Bats are mammals whose forelimbs form wings (Chiroptera
). The order of Chiroptera
is divided into two suborders: Megachiroptera
(fruit bats) and Microchiroptera
(insect-eating bats). Bats are distinct from other mammals because of their ability of powered flight. A majority of the bat species are nocturnal animals. Hanging upside down—a peculiar posture bats assume during winter hibernation—is caused by the specific anatomy of their hind legs [1
]. As insectivores, bats are considered allies of farmers and foresters [2
]. The Spanish guidelines ([2
], Appendix 22, p. 109) put it straight that: “bats play a major role in protecting biological diversity.” Taking into account the above facts and relatively small populations of different bat species, a number of rules and measures are currently in place to protect them [1
The bat protection guidelines defined in References [2
] require, without limitation, that:
new artificial nursery roosts are provided or the existing ones are maintained (including adaptation of pre-war bunkers and air-ride shelters to serve as winter roosting sites),
the timing and methods of construction, repair, and other works do not interfere with the bat breeding and hibernation periods,
observation and recording (monitoring) schemes are in place, covering roosts, refugees, and populations.
Bats are very helpful to humans and their protection is a major environmental issue [1
]. These small mammals play a very important role in limiting the populations of pests and insects and also pollinate and spread plant seeds [10
]. As such, they must be considered a very important asset of the natural environment. Factors harmful to bats include overuse of chemicals, which is noted more frequently these days, which is aggravated by the use of very toxic pesticides and timber fungicides. Moreover, old deciduous forests that provided foraging and nursery roosting sites for some bat species have largely vanished or at least have become degraded. The changes in the social and urban environment have made bats look for new shelters after co-habiting with men for centuries. In the past, spaces such as shacks, damp cellars, lofts, farm buildings, and church towers were frequently used by huge numbers of bats as winter roosts. However, currently, occupied buildings are weatherproofed and insulated (including lofts and cellars), which drives off bats to other potential roosting areas, such as caves, tree hollows, abandoned military bunkers, and air-ride shelters, spaces under bridges, and inside disused wells, i.e., any space where ambient temperature is sufficiently low to enable them go into winter hibernation.
This has been one of the causes why we can find tens of thousands of bats hibernating in pre-war disused German fortifications, bunkers, and air-rider shelters. Besides the above-mentioned alterations to buildings, bat habitats are often disturbed by construction of new roads, motorways, and railway lines, which, by crossing their flyways, many cut them off from their foraging or winter roosting sites. In Poland, pre-war German bunkers are converted to bat habitats as a compensational measure in the case of new road construction projects [11
]. Similar compensation is provided in Ireland [3
]. In other countries, adjacent tree stands are adapted to increase their bat winter roosting capacity as compensatory remediation for habitat losses related to road construction projects [12
]. Such replacement roosts are made by attaching parts of cleared trees, which include natural hollows to the trees that are to remain in place or by making artificial hollows in completely sound trees. While these measures may turn out beneficial, their performance has not yet been confirmed and, as such, they should be monitored and used only with caution.
Taking the above aspects into account, we can state that the sustainable design of roads must include bat-related compensatory remediation. The computing routes of bats cross both the existing and planned roads. The key consideration to be taken into account when designing bat bridges is to ensure appropriate flight height across the road. Hence, the design must be preceded by an environmental survey and the flyway must be precisely determined by a chiropterologist for the observed species. Any built structures located within the flyway should be designed by taking into account this height and, moreover, adequate crossing structures should be provided to prevent collisions between bats and cars travelling down the road.
The importance of the road, the level of service, and the vehicle composition of traffic must all be carefully analyzed [14
] since there is a direct relationship between the minimum vertical clearance of the road and the bat passage height. On single two-lane carriageways of lesser importance, hop-overs, which are a natural type of crossing, should be designed [4
] comprising appropriately shaped groves of adequate height to suit the flyway of the bat species concerned (Figure 1
In recent years, various measures and structures have been tested on the crossings between bat flyways and roads to encourage bats to raise their flight height above the minimum clearance height of the road. The design and guidelines for use of such structures are detailed in References [2
]. In the Netherlands, hop-overs presented in Figure 1
a, without mesh fences, were initially recommended for roads of lesser importance and were found effective for almost all species except for Rhinolophes
. This species tended to lower the height of flight at the median when a double height fence was provided, where bats were meant to cross the road. Therefore, the French guidelines recommend using 6-meter high wire mesh fences (Figure 1
] both for dual carriageways and for roads of lesser importance wherever the presence of Rhinolophes
was confirmed by the field survey.
