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Arboreal animals need trees for some or all of their shelter, food and movement. This diverse group of wildlife
includes mammals, amphibians and reptiles that climb, crawl and glide in trees. Since trees are a critical
resource, arboreal animals are directly affected by habitat loss from road construction. The susceptibility
of arboreal animals to barrier effects and wildlife-vehicle collisions (WVC) will depend on their willingness,
opportunity and ability to cross gaps. Methods to mitigate the impacts of roads and traffic are often unique
and specific to this group of wildlife.
40.1 Always avoid clearing trees where possible.
40.2 Canopy connectivity is important for most arboreal animals.
40.3 Not all arboreal animals need arboreal crossing structures.
40.4 Further research on impacts and mitigation for arboreal species is needed.
Recent studies have quantified the impacts of roads on some arboreal species, primarily mammals, and
successful mitigation techniques are available. However, further research on the use and effectiveness of
mitigation strategies for this group is urgently required, particularly for arboreal amphibians and reptiles.
To read the full-text of this research, you can request a copy directly from the authors.
... Roads disrupt the canopy and can affect arboreal animals through reduced habitat connectivity, habitat loss and degradation, and direct mortality. Arboreal animals can be particularly affected by this type of infrastructure as they feed, nest, or move through the forest canopy (Soanes and van der Ree, 2015). For species that do descend to the ground willing to cross a road, traffic can be an important source of mortality (Srbek-Araujo et al., 2018;Hetman et al., 2019;Chaves et al., 2022). ...
... Preventing these animals from reaching the road to mitigate road fatalities may be challenging due to their climbing capacity. Therefore, maintaining or restoring canopy connectivity via bridges allows movement to continue over a road while also avoiding the dangers of the road if they prefer to move in the canopy (Soanes and van der Ree, 2015;Linden et al., 2020). At the same time, various types of underpasses typically installed for terrestrial animal use may represent a mitigation option for those arboreal species that have the propensity to descend to the ground (Caine, 1996;Oliveira et al., 2012;de Abreu et al., 2017). ...
... Throughout the tropics little information exists on the impact of roads on arboreal animals and few mitigation solutions have been proposed for this group (Soanes and van der Ree, 2015). There is a high diversity of treedwelling species occurring throughout the tropics that need to be targets of road ecology studies and roadkill mitigation (Ascensão et al., 2022). ...
Roads disrupt the canopy and can affect arboreal animals in different ways, such as reducing canopy connectivity, generating habitat loss and degradation, and increasing direct mortality. Since arboreal animals mainly use the canopy for movement, mitigation measures for these species usually focus on maintaining or restoring canopy connectivity to guarantee safe crossings. Here we present a case study of a Brazilian coastal road (ES-060) for which we described the use of a canopy bridge and multiple underpasses by three arboreal mammal species and compared these data with roadkill records of the same species in the vicinity of the crossing structures. Our study includes a 75 m long steel cable canopy bridge, monitored for 3 years, and clusters of different types of underpasses, monitored for 16 years. The use of the crossing structures was monitored with sand track beds installed at entrances on both sides, and roadkill surveys were conducted daily for 16 years. We considered a crossing to be successful if tracks of the same species were recorded on either side of a structure and showed opposite movement trajectories. The canopy bridge survey resulted in an observed rate of 0.16 crossings/month for Callithrix geoffroyi, 7.79 for Coendou insidiosus, and 0.46 for Didelphis aurita, and all types of underpasses combined demonstrated a rate of 0.33, 1.94, and 8.43 crossings/month for each species, respectively. The roadkill surveys resulted in an observed rate of 1.41, 0.78, 2.94 roadkills/month for Callithrix geoffroyi, Coendou insidiosus, and Didelphis aurita, respectively. Even with mitigation structures confirmed to be used by these three species, roadkill hotspots occurred in the road sections with the crossing structures. Our study demonstrated the use of a canopy bridge and different types of underpasses by arboreal mammal species. The canopy bridge was mostly used by Coendou insidiosus, while the underpasses were mainly used by Didelphis aurita. As roadkill hotspots occurred red in the same segments where mitigation crossing structures were installed, our results indicate that some important improvements are needed to mitigate roadkills of arboreal mammals in this area, mainly preventing that these species access the road. We present recommendations for a research agenda to support mitigation planning for arboreal mammals, namely: (1) testing the efficiency of different canopy bridge designs for multispecies mitigation, (2) testing the use of connecting structures, such as ropes that connect to the surrounding forest, to encourage underpass use by arboreal species, and (3) testing fence adaptations to block the access of arboreal mammals to roads.
... The Atlantic Forest biome in South America, considered one of the world's biodiversity hotspots (Myers et al. 2000), is directly affected by the impacts of roads as it has a dense expanding road network (Grilo et al. 2018). While wildlife crossing structures have been tested as a mitigation tool for arboreal species in different parts of the world (Goosem et al. 2005;Donaldson and Cunneyworth 2015;Soanes and van der Ree 2015;Linden et al. 2020), it is still unknown how these structures are efficient for the arboreal South American fauna since no long-term monitoring of canopy bridges testing different types of designs has been carried out so far. ...
