In March 2021, as the COVID-19 pandemic raged, two of us decided to organise a virtual symposium on canopy bridges for the American Society of Primatology and Smithsonian’s Earth Optimism initiative. Afterall, canopy bridges are a conservation solution that invites optimism in the face of all the challenges confronting the natural world. We were astounded by the response to the symposium – we received 540 registration requests from 53 countries, and the seminar itself was a major success, with 130 live attendees (see link to recording on the ASP website: https://www.asp.org/asp-conservation/conservation-videos/). Following that experience, we could tell that interest in the topic of canopy bridges was growing, and we therefore invited our seminar speakers and others – together representing research on five continents – to join us in developing a special issue on the topic for Folia Primatologica. We had the sense that there were many studies waiting to be written, given the discrepancy between the number of studies we had heard of and the number of publications in the scientific literature. At the time of this writing, there were only 32 peer reviewed papers on bridges worldwide, with another 15 in the grey literature. Once again, we were surprised and excited to have a whopping 33 research teams submit proposals for special issue submissions, with 23 of them submitting and finally publishing their full manuscripts in this issue. These 23 additions represent research from 14 countries and five continents and a 72% increase in the number of peer reviewed publications on this topic (fig. 1).
Peer reviewed and grey literature publications on canopy bridges by country before (n = 32) and after (n = 32 + 23) the publication of the Folia Primatologica special issue.
Citation: Folia Primatologica 93, 3-6 (2022) ; 10.1163/14219980-930306IN
Peer reviewed and grey literature publications on canopy bridges by country before (n = 32) and after (n = 32 + 23) the publication of the Folia Primatologica special issue.
Citation: Folia Primatologica 93, 3-6 (2022) ; 10.1163/14219980-930306IN
Peer reviewed and grey literature publications on canopy bridges by country before (n = 32) and after (n = 32 + 23) the publication of the Folia Primatologica special issue.
Citation: Folia Primatologica 93, 3-6 (2022) ; 10.1163/14219980-930306IN
Each canopy bridge project and practitioner has a contribution to make on this relatively unknown and increasingly important topic. Given that many canopy bridge projects are grass-roots initiatives, they are often run by practitioners with little publishing experience or motivation to publish – another reason we wanted to provide the opportunity to publish in a special issue. We were very pleased to see that six (26%) of the 23 papers had first-time first authors, not publishing before because of being early in their careers or because they are conservation practitioners rather than researchers. For nine of the papers (39%) authors would not have or may not have submitted their work to a peer reviewed journal had the opportunity for publication in the special issue not arisen.
A smattering of bridge design types in the special issue publications: a) the Bengal slow loris (Nycticebus bengalensis) using a single rope bridge in northeast Bangladesh (Maria et al., 2022), b) the white-handed gibbon (Hylobates lar) on a single rope bridge behind a lattice bridge (Saralamba et al., 2022), c) mantled howler monkey (Alouatta palliata palliata) using a canopy bridge made of agricultural mesh line with an upper line for the tail in Costa Rica (Azofeifa Rojas and Gregory, 2022), d) Sykes’ monkey (Cercopithecus mitis albogularis) using a horizontal ladder canopy bridge made of rubber and PVC tubing (Cunneyworth et al., 2022), e) kinkajou (Potos flavus) using a rope lattice bridge in Mexico (Hidalgo-Mihart et al., 2022), f) the southern brown howler monkey (Alouatta guariba clamitans) using a rope latter with a crisscross (Monticelli et al., 2022), g) three dimensional canopy bridge installed under a bridge to provide crossings for the hazel dormouse (Muscardinus avellanarius) in the United Kingdom (Moffat et al., 2022), h) Japanese squirrel (Sciurus lis) using the three dimensional Dormouse Bridge in Hokuto, Japan (Minato et al., 2022).
