Pygmy Possum Burramys parvus, revealed that the Great Alpine Road bisected important habitat patches (Mansergh and Scotts 1989). This barrier disrupted the social organisation of the local population, restricting dispersal in males and reducing over-winter survival in females. In 1985, two tunnels (0.9 x 1.2 m cross section) were constructed through the rock strata under the road to restore connectivity. Ongoing research showed that Mountain Pygmy Possums used the tunnels, and that dispersal and over-winter survival rates recovered (Mansergh and Scotts 1989). This information was later used in a population viability analysis, which suggested that while the crossing structure improved the viability of the Mountain Pygmy Possum, the negative effect of the road may have been only partially mitigated (van der Ree et al. 2009).
Larger underpasses can cater for a wide variety of species. When the Black Forest section of the Calder Freeway was upgraded in 1997, VicRoads included a large wildlife underpass to provide connectivity between forest patches that were bisected by the road. The open-span bridge was built as an extended creek crossing, long enough to allow both the creek and the surrounding riparian vegetation to pass under the road. The structure was monitored five years after it was built, and researchers detected several mammal species using the underpass including microbats, koalas, wombats, echidna, possums, gliders and macropods (Abson and Lawrence 2003). A recent review showed that these open-span bridges are an effective way to maintain connectivity across roads for multiple species (Lesbarreres and Fahrig 2012).
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Aerial crossing structures, rope bridges and glider poles were used to reduce the impact of the Hume Freeway in north-east Victoria on arboreal mammals. A before-after research project was established to test their effectiveness, focusing on the ability to restore movement and gene flow in Squirrel Glider populations. Initial studies set out to measure the effect of the four-lane divided highway on Squirrel Gliders before crossing structures were installed. Researchers found that where the gap across the freeway was wider than 50 m, Squirrel Gliders were largely unable to cross; however, where tall trees had been left in the roadsides and centre medians (referred to as 'vegetated medians'), the animals could cross freely (van der Ree et al. 2010). Rope bridges and glider poles were then retrofitted to the freeway in July 2007 at five sites where the barrier effect occurred (Fig. 3). Motion-triggered cameras detected five species of arboreal mammal using the structures to cross the freeway, including the Squirrel Glider, Sugar Glider, Brush-tailed Phascogale, Common Brushtail Possum and Common Ringtail Possum (Soanes et al. 2013; Soanes et al. 2015). Further radio-tracking and genetic research was conducted after the crossing structures were installed, and revealed that rope bridges, glider poles and vegetated medians all facilitated movement and gene flow across the Hume Freeway and that this would not have occurred had crossing structures not been present (Soanes 2014).
There are other methods to reduce the effects of roads on wildlife, such as those designed to alter driver behaviour (e.g. warning signs), or repel animals from the road (e.g. reflectors or ultrasonic warnings). Unfortunately the research to date shows that these are largely unsuccessful (Bender 2003; Ramp and Croft 2006; Bond and Jones 2014). Methods that force drivers to slow down (e.g. reduced speed limits, pinch points and rumble strips) have been shown to be effective at reducing roadkill in 'hot spots' (Jones 2000), but can be difficult (and unpopular) to implement over large areas. A potentially promising method is an automatic-warning system, designed with flashing lights that warn drivers only when an animal is approaching the road (Huijser and McGowan 2003; Bond and Jones 2014).
Opportunities and challenges for research and conservation in Victoria
The importance of managing wildlife in linear habitats along roads, railways and other infrastructure is increasingly being recognised in Australia and worldwide (Gilbert 1998; Carthew et al. 2013; van der Ree et al. 2015). Mammals are not the only species to benefit from roadside habitats. Roadside vegetation provides critical habitat for woodland birds (e.g. Robinson 2006), reptiles (e.g. Brown et al. 2008; Jellinek et al. 2014) and insects (e.g. Major et al. 1999). Victorian roads therefore represent a great opportunity for the conservation of native mammals and other fauna.
The conservation value of roadsides will depend on our ability to manage potential conflicts. The safety risk posed to drivers, risk of bushfire and spread of invasive species are all serious issues that must be considered, but ones that can be controlled through appropriate management, such as barrier fencing and weed control (Gilbert 1998). Roadside habitats are also vulnerable to being incrementally chipped away as roads are upgraded to accommodate higher traffic volumes. For example, during the last 50 years, rural highways such as the Hume, Western and Calder have been progressively widened to create four-lane freeways. However, road upgrades do not necessarily have to lead to the loss of roadside vegetation. Environmentally-sensitive road designs, such as the creation of wide medians to retain woodland vegetation and alignment of road corridors to avoid remnant patches, can help minimise vegetation loss, and are particularly important in landscapes that are otherwise devoid of natural vegetation. Without formal protection and considered planning, the extent and quality of roadside habitats for wildlife can decline rapidly.
Wildlife crossing structures are a promising tool for managing the negative effects of roads, not only for woodland mammals but also for a range of other wildlife (van der Ree et al. 2007; van der Ree et al. 2015). However, a practical challenge to increasing the number of wildlife crossing structures in Victoria is that it would require retrofitting structures to existing roads, a process that can be difficult, disruptive and costly. One option is to take the opportunity to include wildlife crossing structures and other mitigation measures in future road upgrades, thus minimising the cost and disruption. Another possibility is to modify existing structures that were not specifically designed for wildlife, such as drainage culverts and pipes. For example, adding guide fencing, small ledges and 'fauna furniture' to culverts can encourage their use by fauna while maintaining water flows (e.g. Brudin 2003; Foresman 2003).
There are clear opportunities for further road ecology research in Victoria. For instance, there are surprisingly few published studies of roadkill on Victorian roads (although I suspect that many useful datasets are hidden within the 'grey literature'). Research that combines data from roadkill studies, insurance reports and wildlife rescue groups would help provide a better understanding of the extent of the roadkill problem in Victoria. For example, identifying hotspots, the relative risk for each species, and the degree to which roadkill depletes the surrounding population are important gaps in the current knowledge. There is still work to be done to quantify the value of roadside habitats to wildlife populations. Studies that assess the ability of wildlife populations to persist in roadside habitats are needed to guide effective management and mitigation. Can a network of roadside reserves support viable populations in the long-term? Do species use roadsides as permanent habitats (home range), supplementary resources (feeding on grasses etc.), or simply as movement corridors? Finally, we need more studies that test the effectiveness of mitigation measures such as wildlife crossing structures. While it is evident that animals will use these structures, the next step is to determine whether they improve the population persistence within a landscape (van der Grift et al. 2013). Only through careful research and management will we be able to maximise the value of roadside habitats for wildlife, and continue to enjoy our scenic woodland drives.
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Received 25 August 2015; accepted 21 January 2016
Australian Research Centre for Urban Ecology, University of Melbourne
Current address: Clean Air and Urban Landscapes Hub, National Environmental Science Programme
School of Ecosystem and Forest Science, The University of Melbourne. email: email@example.com
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|Publication:||The Victorian Naturalist|
|Date:||Jun 1, 2016|
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