... Wildlife crossing structures are considered efficient mitigation measures to avoid roadkill and barrier effects by reconnecting areas and possibly populations. Mitigating these road impacts is essential for species conservation, including in anthropized landscapes (Soanes and van der Ree 2015). There are eleven types of wildlife crossing designs divided into overpass design and underpass design. ...
... Canopy bridges are crossing structures used for arboreal and semi-arboreal animals, including primate species. Their efficiency and preference by the target species for a certain design should be, ideally, evaluated through long-term monitoring (Beckmann et al. 2010;Clevenger and Huijser 2011;Donaldson and Cunneyworth 2015;Soanes and van der Ree 2015;Linden et al. 2020). They can be made of rope, steel, or wood, with single strands or ladder-like design, and are suspended above the road by timber poles or roadside trees. ...
Canopy bridges are crossing structures specific to mitigate the impact of roads on arboreal animals. Long-term monitoring of such infrastructures together with the analysis of design preferences has never been done in South America. To avoid the roadkills of a threatened primate species, the black lion tamarin (Leontopithecus chrysopygus), in Guareí, São Paulo, Brazil, we installed two designs of canopy bridges: a wood pole bridge and a rope bridge. We aimed to (1) evaluate the functionality (number of species and events) of both designs, (2) test the design preference of each species, and (3) determine if there were seasonal differences in the use of canopy bridges. We monitored the canopy bridges continuously since their installation with camera traps during 3 years. We recorded nine mammal and one lizard species crossing on the canopy bridges as well as 13 bird species using them as perches. Overall, the probability of crossing was higher on the wood pole bridge and the number of crossings, considering both designs, was higher during the dry season. One lizard and seven mammal species used the wood pole bridge, including the black lion tamarin, and six mammal species used the rope bridge. Four out of five species tested, including the black lion tamarin, preferred the wood pole bridge. While replications of this experimental design are necessary to obtain a more robust evaluation of the effectiveness of these canopy bridges, our study suggests that wood pole bridges might be an effective tool to reduce roadkills of the endangered black lion tamarin and possibly other arboreal species.
... Mitigating the impacts of roads, one of the main causes of fragmentation, is critical to conserving treedwelling animals such as Japanese squirrel (Soanes and van der Ree 2016). Many crossing structures have been installed worldwide to mitigate the impacts of roads. ...
... In Taiwan, a different type of structure was installed to protect Taiwanese macaques (Macaca cyclopis) (Weston 2003). The installation of bridges across forestry roads in wet tropical regions of Australia has helped encourage the movement of rainforest ringtail possums, and many bridges have been established, mainly for rainforest ringtail possums (Weston 2003;Goosem et al. 2005; Soanes and van der Ree 2009; Goosem et al. 2011;Weston et al. 2011;Soanes and van der Ree 2016). ...
... The bridge investigated in one study was supported by a wooden pole and two metal stay wires on each side of the road, which were 300 mm in width (Yokochi and Bencini 2015). The mesh size was estimated to be 25 mm, which was much larger than that used in this study; however, many possums used the bridge (Weston 2003;Goosem et al. 2005;Soanes and van der Ree 2009;Goosem et al. 2011;Weston et al. 2011;Yokochi and Bencini 2015;Soanes and van der Ree 2016). Therefore, mesh size should be determined according to the target species. ...
A detached housing subdivision was developed in the 1990s in the city of Otsuki, Japan, located approximately 75 km west of Tokyo. In the environmental impact assessment, Japanese squirrels (Sciurus lis) were found to inhabit the development site, raising the concern that squirrels could be killed when crossing roads in the residential area. As a solution, a squirrel crossing structure was installed in 1997 and monitored periodically. The structure was used by squirrels on 27 of the 32 days monitored. During the inspection in July 2003, the cedar boards of the deck were replaced with wire mesh because of rotting. Based on monitoring from November 2004 to October 2005, Japanese squirrels used the structure on 99 of 176 days monitored, and it appeared to be used consistently, even by successive generations of squirrels. With time, the structure became difficult to maintain and was removed in March 2013.
... Given their dependence on trees, arboreal species are especially vulnerable to discontinuities in the habitat created by roads, which add additional obstacles to the physical challenges already imposed by arboreal locomotion (Asari et al., 2010). Many arboreal species are so well-adapted to treetop life and can be particularly reluctant to cross open clearings (Wilson et al., 2007;Soanes and van der Ree, 2015). These animals are "gaplimited", meaning they will not or cannot cross gaps in tree cover beyond a certain distance. ...
... Arboreal animals are often slow or awkward when moving along the ground and are poor at avoiding traffic and predators. Therefore, susceptibility of these animals to barrier effects and wildlife-vehicle collisions will depend on their willingness, opportunity, and ability to cross gaps (Soanes and van der Ree, 2015). ...
... For most arboreal species, a connected tree canopy is vital (Soanes and van der Ree, 2015). To facilitate the movement of arboreal mammals across linear infrastructure (i.e., roads, pipelines, or power lines) a series of crossing structures specifically constructed for these animals have been used. ...