Besides hop-overs, i.e., natural type crossings, various types of built structures are tested in different countries as compensational mitigation measures. An example of such experimental structures are unique spherical structures installed in two places over the A89 motorway near the town of Balbigny in France (Figure 2
] whose design process included multiple changes and adjustments in relation to earth retaining and vibration damping elements, service lifetime of materials, and the drainage system design. The structural details and engineering challenges are as presented by Riou in Reference [17
]. The bat flight pattern was meticulously examined using a thermal imaging camera [16
]. The camera lens was positioned facing the side of the spherical shell structure to record the flight path along the structure and face the outlet to observe the flight paths inside it as well as to assess the flight path clearance for flights outside the structure. The footages were analyzed to confirm previous observations made on crossings with steel structures located in England [19
] where bats followed a sinusoidal path along the structure, except that the specific spherical shape of the analyzed structure considerably increased reverberation over the median, i.e., halfway along the passage length [18
], which most probably was the cause of slightly raised flight height over the second carriageway. Flights beside the spherical shell structure were noted in many cases when the bat chose not to cross the road or continue foraging activity [18
]. The final conclusions section of the report [18
] suggests the need to verify the performance of spherical shell structures by regular monitoring. The experimental applications in Great Britain include steel structures [19
] and mesh enclosed bridges suspended from trees [22
]. However, they are no longer used due to the reported poor performance [21
]. Gaps in the corridor right at the structure were considered one of the causes.
Another example of special structures are single-span bridges fully enclosed by corrugated sheet cladding installed in Germany over the Biberach by-pass (Figure 2
]. They were mounted fully by crane in two crossing locations. Their side walls are ca. 4 to 5 meters in height. The troughed cladding, mat finished in grey is fixed to steel lattice frame making up a sealed enclosure. A number of follow-up surveys were conducted by the environmentalist Jürgen Trautner [24
] who used, besides ultrasound recording, night-vision devices and infrared cameras. The survey results confirm satisfactory performance of this crossing solution [24
Problems related to environmental compensation are typically involved in motorway and expressway construction projects. Bearing in mind the above-described structures and their shortcomings, follow-up monitoring should be implemented for any new solutions designed to guide bats across roads in order to determine the crossing point performance on a case-by-case basis. This is because a structure that performed well at one place, as confirmed by a follow-up survey, can fail to perform at another.
Let us take, as an example, the environmental challenges encountered during construction of the S3 expressway in Poland, which crossed the flyways to winter roosts in a forested area near Szczecin. As far as commuting routes of bats are concerned, Puszcza Bukowa, which is a special protection area, was found to be of prime importance. It is a large forest that surrounds the south-eastern part of Szczecin. As part of environmental compensation, bat gantries were provided on three bat commuting routes severed by the planned expressway. The monitoring scheme was implemented from 2010 to 2012. The results showed varied use frequency between the structures in the consecutive years [25
]. During the analysis of the available survey data, it became apparent that, in order to establish why a given structure performs well at one location and much less at another, it is necessary to have determined the effect of the relevant parameters of the road (further called determinants) on the frequency of commuting of bats. This relationship is the subject of analysis of this article.
2. Analysis of the Commuting Routes of Bats in the Puszcza Bukowa Forest Before and After Construction of the S3 Expressway
The commuting routes of bats to 13 pre-war bunkers were monitored by chiropterologists before and during the S3 construction project. These bunkers provided winter roosts after their adaptation for this purpose by the Mopek bat conservation association [11
]. The commuting routes of bats before the S3 expressway construction are presented in Figure 3
. The first of them was located at the verge of Puszcza Bukowa ca. 630 m from the A6 motorway (Figure 4
). The middle one crossed a wide forest opening (Figure 5
). The last of the three routes was located ca. 1800 m from the A6 motorway, which was also at the verge of the Puszcza Bukowa forest (Figure 6
). Figure 7
presents the pre-construction and post-construction commuting routes across the S3 expressway with the bat gantries designated as 1, 2, and 3, and, additionally, two wildlife underpasses constructed on the severed forest tracks were designated with the symbol typically used for road structures.