Citation: Folia Primatologica 93, 3-6 (2022) ; 10.1163/14219980-930306IN
A smattering of bridge design types in the special issue publications: a) the Bengal slow loris (Nycticebus bengalensis) using a single rope bridge in northeast Bangladesh (Maria et al., 2022), b) the white-handed gibbon (Hylobates lar) on a single rope bridge behind a lattice bridge (Saralamba et al., 2022), c) mantled howler monkey (Alouatta palliata palliata) using a canopy bridge made of agricultural mesh line with an upper line for the tail in Costa Rica (Azofeifa Rojas and Gregory, 2022), d) Sykes’ monkey (Cercopithecus mitis albogularis) using a horizontal ladder canopy bridge made of rubber and PVC tubing (Cunneyworth et al., 2022), e) kinkajou (Potos flavus) using a rope lattice bridge in Mexico (Hidalgo-Mihart et al., 2022), f) the southern brown howler monkey (Alouatta guariba clamitans) using a rope latter with a crisscross (Monticelli et al., 2022), g) three dimensional canopy bridge installed under a bridge to provide crossings for the hazel dormouse (Muscardinus avellanarius) in the United Kingdom (Moffat et al., 2022), h) Japanese squirrel (Sciurus lis) using the three dimensional Dormouse Bridge in Hokuto, Japan (Minato et al., 2022).
Citation: Folia Primatologica 93, 3-6 (2022) ; 10.1163/14219980-930306IN
A smattering of bridge design types in the special issue publications: a) the Bengal slow loris (Nycticebus bengalensis) using a single rope bridge in northeast Bangladesh (Maria et al., 2022), b) the white-handed gibbon (Hylobates lar) on a single rope bridge behind a lattice bridge (Saralamba et al., 2022), c) mantled howler monkey (Alouatta palliata palliata) using a canopy bridge made of agricultural mesh line with an upper line for the tail in Costa Rica (Azofeifa Rojas and Gregory, 2022), d) Sykes’ monkey (Cercopithecus mitis albogularis) using a horizontal ladder canopy bridge made of rubber and PVC tubing (Cunneyworth et al., 2022), e) kinkajou (Potos flavus) using a rope lattice bridge in Mexico (Hidalgo-Mihart et al., 2022), f) the southern brown howler monkey (Alouatta guariba clamitans) using a rope latter with a crisscross (Monticelli et al., 2022), g) three dimensional canopy bridge installed under a bridge to provide crossings for the hazel dormouse (Muscardinus avellanarius) in the United Kingdom (Moffat et al., 2022), h) Japanese squirrel (Sciurus lis) using the three dimensional Dormouse Bridge in Hokuto, Japan (Minato et al., 2022).
Citation: Folia Primatologica 93, 3-6 (2022) ; 10.1163/14219980-930306IN
Bridge designs and monitoring
This special issue has added to our knowledge on the breadth of bridge design types (fig. 2). Many studies continue to use a two-dimensional horizontal lattice design, either without or with a crisscross X reinforcement (Cunneyworth et al., 2022; Monticelli et al., 2022), like the one developed by Teixeira and colleagues (2013) nearly a decade ago (fig. 2f). Researchers like Ow and colleagues (2022) have modified this design with a fence-like mesh below the lattice, and Hidalgo-Mihart and colleagues (2022) used a lattice design with rope grid reinforcement (fig. 2e), similar to Goldingay and colleagues (2013). Still other studies have turned the two-dimensional design on its side, using a vertical lattice (fig. 2c) (Azofeifa Rojas and Gregory, 2022; Yap et al., 2022). Researchers also have had perhaps a surprising amount of success with simple single-line designs (fig. 2a, b) (Franceschi et al., 2022; Maria et al., 2022; Saralamba et al., 2022) or designs with two lines next to each other (Flatt et al., 2022; Prasetyo et al., 2022). Two studies of dormice on opposite sides of the globe had success with three-dimensional bridges, including triangular, quadrangular, and spherical designs (fig. 2g, h) (Japan: Minato et al., 2022; Great Britain: Moffat et al., 2022). Finally, while natural bridge studies remain scarce and the special issue added only two papers to the handful that exist on this topic (Gregory et al., 2022; Teixeira et al., 2022), Gregory and colleagues (2022) demonstrate that natural canopy bridge studies serve the dual benefit of being effective conservation solutions and also informative about potential use patterns to expect in artificial bridges.