There are few highways in Mexico that have built canopy bridges as a mitigation strategy for maintaining connectivity of arboreal fauna. Main target species have been primates, both, howler (Allouatta pigra, A. palliata) and spider monkeys (Atteles geofforyi), as well as several other arboreal priority species such as the kinkajou (Potos flavus), the northern tamandua (Tamandua mexicana) and the Mexican hairy porcupine (Sphiggurus mexicanus). The Nuevo Xcan-Playa del Carmen highway built 22 canopy bridges along its 54 km length. All bridges were surveyed using camera traps installed at both ends and after an 8,418 trap/night effort, 10 records of four mammal species were recorded using the canopy bridges: the kinkajou, opossum (Didelphis virginiana) and squirrels (Sciurus deppei and S. yucatanensis). More monitoring is required to properly assess the effectiveness of these mitigation measures, as the need for cost/benefit feedback is necessary to enhance further mitigation in this or other projects. Also, long term monitoring is required for properly assessing the use patterns of species. The current study was shortly after the infrastructure became operational, so it covers the adaptation period for several species but its insufficient to properly assess the current use.
... Forests are home to a variety of arboreal animal taxa that depend on trees to varying degrees. Some species spend their entire life elevated in trees whereas others descend to the forest floor more regularly (Soanes and van der Ree, 2015). Those who do come to the ground typically stay close to trees for safety and often show cautious behaviours such as running and displays associated with tension (Gregory et al., 2017). ...
... Those who do come to the ground typically stay close to trees for safety and often show cautious behaviours such as running and displays associated with tension (Gregory et al., 2017). For most arboreal species a connected tree canopy is vital (Soanes & van der Ree 2015). ...
Roads affect the integrity of ecosystems worldwide as a cause of mortality to animals and a barrier to animal movement, decreasing gene flow and increasing local extinction probability. It is estimated that construction of linear infrastructure impacts up to 13% of primate species but research focusing on primate road fatalities and mitigation is not extensive and experimental research on canopy crossing designs for primates is lacking. We used the South African samango monkey (Cercopithecus albogularis) as a model species to test suitable bridge design through field experimentation and behavioural data collection for arboreal guenon roadkill mitigation and a mapped actual roadkill data in the region. We show that canopy overpasses are a viable intervention for mitigating arboreal guenon road fatalities, reducing the probability that monkeys will cross a road on the ground. Samango monkeys clearly preferred a pole bridge over a rope ladder design and canopy bridges were preferred to trees and the ground when the tree canopy was open. Pole bridges were also used by other non-guenon and non-primate species. Although samango roadkills are not predictable in time (no seasonality), adult female and immature fatalities are predictable in space, restricted to bisected riparian zones and roads close to intact forests. Adult male road fatalities can also be expected in seemingly unsuitable habitat areas. Our study shows how important correct interpretation of spatial, temporal and demographic data on road fatalities is and how experimental research prior to installing crossing structures could increase mitigation impact.
... The connectivity of suitable forest patches has also been recognized as an important landscape feature for arboreal mammals (Reunanen et al. 2002;Pardini et al. 2005;Umetsu & Pardini 2007). Efforts to improve habitat quality and maintain vegetation structure, or even the construction of overpasses across roads to connect forested areas, can assist arboreal species conservation (Weston et al. 2011;Soanes & van der Ree 2015). ...
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Habitat fragmentation often induces edge effects that can increase, decrease or have minimal effect upon the population density of a species, depending upon environmental conditions and the requirements of the species. Using a trapping study and generalized linear mixed models, we evaluated edge effects on small tropical mammals living near roads, including two ground-dwelling (Akodon sp. and Cerradomys subflavus) and two arboreal (Marmosops incanus and Riphidomys sp.) species. We examined the relationship of these edge effects to environmental factors at both plot and patch scales. Generalist ground-dwelling species were attracted to edges, with higher population densities recorded in habitats close to road or matrix edges where vegetation density was lower. In contrast, populations of the generalist arboreal species avoided edge habitats, their populations were found in greater density in habitats far from roads/matrix edges. Thus, our results show that patterns of edge habitat utilization were related to the ecological requirements of each species. These findings are especially important in the tropics, where demand for economic growth in many countries has accelerated the fragmentation process and has recently culminated in increased road construction and expansion. Fragmented habitats promote an increase in edge environments, and consequently will reduce the abundance of arboreal small mammal species, such as those used as models in this study.
... 10 Para el caso de los mamíferos arborícolas, su hábitat disminuye drásticamente cuando las vialidades reducen la conectividad del paisaje (Soanes y Van der Ree, 2015) o son víctimas de colisiones cuando se ven obligados a descender a nivel del suelo y trasladarse al fragmento más cercano de vegetación (Pozo-Montuy y Serio-Silva, 2007; Pozo- Montuy et al., 2008). Debido a su carácter arborícola, muchas de estas especies son Impacto de las vías de comunicación sobre la fauna silvestre en áreas protegidas poco ágiles en el suelo y no tienen la capacidad suficiente para evadir a los vehículos que se aproximan (Soanes y Van der Ree, 2015). ...