Significant flight route diversions occurred within the Puszcza Bukowa forest. The two main commuting routes along the forest edge that were identified during the pre-construction survey defined the locations of two bat gantries designated No. 1 and No. 3 (see Figure 7
). Two underpasses were built on two local asphalt paved forest tracks and they were also appropriated by bats as part of their flight route already at the time of the expressway construction. The bat activity surveys conducted both during and after construction [25
] showed that, in the thin part of the forest bats, created a new flyway on which the third gantry structure was built, designated as gantry No. 2 (see Figure 7
). The new commuting routes are marked in Figure 7
with red symbols of bats.
Several research and post-construction surveys were carried out in the project area [26
]. According to the survey data [26
] in March, the greatest number of flights was noted over the third gantry, which is most distant from the interchange. However, taking into account all the surveys, the greatest number of flights was recorded over the second gantry. The least number of flights was recorded over the first gantry. Figure 8
shows the aggregate values [26
] from the corresponding dates and hours of the 2010–2012 bat activity surveys. From the descriptions provided in References [26
], it transpires that bat movements in the vicinity of gantry No. 1 were, for some bat species, associated with their hunting grounds and, in some cases, bats diverted from the path right above the structure or flew next to it all the way. In the summer months, passes over gantry No. 3 were related to foraging activity only in the case of a few species, with the majority of the flights being related to the linear landscape feature. Conversely, most of the flights over gantry No. 2 were related to linear landscape features used by the bats for commuting and migration. Only in March, flights related to foraging were noted over gantry No. 2 in the case of two bat species (Nyctalus noctula
and Nyctalus leisleri
The analysis of the bat activity survey data shows that, in the initial period of operation of the S3 expressway operation, bats discarded their existing commuting routes, as it has been additionally confirmed by other researcher data [18
]. Severing of the linear elements of landscape located on the established commuting routes in relation to the construction works, including clearing of trees, disturbed the bat movements, which resulted in unsatisfactory results of environmental monitoring. In 2011, the former routes were re-established only to a small extent along the forest edge, i.e., over structures No. 1 and No. 3. The re-established routes result in an almost triple, yet still below expectations, increase in the number of confirmed passes. From the evaluation of the survey results [26
], one can see the difficulty in interpreting the observations and obtaining conclusive confirmation of the performance of the structures as a linear feature of the landscape. The conclusions of the bat activity surveys given in reports [26
] are in agreement with the conclusions from observations given in References [18
] in terms of the flyway routing and clearance from the new structure. The common finding was that, besides bats flying right over the structure, there are quite a number of them that choose to fly several meters away from it and follow sinusoidal rather than a straight-line path. Bats can sometimes increase their flight height above the structure or follow a sine path when flying next to it, dropping slightly below the floor mesh. Similar observations were made by authors of Reference [18
] during their study of spherical structures. While all the above-mentioned studies analyzed the situation with separate consideration of the different bat species and their flight techniques, the issue of how readily the same structures are used by bats at different locations was not addressed and the relevant determinants have not yet been sought. One of the key conclusions of the studies reported in References [19
] was the negative impact of gaps in the corridor leading the bats to the crossing structure (i.e., lack of gap-filling vegetation). The second major conclusion is the essential need to maintain the existing commuting routes, as confirmed several times by the study results and as described in the conclusions and recommendations of publications [4
3. Methods of Monitoring and Parameters of Roads that Affect Movement of Bats
Performance or failure to perform the analyzed bat gantries, installed over the S3 expressway, was assessed on the basis of a few parameters defining impact of the road traffic on the routing and frequenting of the commuting routes by bats. The pre-defined impacts of road parameters should be confirmed by the bat activity survey data and the types of commuting movements of different bat species, which should be determined as part of the basic studies to be conducted by chiropterologists. Three monitoring survey methods were used in the research [11
]. In the first method, the most popular among the three, bat detectors are deployed under each structure. The shortcoming of detector-based surveys is that they indicate solely the space without determining the flight height or direction. The frequencies of ultrasounds are recorded and they are, subsequently, used in computer analysis. The primary purpose of the computer analysis was to acquire information on the bat species commuting in the close vicinity of the analyzed crossing structure. The next method is based on identification and counting of roadkill on the road and on its shoulders in the vicinity of the installed crossing structures. In the third method, bats were observed by a chiropterologist in flight (direct observations). The present article uses monitoring data summarized in References [25
The issue of road features having a potential negative impact on the commuting movements of bats was investigated through review of available chiropterological literature in which the researchers described the types and results of the relevant experiments. The literature review concerning the analysis of linear landscape features guiding bats to the crossing structure covered primarily the results of studies of the negative impacts on the movements of bats, published in References [2
], including: road lighting, traffic noise, volume of traffic, or alterations to the linear landscape features. With such a vast area of study, it was necessary to adopt basic assumptions concerning evaluation of the influence of the respective individual features on the performance of the bat gantries under analysis [34
]. The recommendation for the analytical study is to choose sections having the same or almost the same values of parameters representing the different features except for the feature under analysis represented by parameter x
. This approach to experimental research enables estimating the magnitude of influence of the analyzed feature of the road.
