Research articleArtificial lighting reduces the effectiveness of wildlife-crossing structures for insectivorous bats
Introduction
Roads are one of the most pervasive threats to the persistence of wildlife in many parts of the world (Forman and Alexander, 1998; Forman et al., 2003; van der Ree et al., 2011). Wildlife crossing structures are a common solution to addressing the mortality and barrier effects of roads and traffic by facilitating the safe movement of individuals, and their genes, across roads, while reducing exposure to the road and traffic (Soanes et al., 2013, 2018; Sawaya et al., 2014; Smith et al., 2015). To justify the significant construction and maintenance costs, additional uses for these structures have been proposed and implemented, such as providing a safe passage for people while bicycling, walking, horse riding, etc. (Smith et al., 2015; van der Ree and van der Grift, 2015). A dilemma arises when the needs of each user, humans and wildlife, compete. For example, incorporating artificial light at night (ALAN) throughout the structures may be considered essential for human safety (Baumgartner et al., 2011), but may also have adverse effects on nocturnal wildlife (Longcore and Rich, 2004). ALAN in underpasses (i.e. crossing structures that go under the road) has reduced the rate of crossings by Columbia black-tailed deer (Odocoileus hemionus columbianus), deer mice (Peromyscus maniculatus), and opossum (Didelphis virginiana; Bliss-Ketchum et al., 2016). If managers are to install underpasses that are useful for humans as well as wildlife, a better understanding of the trade-offs between human co-use and the effectiveness for wildlife is required (van der Ree and van der Grift, 2015).
Large roads can have numerous negative impacts on insectivorous bats (hereafter referred to as “bats”). Some bat species are susceptible to high rates of road mortality (Lesiński et al., 2011; Medinas et al., 2013), while others avoid approaching and crossing roads (Medinas et al., 2019) due to the gap created in the canopy (Bennett and Zurcher, 2013; Hale et al., 2015), the presence of vehicles (Zurcher et al., 2010), and/or traffic noise (Schaub et al., 2008; Siemers and Schaub, 2011). Crossing structures, such as purpose-built wildlife underpasses, may reduce these impacts and maintain the connectivity of bats in the landscape, as bats can roost under and commute through culvert and bridge underpasses (Abbott et al., 2012; Bhardwaj et al., 2017).
The impact of ALAN on the use of underpasses by bats is currently understudied (see Spoelstra et al., 2018 for exception), however the behaviour of bats around ALAN in other landscapes is insightful. For example, some species are sensitive to light at night and avoid lit landscapes (Stone et al., 2009; Threlfall et al., 2012; Rowse et al., 2016), while others show an attraction to light and exploit lit areas due to the rich food resources they provide (Threlfall et al., 2012; Rowse et al., 2016; Schoeman, 2016). Light pollution in urban landscapes has also been associated with reduced movement by bats (Laforge et al., 2019). In general, larger, faster-flying species of bats tend to be more tolerant of lighting, and in some cases may even exploit stationary lights as a resource for foraging. In contrast, smaller, slower-flying species of bats are less commonly found around lights, and more commonly are associated with light-sensitivity (Jung and Threlfall, 2016; Rowse et al., 2016; Laforge et al., 2019). Therefore, ALAN in underpasses may have contrasting effects on bats, such as improving rates of use of crossing structure, exacerbate road impacts such as road-mortality and barrier effects, or having no effect at all. Understanding this relationship, and the potential influence ALAN has on the use of underpasses by bats is essential in order to design the most effective crossing structures and reduce the impacts of roads on bats.
The aim of this study was to investigate how bats respond to ALAN within underpasses. We experimentally evaluated the changes in the number of bat passes (i.e. sequences of echolocation calls) of nine species and one species group of bats in south-eastern Australia at bridge and culvert underpasses when the structures were lit and not lit. By simultaneously monitoring lit and unlit structures, through a before-during-after control-impact experiment, we were able to evaluate the response of bats to lighting (i.e. attraction, avoidance, or no response; Fig. 1), while controlling for other confounding impacts of roads (e.g. vehicle presence, change in vegetation structure). We expected lighting to change the rate that bats cross through underpasses, and that species would show the same response to lighting in bridges and in culverts. We provide inference of the ecological consequences of lighting underpasses on bats based on changes in the number of bat passes and provide recommendations on lighting regimes in underpasses.
Section snippets
Study area
We conducted this study along the Calder Freeway in Victoria, Australia, approximately 100–130 km to the north west of Melbourne. We studied a 40 km section of this highway, which was upgraded to a four-lane freeway between 2003 and 2009. The freeway has two lanes in each direction (each carriageway approximately 12 m wide, separated by a grassy median approximately 5–20 m in width), a maximum speed limit of 110 km/h and an average daily traffic volume ranging from 5500 to 9100 vehicles/day
Results
When the structures were lit, the overall number of bat passes of all species combined decreased under the structures and increased above the structures, relative to the baseline levels (Fig. 3). After the lighting was removed, the overall number of bat passes was closer to the baseline levels over the next four nights, but did not return to baseline levels entirely. Species varied in their response to lighting in culverts and bridges and therefore these are discussed separately below.
Discussion
In this study, we evaluated the impact of ALAN on the rate of use of underpasses by insectivorous bats while holding other confounding variables, such as vegetation structure and traffic disturbance, constant. We determined that lighting in underpasses reduced the activity levels of most bat species. Similar patterns have been found for other nocturnal terrestrial mammals (Bliss-Ketchum et al., 2016), suggesting that lights are likely to overall have a negative impact on underpass use by a
CRediT authorship contribution statement
M. Bhardwaj: Conceptualization, Methodology, Formal analysis, Writing - original draft, Writing - review & editing, Visualization, Funding acquisition. K. Soanes: Conceptualization, Methodology, Formal analysis, Writing - review & editing, Supervision. J.J. Lahoz-Monfort: Formal analysis, Writing - review & editing, Supervision. L.F. Lumsden: Conceptualization, Methodology, Formal analysis, Writing - review & editing, Resources, Supervision. R. van der Ree: Conceptualization, Methodology,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Funding for this study was provided by The Baker Foundation, Earthwatch Institute, Holsworth Wildlife Research Endowment, and The Albert Shimmins Fund. Research conducted under Scientific Permit 10006093 granted by the Department of Environment, Land, Water & Planning. VicRoads facilitated access to study sites. Thank you to VicRoads, C. Moore and A. Sjölund for support throughout this project. Finally, we would like to thank the four anonymous reviewers whose comments helped to improve our
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