Linear transport infrastructures, such as roads, are a threat to biodiversity and one of the most significant causes of landscape fragmentation (Maxwell et al., 2016). These structures lead to habitat loss, roadkill, and disrupt the movement of animals by creating ecological barriers (Fahrig et al., 1995; Forman and Alexander, 1998). While most taxa are impacted by roads (Forman and Alexander, 1998), some are particularly sensitive to them. According to the European Red List of Amphibians (Temple and Cox, 2009), habitat loss and degradation are the most significant threat to amphibians, affecting 76 of the 85 species found in Europe (Stuart et al., 2004; Maxwell et al., 2016). Many amphibians undertake spring and autumn migrations, which renders them particularly sensitive to roads between the various key areas used during the different phases of their life cycle (Wilbur, 1980; Miaud et al., 2000; Semlitsch, 2008; Joly, 2019; Cayuela et al., 2020). Even outside the migration periods, roads disrupt the movement of individuals between local populations and, furthermore, increase mortality through roadkill, potentially leading to the extinction of local and regional amphibian populations (Fahrig et al., 1995; Dodd et al., 2004; Petrovan and Schmidt, 2016; Testud and Miaud, 2018; Joly, 2019).

To limit roadkill and restore connectivity, mitigation measures have been implemented. These measures are numerous and include, for example, traffic and speed reduction, road closures and wildlife passages (Schmidt et al., 2008; Rytwinski et al., 2016; Testud and Miaud, 2018). The last measure is often associated with fences (e.g., wire netting, plastic mesh, or full panel concrete or metal), constructed to prevent animals from venturing onto roads and guide them instead towards passages (under/overpasses), where they can safely cross a road (Schmidt et al., 2008; Arntzen et al., 2017; Carvalho et al., 2017; Testud and Miaud, 2018). These fences are typically designed for large fauna (large mesh-size), such as ungulates (e.g., deer; Fahrig et al., 1995; Romin and Bissonette, 1996; Clevenger and Waltho, 2000; Forman et al., 2003; Dodd et al., 2004; Glista et al., 2009), but often contain additional components (fences with a small mesh-size), to stop amphibians and small mammals (Morand and Carsignol, 2019). Large fauna fences have been relatively well studied (i) because they are of obvious interest for human safety (Romin and Bissonette, 1996; Schwabe et al., 2002; Forman et al., 2003; Bouffard et al., 2012) and (ii) because their effectiveness can be easily demonstrated by monitoring the number of collisions between ungulates and vehicles.

For small fauna, different types of fences have been used but their effectiveness has rarely been studied. The few studies that have been carried out suggest a low efficacy of such fences (Dodd et al., 2004; Woltz et al., 2008; Brehme et al., 2021; Conan et al., 2022). For example, Arntzen et al. (1995) tested the effectiveness of chicken wire (40 cm height, 10 cm overhang, 1.3 cm mesh size) and plastic mesh fences (50 cm height, 10 cm overhang, 0.3 cm mesh size) to avoid amphibian road crossings (toads and newts, respectively). They found that chicken wire reduced the number of crossings by between 35 and 70% for toads, while plastic fences reduced the crossings of newts by between 34 and 48%. The findings of the above and further studies indicate that plastic mesh fences allow a relatively high number of amphibians to pass and, in some cases, do not reduce roadkill (Arntzen et al., 1995; Baxter-Gilbert et al., 2015; Ottburg and van der Grift, 2019). Plastic fence heights tested in these studies ranged from 40 cm for amphibians to 80 cm in reptiles and no overhang was used. Another type of small wildlife fence regularly found along roads is a wire netting fence (with adapted mesh size). However, this type suffers from the same defects as the plastic mesh fence; it deteriorates quickly and is easily climbed and passed by small-mammals and amphibians (Conan et al., 2022). Non-mesh (full panel) fences, constructed with different materials (e.g., PVC, concrete and metal) and a typical height of 40–60 cm (Morand and Carsignol, 2019), have also been used alongside roads. Such fences are fairly durable, overcoming the problem of fast deterioration and are, therefore, increasingly recommended (Dodd et al., 2004; Morand and Carsignol, 2019; Conan et al., 2022). However, to the best of our knowledge, only one study tested their efficacy for amphibians under controlled conditions in the laboratory (Zbierski and Schneeweiß, 2003). That study, which tested the efficacy of different shapes of full panel concrete fences (40–60 cm height without overhang) for European tree frogs (Hyla arborea), found that the most effective shapes are difficult to build and expensive to maintain (Zbierski and Schneeweiß, 2003).