Environmental impacts derived from terrestrial communication corridors overlap each other and, in most cases, it is difficult to establish a single cause-effect relationship. This chapter reviews the variables involved in this relationship, among which are those related
to fauna wildlife (v. g. species ecology and animal behaviour), traffic (speed and intensity), and environmental factors (road configuration and landscape characteristics). These variables may have a synergistic, antagonistic or neutral character, so the response from the fauna may present spatial and temporal differences, and will be specific for each
place and/or project.
... Restrictions on individual movement such as daily displacement, home range, seasonal migration, or dispersal [24,25] ultimately have negative consequences for population size and persistence [19,26]. Areas free of linear infrastructure are essential for arboreal species in tropical forests [27,28] because these species rely on canopy connectivity to move through the landscape to perform vital activities, e.g., feed, find mates, and reproduce . ...
Linear infrastructures are a primary driver of economic development. However, they also can negatively affect wildlife by mortality and the barrier effect. In this paper, we address how paved and unpaved roads, high-tension power lines, and gas/oil pipelines affect home range size, core areas, and movement in an endangered primate, the golden lion tamarin (GLT). Location data were recorded using radio telemetry on 16 groups in two protected areas and in privately owned forest fragments. The GLT’s home range, not core area, increased in size for the groups that occupied locations far from linear infrastructures; home range was also significantly influenced by available forest size. None of the home ranges contained a road, but home ranges did contain power lines. GLTs used the surrounding landscape near all types of infrastructure. Movement analysis showed that most of the step lengths (distances between subsequent locations) were less than 100 m between two consecutive locations, but step length was longer for roads and longer for groups in fully forested habitats. Tamarins avoided paved roads when in close proximity to this type of infrastructure; this behavior increased in areas without adequate adjacent forest habitat. Our results show that linear infrastructures differ in their level of impact: roads can act as a barrier, whereas other types of infrastructure have minimal effect on movement and home range. We discuss these differences in impact in terms of structure, maintenance schedules, and edge effects of infrastructure.
... These impacts can be even more pronounced in tropical forests, due to their extreme physical and ecological complexity 4 . In addition, many bird and small mammal species in tropical forests either avoid edges and/or are adverse to crossing clearings 5-7 , and arboreal species can be particularly reluctant to cross open clearings 8,9 . For many species, crossing an open clearing on the ground represents a major predation threat, exemplified in a study by Campbell et al. 10 by lower rates of terrestrial behaviour where predation pressure was higher. ...
Linear infrastructure development and resulting habitat fragmentation are expanding in Neotropical forests, and arboreal mammals may be disproportionately impacted by these linear habitat clearings. Maintaining canopy connectivity through preservation of connecting branches (i.e. natural canopy bridges) may help mitigate that impact. Using camera traps, we evaluated crossing rates of a pipeline right-of-way in a control area with no bridges and in a test area where 13 bridges were left by the pipeline construction company. Monitoring all canopy crossing points for a year (7,102 canopy camera nights), we confirmed bridge use by 25 mammal species from 12 families. With bridge use beginning immediately after exposure and increasing over time, use rates were over two orders of magnitude higher than on the ground. We also found a positive relationship between a bridge’s use rate and the number of species that used it, suggesting well-used bridges benefit multiple species. Data suggest bridge use may be related to a combination of bridge branch connectivity, multiple connections, connectivity to adjacent forest, and foliage cover. Given the high use rate and minimal cost, we recommend all linear infrastructure projects in forests with arboreal mammal populations include canopy bridges.
Roadkill (the mortality of animals through wildlife–vehicle collisions) is one of the main impacts of roads on wildlife. Studies quantifying the location and rate of roadkill to identify ‘hot spots’ are often used to guide the location of mitigation efforts, such as fencing or wildlife crossing structures. However, sometimes quantifying rates of roadkill can be challenging, particularly for species that are small and difficult to detect. In our study, a squirrel glider that was trapped and radio-collared in north-east Victoria was found as roadkill more than 500 km away, suggesting that a vehicle struck the animal and carried the carcass away from the site of impact. Our observation is the first evidence that this occurs for squirrel gliders.
11 We review eight years of monitoring data to quantify the number of predation attempts on 12 arboreal marsupials using canopy bridges and glider poles across a major road in southeast 13 Australia. We recorded 13,488 detections of arboreal marsupials on the structures, yet only 14 a single (and unsuccessful) predation attempt was recorded. 15
The Belt and Road Initiative, due to its diverse and extensive infrastructure investments, poses a wide range of environmental risks. Some projects have easily identifiable and measurable impacts, such as energy projects’ greenhouse gas emissions. Others, such as transportation infrastructure, due to their vast geographic reach, generate more complex and potentially more extensive environmental risks. The proposed Belt and Road Initiative rail and road investments have stimulated concerns because of the history of significant negative environmental impacts from large-scale
transportation projects across the globe. This paper studies environmental risks—direct and indirect—from Belt and Road Initiative transportation projects and the mitigation strategies and policies to address them. The paper concludes with a recommendation on how to take advantage of the scale of the Belt and Road Initiative to address these concerns in a way not typically available to stand-alone projects. In short, this scale motivates and permits early integrated development and conservation planning.
Millions of dollars are spent on wildlife crossing structures intended to reduce the barrier effects of roads on wildlife. However, we know little about the degree to which these structures facilitate dispersal and gene flow.
