Brendan D. Taylor

Roads and wildlife: impacts, mitigation and implications for wildlife management in Australia

Roads can disrupt the population processes of vertebrate wildlife species through habitat fragmentation and vehicle collision. The aims of this review were to synthesise the recent literature on road impacts on wildlife, to identify gaps in our understanding of this topic and to guide future research and management in Australia. We reviewed 244 published studies from the last decade on road and vehicle impacts on wildlife conducted worldwide. A geographic bias was evident among the studies, with 51% conducted in North America, 25% in Europe, 17% in Australia and 7% across several other countries. A taxonomic bias was evident towards mammals (53%), with far fewer studies on birds (10%), amphibians (9%) and reptiles (8%), and some (20%) included multiple taxonomic groups. Although this bias is partly explained by large insurance and medical costs associated with collisions involving large mammals, it is also evident in Australia and signals that large components of biodiversity are being neglected. Despite a prevalence of studies on wildlife road mortality (34%), population impacts are poorly described, although negative impacts are implicated for many species. Barrier effects of roads were examined in 44 studies, with behavioural aversion leading to adverse genetic consequences identified for some species. The installation of road-crossing structures for wildlife has become commonplace worldwide, but has largely outpaced an understanding of any population benefits. Road underpasses appear to be an important generic mitigation tool because a wide range of taxa use them. This knowledge can guide management until further information becomes available. Global concern about the decline of amphibians should lead to a greater focus on road impacts on this group. Priorities for research in Australia include (1) genetic studies on a range of taxa to provide an understanding of life-history traits that predispose species to barrier effects from roads, (2) studies that examine whether crossing structures alleviate population impacts from roads and (3) studies that describe the behavioural response of frogs to crossing structures and that identify factors that may promote the use of suitable structures. A national strategy to mitigate the impacts of roads on wildlife populations is long overdue and must ensure that research on this topic is adequately funded.

Cutting the carnage: wildlife usage of road culverts in north-eastern New South Wales

Culverts have been used for a number of decades in Europe and the USA to reduce wildlife road-kills. In Australia, culverts have been employed by road authorities only relatively recently. This study used sand-strip surveys to investigate wildlife usage of nine purpose-built culverts along a 1.4-km section of the Pacific Highway at Brunswick Heads, north-east New South Wales. Surveys during two eight-day periods in spring and summer 2000 found 1202 traverses by wildlife through the culverts. Frequent culvert users were bandicoots (25% of traverses), rats (25%), wallabies (13%) and cane toads (14%). All culverts were used by these species, suggesting that at least several individuals of each species were involved. Infrequent users (each <2% of crossings) were possums, echidnas, lizards, birds and introduced carnivores. A koala was recorded crossing on two occasions. The long-nosed potoroo was observed in the surrounding habitat but was not confirmed traversing the culverts. Surveys for road-kills on this road section suggest that the exclusion fence bordering the highway prevented mammal road-kills and channeled mammals to the culverts. A single survey on a wet night found many frogs crossing the road surface and many were killed. This study confirms that culverts and exclusion fencing facilitate safe passage across a road for a range of wildlife species. This suggests that this form of management response to extensive road mortality of wildlife is appropriate and should be adopted more widely. However, this form of mitigation is not effective for frogs.

Can Road-Crossing Structures Improve Population Viability of an Urban Gliding Mammal?

Tree-dwelling mammals are potentially highly vulnerable to discontinuities in habitat created by roads. We used population modeling to assess the viability of a metapopulation of Australias largest gliding marsupial, the greater glider (Petauroides volans), occurring in forest remnants in the fastest- urbanizing region of Australia, where habitat is dissected by major roads. Crossing structures for arboreal mammals (consisting of a land bridge with wooden poles for gliding and adjacent rope canopy bridges) have been installed over an arterial road that separates two of these remnants (one large, one small). It is currently unknown whether this species 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. Our modeling reveals that even a relatively low rate of dispersal facilitated by these structures would substantially reduce the probability of extinction of the smaller subpopulation. This rate of dispersal is plausible given the small distance involved (about 55 m). The inclusion of wildfire as a catastrophe in our 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 remnants that straddle two motorways. These motorways create discontinuities in forest cover that are beyond the gliding ability of this species. Crossing structures will be required to enable inter-patch movement. A priority for future research should be whether the greater glider will use road-crossing structures. Loss of habitat and habitat connections is continuing in this landscape and 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 intersect wildlife habitat.