Our experiments showed that the height of the fence types tested was sufficient to stop adult individuals of species representing the running and short-distance jumping mode (Natterjack and European green toads), as well as all juveniles from crossing. Addition of an overhang (in case of the concrete fence) further reduced height at which these species were able to cross the fence. By contrast, proficient jumping and climbing species (American tree frogs and agile frogs) were not stopped by the greatest fence height tested. However, addition of an overhang stopped the proficient climbers (tree frogs) at a height below the maximal tested height, while it had no effect on agile frogs. An alternative galvanized metal fence (with overhang), which we tested with some species, performed similar to the concrete fence with overhang. Finally, climbing juveniles were only able to pass a concrete fence at about half its maximal height and only when it was wet.

In our setup, amphibians were highly motivated to cross the barrier separating the departure and arrival compartment. When failing repeatedly to cross at a particular height, individuals nevertheless continued to try. Crossing attempts in such cases ranged between 3 and 13 per hour for the two toad species tested. Furthermore, if amphibians failed to cross at a particular height, they were given two more nights (3 consecutive nights per test height in total). A sufficient time frame for testing is important, since motivation to cross might not be identical throughout trials. For example, we observed that individuals of all adult species, except Natterjack toads, failed to cross the fence at a lower height but managed at a greater height soon after. Accordingly, results from a previous study that are based on only 6 h of testing and without replicates should be treated with caution, as they might underestimate the crossing capacity of species (Zbierski and Schneeweiß, 2003). In the wild, the motivation to cross might differ with season, breeding status, rain and further parameters. Our experimental setup consisted of a small test arena and we used a number of stimuli to ensure sufficient motivation for crossing. Hence, we believe that our setup was well suited to test the crossing capacity of amphibians.

As expected, we found that the effective fence height (H_eff), i.e., the minimal height at which amphibians failed to cross, differed according to locomotion mode. However, in contrast to our most fundamental expectation, a concrete fence without overhang (Crt; maximal height tested: 40 cm) was only sufficient to stop the adults of the less nimble species, like toads (green toads and Natterjack toads) and all juveniles but failed to prevent the crossing of agile frogs and tree frogs (Figure 4). In case of the toads tested, larger individuals succeeded in crossing at a greater fence height (Figure 6). As expected, the addition of a 10 cm overhang (Crt+) increased the efficacy of the concrete fence, lowering effective fence height (H_eff) for the adults of all species where an overhang was tested, albeit, not for agile frogs (Figure 4). Hence, in case of most European amphibian species, road managers are left with the choice of a greater concrete fence height without overhang or a lower fence height with overhang. However, while an increase in fence height to, for example, 50 cm, may stop agile frogs, this will unlikely stop American tree frogs, unless an overhang is also installed. Accordingly, when more nimble species (agile frogs, tree frogs) are present, we strongly recommend the addition of an overhang to a fence. Such addition stopped the American tree frog, a species able to climb trees (Gourevitch and Roger Downie, 2018), at a fence height of 36 cm. Unfortunately, we were unable to test a fence height greater than 40 cm, so H_eff for agile frogs remains unknown, as they jumped across the greatest height tested, even in the presence of an overhang. This is likely of relevance for all proficient jumping amphibian species. Similar to previous recommendations (Morand and Carsignol, 2019), we therefore suggest a fence height of a least 50 cm with the addition of a horizontal overhang. The width of the overhang is crucial and anything less than 10 cm might not work well for frog species (Conan et al., 2022).