Our study incorporates two elements that are rarely used in the evaluation of wildlife crossing structures: an experimental design including a before and after comparison, and the use of genetic techniques to demonstrate effects on gene flow at both population and individual levels. We evaluated the effect of wildlife crossing structures (canopy bridges and glider poles) on a gliding mammal, the squirrel glider (Petaurus norfolcensis). We genotyped 399 individuals at eight microsatellite markers to analyse population structure, first-generation migrants and parentage relationships.
We found that the freeway was not a complete genetic barrier, with a strong effect evident at only one site. We hypothesise that the presence of corridors alongside the freeway and throughout the surrounding landscape facilitated circuitous detours for squirrel gliders.
Installing a crossing structure at the location with a strong barrier effect restored gene flow within just five years of mitigation.
Synthesis and applications. Our study highlights the importance of using genetic techniques not just to evaluate the success of road crossing structures for wildlife, but also to guide their placement within the landscape. Managers wishing to reduce the effects of linear infrastructure on squirrel gliders and other arboreal mammals should aim to preserve and enhance vegetation along roadsides and within centre medians, as well as mitigate large gaps by implementing wildlife crossing structures.
Although roads are often assumed to be barriers to the dispersal of arboreal species, there has been little empirical testing of this assumption. If arboreal animals are unable to cross roads, population subdivision may occur, or resources may become inaccessible. We tested the hypothesis that Route Nationale 4 (RN4), a paved highway, was a barrier to movement and dispersal of the Endangered golden-brown mouse lemur Microcebus ravelobensis in Ankarafantsika National Park, in north-west Madagascar. During June–August 2015 we conducted a capture–mark–recapture study at three sites: two adjacent to RN4 and one within intact forest without a potential barrier. During 2,294 trap nights we captured 120 golden-brown mouse lemurs 1,032 times. In roadside habitats we captured significantly more males than females, whereas the opposite was the case in interior forest habitat. We detected eighteen crossings of highway transects by nine individuals; however, all potential dispersal events involved males. In roadside habitat, movement was significantly inhibited in both males and females. We present some of the first data on the effects of roads on movement patterns in arboreal Malagasy mammals, showing species- and sex-biased effects of roads as dispersal barriers. Our findings indicate that roads may not be complete barriers to dispersal in lemurs. We recommend that conservation managers and scientists examine explicitly the effects of roads and natural arboreal bridges in Madagascar in future studies.
La expansión de las redes de transporte es una de las mayores amenazas a la biodiversidad, provocando, entre otras cosas, un aumento en el riesgo de mortalidad por accidentes para la fauna. El objetivo del presente trabajo es evaluar los atropellamientos ocurridos entre los años 2012 y 2016 en un tramo de 34 km sobre la ruta Nacional N°12, en el bosque atlántico del Alto Paraná (Argentina). Este tramo atraviesa varias reservas naturales y soporta un flujo turístico masivo. Se comparó el número de atropellamientos entre estaciones del año, tipo de cobertura vegetal y presencia de áreas protegidas, utilizando modelos lineales generalizados mixtos. Se mapeó la distribución de los accidentes para identificar sitios críticos. Durante los 47 meses del estudio, detectamos 1784 ejemplares atropellados: 67.5% mamíferos, 25.2% aves y 7.3% reptiles. La especie con más accidentes registrados fue la comadreja overa (Didelphis albiventris), seguida del lagarto overo (Tupinambis merianae) y el alicuco común (Megascops choliba). Solamente los reptiles mostraron diferencias significativas en los atropellamientos por estación. El 72.6% de los accidentes ocurrieron en zonas con presencia de bosque nativo. A su vez, el 70.7% de los accidentes ocurrió dentro de áreas protegidas. Se identificaron 13 kilómetros críticos donde se propone la aplicación de medidas, como la colocación de reductores de velocidad y la adecuación de estructuras de desagüe de arroyos como pasafaunas subfluviales. Una vez implementadas las medidas propuestas, debería evaluarse su efectividad a largo plazo.
Context: Wildlife crossing structures are installed to mitigate the impacts of roads on animal populations, yet little is known about some aspects of their success. Many studies have monitored the use of structures by wildlife, but studies that also incorporate individual identification methods can offer additional insights into their effectiveness.
Aims: We monitored the use of wildlife crossing structures by arboreal marsupials along the Hume Freeway in south-eastern Australia to (1) determine the species using these structures and their frequency of crossing, (2) determine the number and demographic characteristics of individuals crossing, and (3) use the rate of crossing by individuals to infer the types of movement that occurred.
Methods: We used motion-triggered cameras to monitor five canopy bridges and 15 glider pole arrays installed at 13 sites along the Hume Freeway. The five canopy bridges were also monitored with passive integrated transponder (PIT)-tag readers to identify the rate of use by individuals.
Key results: Five species of arboreal marsupial were detected using canopy bridges and glider poles at 11 sites. Our analysis suggested that increasing the number and the distance between poles in a glider pole array reduced the rate of use by squirrel gliders. The PIT tag and camera footage revealed that the structures were used by adult males, adult females and juveniles, suggesting that all demographic groups are capable of using canopy bridges and glider poles. At two canopy bridges, multiple squirrel gliders and common brushtail possums crossed more than once per night.