Gliding performance and its relevance to gap crossing by the squirrel glider (Petaurus norfolcensis)

Gliding mammals occur worldwide and many are subject to increasing levels of habitat fragmentation. Knowledge of their ability to cross tree-gaps by gliding is quite poor. We describe aspects of the gliding performance of the squirrel glider (Petaurus norfolcensis) based on recorded parameters of 85 glides of 73 individuals. Animals launched from a horizontal position ~1.7 m below the top of a tree and 2.3 m out from the main trunk. All but one glide was to the trunk of a tree, landing 5.7 m above the ground. Animals glided a mean of 21.5 ± 0.9 m (range 9–47 m) in a horizontal plane, with no significant difference between the sexes. Horizontal glide distance appears to reflect tree spacing where individuals were released. The mean glide angle was 28.5 ± 0.8°, with no significant difference between the sexes. We predict that trees beside roads that create a tree-gap of 20 m (two-lane road) or 43 m (four-lane road) will need to be a least 13 m and 25 m tall, respectively, to enable animals to safely glide across a road. Where trees are absent, 12-m-high wooden poles could be installed, requiring some in the median strip of four-lane roads.

Wooden poles can provide habitat connectivity for a gliding mammal

Gliding mammals may be susceptible to habitat fragmentation due to increased vulnerability to predators and road mortality if forced to cross roads and other canopy gaps on the ground. We document three trials where 6–12-m-high wooden poles, also known as glide poles, were installed to provide a link for gliding mammals across 50–75-m-wide canopy gaps, over open pasture or over roads. We used hair-traps over periods of 10–42 months to determine whether squirrel gliders (Petaurus norfolcensis) used the poles. Squirrel glider hair was detected on at least one pole during 69–100% of sampling sessions. At two road locations where poles were installed on wildlife land-bridges, hair was detected on poles in the middle of the bridge in 7–18 sessions, suggesting that complete crossings may have occurred. At one road location a camera-trap recorded a squirrel glider ascending a middle pole on five of 20 nights. Repeated use of the wooden poles by squirrel gliders at three locations suggests that tall wooden poles can restore habitat connectivity for a gliding mammal. We recommend further trials to extend our knowledge of the usefulness of this management tool for a range of gliding mammal species.

Will arboreal mammals use rope-bridges across a highway in eastern Australia?

Artificial structures designed to promote road-crossing by arboreal mammals are increasingly being installed in Australia but there is a limited understanding of their usefulness. We studied five 50–70-m-long rope-bridges (encompassing three designs) erected across the Pacific Highway, a major freeway in eastern Australia. Native arboreal mammals showed a willingness to explore these structures, being detected by camera traps on four rope-bridges. The vulnerable squirrel glider (Petaurus norfolcensis) crossed on one rope-bridge at least once every 4.5 weeks over a 32-week period. The feathertail glider (Acrobates pygmaeus), common ringtail possum (Pseudocheirus peregrinus) and the common brushtail possum (Trichosurus vulpecula) were detected on one of two rope-bridges that extended under the freeway at creek crossings. The feathertail glider was detected on all three rope-bridge designs. Our results suggest that rope-bridges have the potential to restore habitat connectivity disrupted by roads for some arboreal mammals. Further research is needed to refine the design and placement of rope-bridges as well as to determine whether these structures promote gene flow.

How many frogs are killed on a road in North-east New South Wales?

Road impacts on Australian frogs are poorly documented. This limits our ability to predict which species may be vulnerable to impacts and to develop strategies for mitigating impacts. We conducted foot-based surveys for road-killed frogs along two 100-m sections of a road that traverses known frog habitat near Lennox Head, in north-eastern New South Wales. More than 1000 dead frogs were counted over 13 mornings. Two threatened species, the Wallum Sedge Frog Litoria olongburensis and the Wallum Froglet Crinia tinnula, accounted for at least 60% of these roadkills. It is estimated that in an average summer period, there would be ≯40,000 frogs killed on the 4-km span of road through the heathland habitat. That is, ≯10,000 road-kills of each of the Wallum Sedge Frog and Wallum Froglet. Research is needed to ascertain how severe this impact is on these isolated populations and to determine how road mortality of frogs can be minimized.