When installing canopy bridges, cost and maintenance can be a big concern and even inhibitory for implementation. Several papers in the special issue provide valuable information on the cost of the canopy bridges studied (Azofeifa Rojas and Gregory, 2022; Cunneyworth et al., 2022; Flatt et al., 2022; Minato et al., 2022). Azofeifa Rojas and Gregory (2022) also provide information on the maintenance schedule used to keep bridges functional.
Perhaps of most interest to the progress of canopy bridge science were studies that took an experimental approach to comparing different bridges (Flatt et al., 2022; Moffat et al., 2022; Narváez-Rivera and Lindshield, 2022; Prasetyo et al., 2022; Saralamba et al., 2022). Also worth highlighting is the fact that some species seem to be more amenable to using bridges, including howler monkeys, Alouatta spp. (Azofeifa Rojas and Gregory, 2022; Monticelli et al., 2022), while others required long habituation periods (Monticelli et al., 2022) or seem to be entirely averse to using bridges, like spider monkeys, Ateles spp. (Aureli et al., 2022; Hidalgo-Mihart et al., 2022).
The contributions in this special issue demonstrate the advances that researchers have made in understanding bridge use by employing camera traps for monitoring rather than traditional observation (Azofeifa Rojas and Gregory, 2022; Flatt et al., 2022; Hidalgo-Mihart et al., 2022; Minato et al., 2022; Monticelli et al., 2022; Yap et al., 2022). Furthermore, with the study by Franceschi and colleagues (2022), we also learned about effective arboreal application of a traditionally terrestrial method for monitoring bridges – a sand trap bed used to monitor a bridge – when camera trap theft is a concern.
World map indicating the countries in which 22 of the special issue canopy bridge studies were conducted (one paper covers the Americas in general).
Citation: Folia Primatologica 93, 3-6 (2022) ; 10.1163/14219980-930306IN
World map indicating the countries in which 22 of the special issue canopy bridge studies were conducted (one paper covers the Americas in general).
Citation: Folia Primatologica 93, 3-6 (2022) ; 10.1163/14219980-930306IN
World map indicating the countries in which 22 of the special issue canopy bridge studies were conducted (one paper covers the Americas in general).
Citation: Folia Primatologica 93, 3-6 (2022) ; 10.1163/14219980-930306IN
Geography of bridge studies
Researchers in Australia have historically led the way in canopy bridge research (fig. 1; e.g., Weston et al., 2011; Soanes et al., 2013; Yokochi and Bencini, 2015) before this special issue, having produced five times as many peer-reviewed publications as the country with the second highest number of publications: Brazil. While the special issue includes no contributions from Australia, it establishes that the number of canopy bridge studies in Brazil is steadily growing (Buss et al., 2022; Franceschi et al., 2022; Monticelli et al., 2022; Teixeira et al., 2022), with a review paper indicating that the country has nearly 150 canopy bridges (Teixeira et al., 2022). Research from Costa Rica is also well-represented in this special issue (Azofeifa Rojas and Gregory, 2022; Fan and Lindshield, 2022; Flatt et al., 2022; Narváez-Rivera and Lindshield, 2022), and while some countries have required linear infrastructure projects to include canopy bridges (e.g., Australia, Peru), remarkably, Costa Rica seems to be the first country in the process of passing legislation requiring canopy bridges over all new roads nation-wide (Azofeifa Rojas and Gregory, 2022).
We are excited to see that with this issue, research from six previously unrepresented countries has been added to the canopy bridge literature (figs. 1 and 3; Thailand: Aggimarangsee et al., 2022; Saralamba et al., 2022; Mexico: Hidalgo-Mihart et al., 2022; Bangladesh: Maria et al., 2022; Singapore: Ow et al., 2022; Argentina: Raño et al., 2022; Malaysia: Yap et al., 2022). The special issue includes two papers from Africa (Cunneyworth et al., 2022; Linden et al., 2022) but given the high number of arboreal species on the continent, it still remains under-represented in the world of canopy bridge research.
