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Reducing Road Impacts on Tree-Dwelling Animals

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Abstract

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.

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... 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). ...
Article
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. ...
Article
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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. ...
Article
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. ...
Article
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). ...
Article
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). ...
Article
Download this article in Blog Alex Bager (http://bab.empreendedor-academico.com.br/) 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). ...
Chapter
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... 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 [29]. ...
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... 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. ...
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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.
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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.
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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.
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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.
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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
  • H Bekker
Bekker, H. 2005. Taking the high road: Treetop bridges for arboreal mammals. International Conference on Ecology and Transportation, San Diego, CA.