Fine-Scale Genetic Response to Landscape Change in a Gliding Mammal

Understanding how populations respond to habitat loss is central to conserving biodiversity. Population genetic approaches enable the identification of the symptoms of population disruption in advance of population collapse. However, the spatio-temporal scales at which population disruption occurs are still too poorly known to effectively conserve biodiversity in the face of human-induced landscape change. We employed microsatellite analysis to examine genetic structure and diversity over small spatial (mostly 1-50 km) and temporal scales (20-50 years) in the squirrel glider (Petaurus norfolcensis), a gliding mammal that is commonly subjected to a loss of habitat connectivity. We identified genetically differentiated local populations over distances as little as 3 km and within 30 years of landscape change. Genetically isolated local populations experienced the loss of genetic diversity, and significantly increased mean relatedness, which suggests increased inbreeding. Where tree cover remained, genetic differentiation was less evident. This pattern was repeated in two landscapes located 750 km apart. These results lend support to other recent studies that suggest the loss of habitat connectivity can produce fine-scale population genetic change in a range of taxa. This gives rise to the prediction that many other vertebrates will experience similar genetic changes. Our results suggest the future collapse of local populations of this gliding mammal is likely unless habitat connectivity is maintained or restored. Landscape management must occur on a fine-scale to avert the erosion of biodiversity.

Horizontal or vertical? Camera trap orientations and recordingmodes for detecting potoroos, bandicoots and pademelons

Camera traps can detect rare and cryptic species, and may enable description of the stability of populations of threatened species. We investigated the relative performance of cameras oriented horizontally or vertically, and recording mode (still and video) to detect the vulnerable long-nosed potoroo (Potorous tridactylus) as a precursor to population monitoring. We established camera traps for periods of 13–21 days across 21 sites in Richmond Range National Park in north-east New South Wales. Each camera trap set consisted of three KeepGuard KG680V cameras directed at a bait container – one horizontal and one vertical camera in still mode and one horizontal camera in video mode. Potoroos and bandicoots (Perameles nasuta and Isoodon macrourus) were detected at 14 sites and pademelons (Thylogale stigmatica and T. thetis) were detected at 19 sites. We used program Presence to compare detection probabilities for each camera category. The detection probability for all three taxa groups was lowest for the vertical still and similar for the horizontal cameras. The detection probability (horizontal still) was highest for the potoroos (0.43) compared with the bandicoots (0.16) and pademelons (0.25). We estimate that the horizontal stills camera could achieve a 95% probability of detection of a potoroo within 6 days compared with 8 days using a vertical stills camera. This suggests that horizontal cameras in still mode have great potential for monitoring the dynamics of this potoroo population.

Facilitated movement over major roads is required to minimise extinction risk in an urban metapopulation of a gliding mammal

Abstract Context. Urbanisation is recognised as a primary cause of biodiversity loss. Roads are an inherent element of this, creating partial or complete barriers to animal movement. Urban landscapes of eastern Australia are typified by a dense road network interspersed with remnant patches of bushland. Inter-patch movement by tree-dependent gliding mammals may be halted and, consequently, population viability threatened, when canopy gaps over roads exceed gliding ability. Aims. We test the notion that a metapopulation of the squirrel glider (Petaurus norfolcensis) in southern Brisbane can persist within a highly fragmented urban landscape with large road canopy gaps. Methods. We used the population modelling software VORTEX to investigate the influence of inter-patch movement (dispersal) and wildfire on the probability of extinction. Wildfire is an inherent characteristic of this landscape. Key results. Our modelling suggests that a lack of inter-patch movement as a result of road barriers, in tandem with wildfire, is associated with a high probability of local extinction. However, a small rate of inter-patch movement can substantially reduce the likelihood of extinction. Conclusions. Road-crossing structures are the most plausible means available to link remnants to enable inter-patch movement for squirrel gliders in this landscape because of inadequate road-side tree height. Simulation studies such as the present study that test population viability are critical to convince land managers that action must be taken. Implications. The need to conserve urban biodiversity will increase over time, so land managers must consider the likely benefits to population persistence conferred by installing wildlife crossing structures into existing roads.