Conclusions: Given that previous studies have shown that the freeway is a barrier to movement, and that many of the species detected crossing are subject to road mortality, we conclude that canopy bridges and glider poles benefit arboreal marsupials by providing safe access to resources that would otherwise be inaccessible.
Implications: Although the factors influencing crossing rate require further study, our analysis suggests that glider pole arrays with fewer poles placed closer together are likely to be more successful for squirrel gliders. The individual identification methods applied here offer insights that are not possible from measuring the rate of use alone and should be adopted in future monitoring studies.
Roads and traffic reduce landscape connectivity and increase rates of mortality for many species of wildlife. Species that glide from tree to tree may be strongly affected by roads and traffic if the size of the gap between trees exceeds their gliding capability. Not only are wide roads likely to reduce crossing rates, but mortality may also be increased if gliders that do cross have poor landing opportunities. The road-crossing behavior of 47 squirrel gliders (Petaurus norfolcensis) was investigated in southeast Australia using radio-tracking. The proportion of gliders crossing one or both roadways of a freeway where trees were present or absent from the center median was compared to that at single-lane country roads (control). The proportion of gliders crossing the road at control sites (77%) was similar to the proportion that crossed one or both roadways at the freeway with trees in the median (67%), whereas only a single male (6%) crossed the freeway where trees were absent from the median. The frequency of crossing for each individual was also similar at control sites and freeway sites with trees in the median. The almost complete lack of crossing at sites where trees were absent from the median was attributed to the wider gap in canopy (50 - 64 m vs. 5 - 13 m at sites with trees in the median). This suggests that traffic volume, up to 5,000 vehicles per day on each roadway, and the other characteristics of the freeway we studied are not in themselves complete deterrents to road crossing by squirrel gliders. This study demonstrates that retaining and facilitating the growth of tall trees in the center median of two-way roads may mitigate the barrier effect of roads on gliders, thus contributing positively to mobility and potentially to connectivity. This information will be essential for the assessment of road impacts on gliding species using population viability models.
The effects of habitat fragmentation and deforestation are exacerbated by some elements, such as roads and power lines, which may become filters or barriers to wildlife movements. In order to mitigate mortality and restore connectivity, wildlife passages are being constructed as linear corridors. The installation of these mitigation measures must be followed by systematic monitoring, in order to evaluate their use and effectiveness, to assist in their management, and to convince stakeholders of their value. In this paper we present the results of a monitoring study of the use of rope overpasses developed near a protected area in Porto Alegre, southern Brazil. The canopy bridges were installed by the Urban Monkeys Program in places where electric hazards and road-kills of brown howler monkeys (Alouatta guariba clamitans Cabrera, 1940) were recorded. Camera traps were installed at each bridge, and local people were selected and trained to monitor overpass use over 15 months, from August 2008 to October 2009. Three species were recorded using canopy bridges: brown howler monkey (Alouatta guariba clamitans Cabrera, 1940), white-eared opossum (Didelphis albiventris Lund, 1840) and porcupine (Sphiggurus villosus Cuvier, 1823). Rope bridges with the highest number of species recorded had more forest cover and lower urban area around them than overpasses little used. Our results indicate that overpasses, in Porto Alegre, work as a linear corridor between forest remnants, although the outcomes for individual survival, group persistence, population demography or gene flow have not been measured. Furthermore, canopy bridges may be important to mitigate the impact of roads and power lines on wildlife, but electric cables also need to be completely isolated when present, to warrant animals' physical integrity.
We investigated the use of inexpensive aerial bridges (rope canopy bridges) above roads and a highway by arboreal mammals in the Wet Tropics of Queensland, Australia. Three rope bridge designs were trialed, including a single rope, ladder-like bridges and tunnel-shaped bridges. Nine mammal species were recorded using canopy bridges, including five species or subspecies endemic to the Wet Tropics and three species listed as rare under State nature conservation legislation. Most of these species suffer severely from either the fragmentation or mortality impacts caused by roads. Over 50 crossings above a 15-m-wide tourist road were observed on an elevated ladder-like bridge. Longer (∼40m) rope bridges were used on several occasions by four species. Our observations suggest that canopy bridges can assist rare arboreal mammals to cross roads in the Wet Tropics, thereby reducing both the risk of road-kill and the potential for subpopulation isolation. Further research is required to ascertain the level of benefit afforded by canopy bridges for arboreal mammal populations. It is likely that rope canopy bridges will have broad application for a range of arboreal mammal species.
Wildlife crossing structures are commonly used to mitigate the barrier and mortality impacts of roads on wildlife. For arboreal mammals, canopy bridges, glider poles and vegetated medians are used to provide safe passage across roads. However, the effectiveness of these measures is unknown. We investigate the effect of canopy bridges, glider poles and vegetated medians on squirrel glider movement across a freeway in south-east Australia. We monitored structures directly using motion-triggered cameras and passive integrated transponder (PIT) scanners. Further, post-mitigation radio-tracking was compared to a pre-mitigation study. Squirrel gliders used all structure types to cross the freeway, while the unmitigated freeway remained a barrier to movement. However, movement was not restored to the levels observed at non-freeway sites. Nevertheless, based on the number and frequency of individuals crossing, mitigation is likely to provide some level of functional connectivity. The rate of crossing increased over several years as animals habituated to the structure. We also found that crossing rate can be a misleading indicator of effectiveness if the number of individuals crossing is not identified. Therefore, studies should employ long-term monitoring and identify individuals crossing if inferences about population connectivity are to be made from movement data alone.