When the installation of a concrete fence is not possible (due to high costs and/or the destructive nature of its construction), road managers have the option to install other fence types. These might be less expensive to install but might also be less durable. As a first choice, road managers often consider the use of wire-netting fences to stop small animals. However, they are fragile and, most importantly, useless in preventing the crossing of green frogs, European green toad and many small mammalian species (Conan et al., 2022). A better option might be the use of galvanized metal fences of the type we tested (i.e., with a 5 cm horizontal overhang and a 2 cm downward component; Figure 3). Unfortunately, we were only able to test this fence type with green toads and tree frogs but H_eff was similar to that of the concrete fence with overhang (Figure 4). However, the overhang was problematic for the tree frogs, since they could attach themselves to the downward bent and then cross over the fence, reducing the efficacy of this fence type. Unfortunately, the 2 cm downward bent of the overhang was a structural requirement for the prototype we tested. Nevertheless, increasing the horizontal dimension of this overhang (e.g., from a width of 5 to 10 cm) might improve its efficacy and should be tested.

The 40 cm concrete fence stopped all juvenile amphibians, despite their similar locomotion mode to that of tree frogs (i.e., climbing), which were able to pass such a fence. Juveniles of all tested species were unable to reach a height greater than 10 to 20 times their body size (SVL ~1 cm). Unfortunately, we were unable to test the effect of an overhang with juveniles. Such structural addition will limit the presence of water along a fence during rainy weather (Schmidt et al., 2008), reducing fence wetting. The latter is of great importance, as fence wetting will likely ease fence crossing (Figure 5).

Our results are supported by previous studies indicating that durable and opaque structures are required to stop amphibians from crossing into roads and to guide them to the nearest wildlife passage (Brehme et al., 2021; Conan et al., 2022). A concrete fence equipped with a 10 cm overhang is an effective fence to stop the majority of amphibians present in Europe but only if it is well managed. In this context, we occasionally observed American tree frogs climbing the side walls of the arena, rather than the fence itself, from which they jumped over the fence, sideways. This is of great relevance for the situation on site. If the growth of sufficiently large/high vegetation near a fence is not prevented, tree frogs and similar climbing species will be able to overcome any type of fence. Hence, maintaining the vegetation near fences sufficiently small/low is essential to ensure the effectiveness of fences, especially in the case of climbing species (Speybroeck et al., 2018).

In our study we only addressed the situation of amphibian species. However, amphibians are not the only animals concerned by roadkill. Road mortality is also of concern for numerous other small animal species, such as small mammals, reptiles and also insects (Forman and Alexander, 1998; Dodd et al., 2004; Baxter-Gilbert et al., 2015). However, few studies have addressed the efficacy of fences for these groups (Dodd et al., 2004) but instead have focused on the efficacy of the ensuing tunnels to facilitate road crossing (Dodd et al., 2004; Glista et al., 2009). Furthermore, the lack of rigorous experimental testing of fences often leads to approximate measures by road managers which are based on practice rather than scientific evidence, and this includes the situation for amphibians (Morand and Carsignol, 2019).

In conclusion, a 40 cm concrete fence with a 10 cm overhang was effective to stop some of the amphibian species tested at their adult stage and all juveniles (Bufonidae, Hylidae). However, species with a great ability to jump were not stopped by such a barrier (e.g., Rana dalmatina). Accordingly, for the latter group of species we recommend a concrete fence height of at least 50 cm (see also Morand and Carsignol, 2019). If, for some reason, a full panel concrete fence cannot be used, we advise the use of a full panel galvanized metal fence with a minimal height of 50 cm and an overhang of at least 10 cm.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

All procedures were carried out after obtaining the authorizations for capture and transport (2019-DREAL-EBP-0031) and a certificate for the detention of the various species in captivity (DDPP67-SPAE-FSC-2019-04). The experimental protocol was approved by the French Ethical Committee (CREMEAS) and the French Ministry of Environment: ‘Ministère de la Transition Écologique’ under agreement number (APAFIS #18546-2019011810282677.v7).

AC, JJ, and YH conceived the study and designed the methods. AC, MLB, ND, and LG collected the data. AC analyzed the data. AC drafted the manuscript with contributions from MLB, ND, LG, JF, ME, JJ, and YH. All authors approve this version of the manuscript for publication. They agree to be accountable for the aspects of the work that they conducted and will ensure that any questions related to the accuracy or integrity of any part of their work are appropriately investigated and resolved.

This study was conducted within the framework of the CERISE project and was funded by the French Minister of Ecology (DREAL Grand-Est), the Region Grand-Est, the European collectivity of Alsace (CeA), and the Eurometropole of Strasbourg (EMS). The funders had no role in any aspect of the study and agreed to its publication.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.