Cross-disciplinary approaches and local stakeholder engagement have shown to be crucial for implementation and long-term success of conservation actions (e.g., Jones et al., 2015; White, 2015). In this regard it is encouraging news for conservation and a testament to the hard work of researchers who conserve their local habitats, that for 78% of the 23 papers (N = 18), the first author resides in the country of research, and an even higher percent (82%, 112 of the 136) of the total number of authors reside in-country. Author affiliations on the special issue publications illustrate the wide variety of professionals involved in canopy bridge research and installation, with authors representing universities, NGOs, government, corporations, and even high schools. The combinations of researchers illustrate how technically complex some canopy bridges studies are – for example, Minato and colleagues (2022) were a team of 25 co-authors from the academic, engineering, industry, and government sectors – and how grassroots others are. The study by Raño and colleagues (2022) is an example of a high school applied education program providing an excellent collaboration opportunity for researchers, local government, educators, and teenagers.
The publications in this issue provide some insight into the variety of taxa for which bridges are providing a solution. The majority of the studies in this issue targeted primate species, from lorises (Maria et al., 2022; Nekaris et al., 2022) to apes (gibbons and orangutans: Prasetyo et al., 2022; gibbons: Saralamba et al., 2022), but several studies found non-primates to use the bridges (Azofeifa Rojas and Gregory, 2022; Fan and Lindshield, 2022; Franceschi et al., 2022; Monticelli et al., 2022; Teixeira et al., 2022), and Gregory and colleagues (2022) discovered that their natural bridges provided a crossing option for a surprising 27 species of 12 families. Two other studies from opposite sides of the globe were focused on local species of Dormice (fig. 3g and h; Minato et al., 2022; Moffat et al., 2022).
Bridges and communities
With this issue, our knowledge of the challenges linear infrastructure presents to arboreal mammals and our understanding of the urgent need for implementing mitigation measures is expanded with papers on road and power line mortalities (Linden et al., 2022), the role canopy bridges can play in reducing these (Azofeifa Rojas and Gregory, 2022) and even unexpected use of underpasses by arboreal mammals (Franceschi et al., 2022). In one case, a team of researchers even suggested that despite the danger of electrocution, electrical lines can serve as canopy bridges when the ground presents an even greater danger from packs of domestic dogs (Aggimarangsee et al., 2022).
Canopy bridges are a solution to fragmentation that is typically human-induced. This means that they can either reduce human-wildlife conflict or become a source of it when local communities are not in support of bridges. Several authors discuss improving canopy bridge outcomes by engaging with local communities to understand their perceptions of the bridges (Nekaris et al., 2022) and/or to receive their input in canopy bridge designs (Fan and Lindshield, 2022; Raño et al., 2022). Other authors fomented community support for bridges by building them on private land and engaging landowners in bridge monitoring (Azofeifa Rojas and Gregory, 2022), or they used canopy bridges as an education tool to encourage engagement of children (Raño et al., 2022). Other projects have benefitted from citizen data collection (Saralamba et al., 2022; Yap et al., 2022) to monitor bridge use.
Conclusion
With this substantial addition to the canopy bridge literature, we have gained considerable knowledge on what works with this potential solution to landscape fragmentation. But perhaps even more so, the special issue demonstrates that we have only scratched the surface. Questions remain about which bridge design will be preferred by which species, which substrate is preferred, where bridges should be placed, among many others. The more canopy bridge practitioners do to answer these questions, the more established the science of canopy bridge research will become, the more effective our mitigation solutions will become, and the more bridges will become common practice, as over and underpasses have come to be for terrestrial species (e.g., Denneboom et al., 2021). With what we envisioned to be a publishing window of opportunity for canopy bridge researchers, we hope we have also provided a window of information into the unexplored science of arboreal animal use of canopy bridges.
Corresponding author; e-mail: gregoryt@si.edu
Acknowledgements
We thank the many authors of the manuscripts in this special issue for their contribution to conservation and the literature on the growing field of canopy bridge research. We also thank staff at Brill, particularly Wendel van der Sluis, for their support in putting together this immense special issue.
Statement of ethics
This study did not involve contact with animals or people.
Conflict of interest statement
The authors have no conflicts of interest to declare.
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