Fragmentation is a severe threat to tropical rainforests. However the habitat loss and less extensive fragmentation caused by roads can also be a threat, not only through allowing access to remote areas, but also through a suite of insidious associated impacts. These include abiotic and biotic edge effects adjacent to road clearings, the disturbance impacts caused by vehicle operation, invasions by weeds, feral and alien fauna and disease, and faunal mortality from vehicle collisions. In combination, these can create a significant barrier to movements of rainforest biota. Impacts can be ameliorated through clever road design and sustainable vehicle operation.
The gliding angle of the Mahogany Glider Petaurus gracilis and the Sugar Glider Petaurus breviceps was determined from field studies by measuring the height of launch and landing of glides and the distance travelled. This showed no significant difference between these two species in glide ratio, which averaged 1.91 and 1.82 m distance per 1 m loss in altitude, respectively, nor in glide angle which averaged 28.26° and 29.69° for the Mahogany Glider and Sugar Glider, respectively. Significant differences were found between them for height of launch (19.75 and 11.96 m, respectively), height of landing (4.48 and 1.95 m, respectively), diameter at breast height of landing tree (44.12 and 23.22 cm, respectively), and glide distance (29.71 and 20.42 m, respectively). An examination of the ratio of interorbital width to maximum skull width of gliding and nongliding possums was measured from museum skulls to examine whether gliders have eyes wider apart, to allow triangulation of distance in preparation for gliding. Gliding possums showed a trend toward having a larger interorbital width than nongliding possums, although there appear to be several factors acting on the interorbital width. Museum study skins of all gliding marsupials were measured to determine the relationship between patagium surface area and body mass which showed a clear relationship (r2 = 0.9688). A comparison of gliding behaviour, patagium, development of limbs, tail morphology and mass was also made between gliding marsupials and other gliding mammals.
Roads and traffic are prominent components of most landscapes throughout the world, and their negative effects on the natural environment can extend for hundreds or thousands of meters beyond the road. These effects include mortality of wildlife due to collisions with vehicles, pollution of soil and air, modification of wildlife behavior in response to noise, creation of barriers to wildlife movement, and establishment of dispersal conduits for some plant and animal species. In southeast Australia, much of the remaining habitat for the squirrel glider, Petaurus norfolcensis, is located in narrow strips of Eucalyptus woodland that is adjacent to roads and streams, as well as in small patches of woodland vegetation that is farther from roads. We evaluated the effect of traffic volume on squirrel gliders by estimating apparent annual survival rates of adults along the Hume Freeway and nearby low-traffic-volume roads. We surveyed populations of squirrel gliders by trapping them over 2.5 years, and combined these data with prior information on apparent survival rates in populations located away from freeways to model the ratio of apparent annual survival rates in both site types. The apparent annual survival rate of adult squirrel gliders living along the Hume Freeway was estimated to be approximately 60% lower than for squirrel gliders living near local roads. The cause of the reduced apparent survival rate may be due to higher rates of mortality and/or higher emigration rates adjacent to the Hume Freeway compared with populations near smaller country roads. Management options for population persistence will be influenced by which of these factors is the primary cause of a reduced apparent survival rate.
More than 300 species of Australian native animals — mammals, birds, reptiles and amphibians — use tree hollows, but there has never been a complete inventory of them. Many of these species are threatened, or are in decline, because of land-use practices such as grazing, timber production and firewood collection. All forest management agencies in Australia attempt to reduce the impact of logging on hollow-dependent fauna, but the nature of our eucalypt forests presents a considerable challenge. In some cases, tree hollows suitable for vertebrate fauna may take up to 250 years to develop, which makes recruiting and perpetuating this resource very difficult within the typical cycle of human-induced disturbance regimes. Tree Hollows and Wildlife Conservation in Australia is the first comprehensive account of the hollow-dependent fauna of Australia and introduces a considerable amount of new data on this subject. It not only presents a review and analysis of the literature, but also provides practical approaches for land management.
The Ambatovy Project includes a large, open-pit nickel mine located in Madagascar's eastern humid forest, and an associated pipeline to remove laterite slurry off site. The area is recognized for its high biodiversity exemplified by the presence of at least 13 lemur species in forests surrounding the mine site. In order to reduce potential habitat fragmentation impacts on the lemur populations as a consequence of recent access road construction, seven crossing structures (referred to as 'lemur bridges') were erected within the mine footprint area and along the slurry pipeline that will remain in place until rehabilitated forest allows for movement over roads via the forest canopy. Two bridge designs were used due to differences in road width and vehicle traffic type. Lemur bridges have been monitored since their construction in January-February 2009. To date (10 August 2010), bridges have been used by six lemur species. Mine footprint type bridges (suspension bridge design) have been used more frequently than slurry pipeline bridges (plank bridge design) and, overall, there has been an increase in bridge use in 2010 when compared to 2009 (from 8% to 24% of total observations where lemurs are present in proximity to bridges). These results suggest that although a certain time period may be required for lemurs to locate and habituate to bridges, these crossing structures offer an effective mitigation measure to assist in reducing the impacts of habitat fragmentation.