According to the conclusions of studies on the negative impact of the volume of traffic on the number of bat flights [4
], this factor should be considered relevant to evaluation of the bat gantries’ performance. Taking this into account, the essential requirement of equal traffic volume at the analyzed locations should be considered satisfied since the gantries under analysis are located on one section of the S3 expressway at ca. 1.2 km intervals.
On the analyzed commuting routes, the linear landscape features have not been altered, except for clearing of trees in the right-of-way belt. Note that clearing width was the same at the three analyzed locations. This also ensures constancy of this parameter. The varying parameters were the landscape features at the respective gantries. The landscape features have a bearing on the illumination of the road surroundings and also on the traffic noise propagation.
The symmetric layout of landscape features along the S3 expressway and along the planned commuting route is ensured only at gantry No. 2 (Figure 5
and Figure 7
). At gantry No. 3, which is aligned with the forest edge, the landscape features are symmetric along the S3 expressway only in the summer and autumn (Figure 6
and Figure 7
). The big wooded area near gantry No. 3 planted with deciduous trees, that are bare in November and March, does not ensure symmetry of landscape features on the two sides of the constructed road. This lack of symmetry is also the case along the commuting route surrounded by the wooded area on one side and by farm fields on the other. In the case of gantry No. 1, we are dealing with no symmetry, as far as the layout of landscape features is concerned. In addition, part of the area is open and, thus, exposed to light pollution from streetlights of the three-level interchange and from the headlights of cars travelling on it (Figure 4
and Figure 7
). Gantry No. 1 is aligned with the forest edge. This means a similar lack of symmetry of landscape features along the commuting route. The three-level interchange is another feature responsible for the lack of symmetry along the S3 expressway. The ramps on the interchange are constructed at grade, in cuts, or on high fills. Additionally, the A6 motorways have varying grade lines, starting from grade level in the western part and dropping to ca. 4–6 m below grade in the eastern part. In the author’s opinion, the above-mentioned asymmetric layouts of the landscape features are also relevant to the performance of the constructed gantries.
A significant problem in analyzing the performance of bat gantries is the effect of road lighting and car headlights. The negative impact of road lighting on the movements of bats is broadly described in References [29
]. For example, the authors of Reference [30
] studied the effect of the type of lighting and illumination level on different bat species and how road lighting influences the activity of bats on their established commuting routes. According to the authors, the test data demonstrate that “… light pollution may have significant negative impacts upon the selection of flight routes by bats” [30
], (p. 1127). In the summary, the authors formulate a hypothesis that “… light pollution may force bats to use suboptimal flight routes, potentially causes isolation of preferred foraging sites …” [30
], (p. 1127). Therefore, taking into account the above conclusions and recommendations, it appears necessary to consider the effect of light pollution as an obligatory requirement in order to ensure sustainable design of bat gantries, including both selection of the location and the type of structure assisting the bats in crossing the road.
According to the Australian guidelines [31
], (p. 90) car lights “… can be visible up to 90 meters into the forest and further in open areas …” (Figure 9
). Scattered light of head-lights and tail-lights of cars travelling across a forest create undesired reflections, disturb all animal species, can scare animals, and even make them abandon their habitats.
Consequently, sustainable design of roads and the features related to conservation of bat flyways should take into consideration landscape features on both sides of the road in the nearest vicinity of the crossing (including wooded areas, groves, farm fields, and linear features) and appropriate compensational measures (physical) should be planned to considerably mitigate the negative impacts of artificial lighting on wildlife, including installation of screens, earth berms (Figure 10
), or lowering of the road grade line below the surrounding area. The areas requiring particular attention in terms of environmental compensation are clearly defined by the environmental conditions. These are nesting sites and waterlogged areas with a likely presence of local habitats, which can be potential foraging areas for bats.
Environmental compensation can be provided, for example, by screening structures protecting the habitat areas. In the case of bats, such screens additionally lead them to foraging grounds and to the crossing structure as well. This is, however, a costly option and the environmental benefits of it are still under evaluation. The total screen height should be determined depending on the topography of the area with 1.4 meters being the minimum height to effectively block the headlight beams [31
]. Higher screens are required where the roads run on slopes and the headlight beams can reach much deeper into the open area. Negative impacts of artificial light can be mitigated by planting gap-filling vegetation near the carriageway or by lining the road with earth beams (Figure 10
The next parameter of roads that must be considered in designing and erecting features associated with the protection of bat commuting routes is propagation of traffic generated noise into the surrounding area along the road. Higher noise levels can affect auditory perception of predators, which increases the predation rate, and the mating calls of males, which affects breeding success. The aspects of the negative impact of road traffic noise on bats are discussed in References [31
]. In the conclusions of the experimental research described in Reference [32
], the authors point out that, in a sustainable design of motorways, it is necessary to take into account potential foraging sites and winter roosts rather than only the commuting routes of bats. From the research data, it can be seen that a high level of traffic noise makes the bats discard any foraging areas within the strip of 10 to 15 meters from the road. In addition, foraging sites spaced 50 meters away from the motorway were not accepted by bats due to a high level of traffic during the night [32
According to the previously mentioned conclusions, the existing roosts of bats and location of crossing structures to be provided on their commuting routes should be analyzed in terms of the traffic noise level. It is worth mentioning that vegetation, whether natural or artificial (planted in gaps), is a “soft” kind of noise mitigation measures. The plantings to fill the gaps should always match the existing vegetation. Deciduous trees and shrubs are more effective in attenuating noise, as compared to coniferous trees. Yet, the latter are useful as a supplementary measure in the periods when the former are devoid of leaves [31
]. Hence, the gap bridging vegetation near the planned structures assisting bats in making their way across the road should include both coniferous and deciduous plants. The Australian guidelines [31
] depict the negative impact of the sound wave propagation similar to the impact of artificial light presented in Figure 8
. The existing forests and artificial vegetation in the form of small groves were composed of a few rows of plants at the forest edge along the constructed road, which reduce the noise level considerably while, on the other hand, individual trees growing along the road have no significant effect on noise reduction in their vicinity. The sound wave propagation is considerably disturbed (meaning much greater noise reduction) when the sound wave meets on its way vegetation composed of plants of different heights and varied species composition. The author’s own studies demonstrate that the greater the variation of vegetation, including both height and species’ composition, the better the noise reducing performance of this mitigation measure [36
]. Moreover, the noise reducing effect can be considerably improved when two or three rows of tall tree species are supplemented with shrubs in the undergrowth level.
Lowering the grade line below the surrounding area or lining the road with earth berms in flat areas can also do the job. However, the cost of such measures is high. For example, earth berms must be sufficiently high to be effective and this height translates to greater footprints and increased cost of a land purchase.
The negative impacts of road lighting and traffic noise can be mitigated on bat commuting routes by natural type structures, such as hop-overs. The design originally proposed by the Dutch researchers in 1991 [29
], have been since then supplemented in many aspects by different guidance documents published in the Netherlands and in other countries [4
]. Coming to the end of this part of the article, which concerns assessment of road characteristics in terms of their effect on noise reduction, it is worth noting that there are on-going research projects related to the application of pavement surfaces attenuating traffic noise and various types of quiet tires that can result in the reduction of road traffic noise. However, these new developments are still in the experimental stage, have not yet been completed, and, in the analyzed case, are only secondarily related to bat crossings’ performance.