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 Australia's largest gliding marsupial, the greater glider (Petauroides volans), occurring in forest remnants in the fastesturbanizing 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.
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.
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.
Tree‐dwelling mammals may be vulnerable to road mortality if forced to cross canopy gaps on the ground. This group of mammals has received scant attention worldwide despite major road projects potentially causing severe fragmentation to their habitat. Gliding mammals may be enabled to cross road gaps that exceed their gliding capability by the installation of tall wooden poles to act as “stepping stones.” We investigated whether such glide poles installed across two land‐bridges in eastern Australia could restore landscape connectivity for small gliding petaurid marsupials. Hair‐traps revealed repeated use of all poles at both locations over periods of 1–3 years. Camera traps at one site suggest a crossing frequency on the poles by the squirrel glider (Petaurus norfolcensis) of once every 3.8 nights. Radio‐tracked animals did not glide directly over the road but instead used the poles to cross on the bridge. Hair‐traps and camera traps installed within the middle of two reference land‐bridges that lacked glide poles failed to detect crossings by gliding mammals despite their presence in adjacent forest. These observations suggest that glide poles can facilitate road crossing and thereby restore habitat connectivity for gliding mammals. This lends support to the notion that glide poles have the potential to mitigate road‐induced habitat fragmentation for gliding mammals worldwide.
Single trees and small patches of trees in farmland are conspicuous components of agricultural landscapes around the world, but their value for the conservation of biodiversity is not well known. In this study, arboreal mammals were censused by using hair-sampling tubes in small patches of woodland (single trees to patches <1.0 ha) in cleared farmland adjacent to a linear network of woodland known to support resident populations of arboreal mammals. Ninety-one small isolates were stratified by size (single trees or small patches) and distance from the linear network to test the capacity of animals to cross habitat ‘gaps’. The genus Petaurus (small gliding marsupials), the most commonly detected taxon, was recorded in 31% of hair-tubes (98 of 316). It occurred in 21% of sites in isolated trees and patches, and in all linear strips. A logistic regression model demonstrated that Petaurus sp. was most likely to occur in isolates in close proximity to linear strips and other patches of woodland. Ninety-five per cent of sites at which this taxon occurred were within 75 m of the linear network. This threshold corresponds with the maximum distance that animals can glide in a single movement between trees. The size of isolates did not influence utilisation rates. Such isolates are smaller than a single home range and were probably used to supplement home ranges centred on the linear network, by providing additional foraging habitat and den sites. Protection and restoration of isolated trees and small woodland clumps in cleared landscapes contributes to mammal conservation and this study provides quantitative data that can assist landscape design and habitat restoration in rural environments.
Linear infrastructure such as roads, highways, power lines and gas lines are omnipresent features of human activity and are rapidly expanding in the tropics. Tropical species are especially vulnerable to such infrastructure because they include many ecological specialists that avoid even narrow (<30-m wide) clearings and forest edges, as well as other species that are susceptible to road kill, predation or hunting by humans near roads. In addition, roads have a major role in opening up forested tropical regions to destructive colonization and exploitation. Here, we synthesize existing research on the impacts of roads and other linear clearings on tropical rainforests, and assert that such impacts are often qualitatively and quantitatively different in tropical forests than in other ecosystems. We also highlight practical measures to reduce the negative impacts of roads and other linear infrastructure on tropical species.
Nocturnal mammals are poorly studied in Central Africa, a region experiencing dramatic increases in logging, roads, and hunting activity. In the rainforests of southern Gabon, we used spotlighting surveys to estimate abundances of nocturnal mammal species and guilds at varying distances from forest roads and between hunted and unhunted treatments (comparing a 130-km(2) oil concession that was nearly free of hunting, with nearby areas outside the concession that had moderate hunting pressure). At each of 12 study sites that were evenly divided between hunted and unhunted areas, we established standardized 1-km transects along road verges and at 50, 300, and 600 m from the road. We then repeatedly surveyed mammals at each site during 2006. Hunting had few apparent effects on this assemblage. Nevertheless, the species richness and often the abundance of nocturnal primates, smaller ungulates, and carnivores were significantly depressed within approximately 30 m of roads. Scansorial rodents increased in abundance in hunted forests, possibly in response to habitat changes caused by logging or nearby swidden farming. In multiple-regression models many species and guilds were significantly influenced by forest-canopy and understory cover, both of which are altered by logging and by certain abiotic variables. In general, nocturnal species, many of which are arboreal or relatively small in size (<10 kg), were less strongly influenced by hunting and more strongly affected by human-induced changes in forest structure than were larger mammal species in our study area.
Taking the high road
Bekker, H. 2005. Taking the high road: Treetop bridges for
arboreal mammals. International Conference on Ecology
and Transportation, San Diego, CA.