Following presentation of the methods and road characteristics that can adversely impact the crossing movements of bats, the further part of this article will analyze the effect of light pollution, noise, and artificial gap-filling vegetation at the installed gantries on their performance at three different locations, which correlates them with the reported monitoring survey results [26
5. Discussion and Conclusions
Analyzing the above data, a hypothesis can be put forward that the existing lighting of the expressway, noise, location of the structure in relation to the three-level interchange, landscape features along the bat commuting route, and location of the structure in relation to the forest were the factors that might have had an important effect on the effectiveness of the installed structures. The results of the analysis of an impact of a given factor on the gantry structure performance presented in chapter 5 are compiled in Table 1
, where, if the impact of a given factor was negative, the factor received a minus (−) while, if the impact was positive, the factor received a plus (+).
A comparative analysis of the investigated factors that could potentially have an effect on the effectiveness of a gantry structure revealed that they could be subdivided into general factors and the main determinants of the expected good performance of the gantry structures were used (Figure 28
). When designing a road and planning locations of structures intended to assist bats in making their way across the road, if general conditions are met as a prerequisite, in order to achieve good effectiveness of a gantry structure, it is necessary to ensure that other relevant factors, being the main determinants, are satisfied.
This means the necessity to analyze the existing bat commuting routes and plan the location of the crossing structure in the forest opening and over the planned road, placed in a cutting over this section, and take every effort to provide suitable improvements along the approach route to the structure in the form of symmetrical guiding vegetation plantings or, if need be, filling vegetation in forest openings. This will encourage bats to increase the flight height and will protect them from the noise and light pollution.
If the existing commuting route does not lead through the forest opening but along the forest edge, it will also be beneficial to design the road in a cutting and provide suitable landscape features comprising guiding and filling vegetation. As for the filling vegetation, it can be provided in the forest area in the immediate vicinity of the road and possibly in the green belts along the cutting edge. If the existing bat foraging areas are located in the immediate vicinity of the structure, filling vegetation should be designed along the approach routes.
The analyses show that the more symmetrical is the layout of the surrounding landscape features in relation to the road and to the commuting route, the more protection from noise and light pollution they offer to the bat gantry, which improves its performance. The results of analyses regarding maintaining the symmetrical landscape features when designing structures to support bats in crossing the roads can be used in different countries for structures other than those investigated in this article.
The results of analyses of bat gantry area illumination show that the closer the edge of the forest is situated to the gantry structure and the more symmetrical are the landscape features, the less is the approach section affected by the headlight beams and noise. The situation is similar with noise propagation. The more artificial vegetation is present in the nearest vicinity of the crossing structure on both sides of the road and of the flyway, the greater is the noise reduction on the bat flyway. Vegetation planted symmetrically on both sides of the road to fill gaps in the forest openings makes the bats increase the height of flight and restricts access of light and noise to the approach route and to the place where bats rise in altitude to fly over the structure, which encourages the bats to use the route. On the other hand, aligning the crossing structures with the forest edge, with an asymmetrical landscape feature on both sides of the flight route, does not guarantee the desired performance of the structure. Several other conditions need to be met in this case so that the structure is accepted by bats. Conversely, without such a symmetry along the approach section, the structure may fail to perform.
If installation of a bat gantry is planned where the road intersects a forest opening, then a special lamp covers restricting propagation of light into the shoulder and the adjacent area should be specified for the road section in the nearest vicinity of the location. The light emitted by road lamps should illuminate only the surface of the road. If there is a fencing along the edge of the road, filling vegetation should be plated over the length of trapezoidal guiding corridors to fill the gaps between the guiding plantings, which, in this way, restricts the access of light emitted by road lamps. Elderberry (Sambucus nigra) and violet willow (Salix daphnoides) can be used for this purpose, which are fast growing species with good screening properties. In addition, they feature high resistance to traffic pollution.
Bat gantries should not be located where road elements with a potential to have an adverse impact on the commuting route are planned to be installed on or are at the route, including road lighting elements associated with interchanges or service areas, as this will compromise their performance below an acceptable level.