Transport infrastructures, such as highways, disrupt animal migrations and cause roadkill. To mitigate the latter problem, fences have been built but their effectiveness has rarely been tested under controlled conditions. Here, we tested the effectiveness of the most commonly used fence in France and probably in Europe (wire netting fence) to block animals. We tested the wire netting fence, with and without a structural modification (i.e. an overhang), with three small mammalian species (the European hamster: Cricetus cricetus Linnaeus, 1758; the common vole: Microtus arvalis Pallas, 1778 & the wood mouse: Apodemus sylvaticus Linnaeus, 1758) and two amphibian species (the marsh frog: Pelophylax ridibundus Pallas, 1771 & the European green toad: Bufotes viridis Laurenti, 1768). During testing, all small vertebrate species tested were placed into an arena, from which they could only escape by crossing the wire netting fence. Without an overhang, almost all adult individuals of all tested species were able to climb over a 30 to 40 cm high wire netting fence. Furthermore, the addition of an 8 cm long overhang at the top of the fence stopped the amphibian species tested but not the most agile mammalian species, such as the hamster and the wood mouse. Based on these results, we do not support the construction of wire netting fences along roads as a measure to stop small animals from crossing. We recommend the use of more effective and durable fences, which, in addition, can be associated with wildlife passages to reconnect isolated populations.

Amphibians, fences, roadkill, small mammals

Millions of animals around the world are killed daily by wildlife-vehicle collisions, affecting populations of most taxa (Bruinderink and Hazebroek 1996; Laist et al. 2001; Rao and Girish 2007). Even on roads with only moderate traffic, roadkill can severely impact population viability (Eigenbrod et al. 2007). These impacts can be accentuated in highly modified landscapes, when terrestrial fauna has to cross roads on a daily basis and/or during dispersion and migration movements (Billeter et al. 2008; Bissonnette and Rosa 2009). To limit roadkill, different mitigation measures, such as the construction of fences and wildlife passages, have been implemented, especially in industrialized regions (e.g. Australia, Western Europe and North America). Initially installed to limit vehicle collisions with large mammal species, like ungulates (e.g. deer, wild boar), the primary aim of fence construction concerned human safety and the limitation of economic losses associated with these collisions (Romin and Bissonette 1996; Schwabe et al. 2002; Forman et al. 2003; Bouffard et al. 2012). The installation of fences on both sides of the road or along railways is not mandatory but global recommendations advise such measures, if the risk of collision and/or wildlife mortality is high (Iuell et al. 2003). Large-fauna fences or wire netting must be tall enough to prevent animals from jumping over it (i.e. >1.8 m, Iuell et al. 2003; Morand and Carsignol et al. 2019), while mesh size of such fences is typically relatively large (in general greater than 60×60 mm).

When research demonstrated the major role that roads play in habitat fragmentation and its negative impact on the populations of many species (and not only large mammals), further mitigation measures were implemented. These mitigation measures are designed to reduce roadkill (i.e. fences) and restore population connectivity (i.e. wildlife passages) to allow safe movements on different parts of the habitats separated by the road for various small fauna, such as reptilian (turtle), amphibian (frog and toad), small (shrew) and medium-sized mammalian species (hares, foxes, badgers, etc.) (Aresco 2003; Glista et al. 2009; Klar et al. 2009; Jarvis et al. 2019; Plante et al. 2019). For these species, different fences (made from wire netting, concrete, PVC or metal) were designed and often installed alongside the large-fauna fences. In Western Europe, wire netting fences (with a typical mesh size of 6.5×6.5 mm and a height of 40 to 60 cm) are most often used to block small fauna, and are usually attached to the large-fauna fences (Iuell et al. 2003; Puky 2003; Beebee 2013; Morand and Carsignol et al. 2019). Similar to the large-fauna fences, these small-fauna fences coupled with a wildlife passage are needed to manage the reconnection of populations in areas of high biodiversity (Clevenger et al. 2001; Iuell et al. 2003; Beebee 2013; Testud and Miaud 2018).

Some species are more sensitive than others and require special attention during the planning of infrastructure, such as roads. For example, amphibians are particularly vulnerable to roadkill due to their mass migration strategy (Joly 2019; Cayuela et al. 2020) and their immobility when facing motor vehicles (Gibbs and Shriver 2005; Mazerolle et al. 2005). Amphibians are of particular concern, since globally 41% of all amphibian species are threatened with extinction (IUCN 2021). Small mammals are also of concern, since they often use the side of roads as refuge, especially in a highly modified landscape (Ruiz-Capillas et al. 2013; Jumeau 2017). While the impact of road mortality on populations of these small mammalian species may not appear problematic, because their population densities are generally high, this is not always the case. Studies show that some of these species are declining at an alarming rate and are now considered endangered, like the garden dormouse (Eliomys quercinus) or the European hamster (Cricetus cricetus) (Surov et al. 2016; Bertolino 2017). Hence, roadkill of wildlife is an important issue that has to be addressed in an overall conservation strategy (O’Brien 2015; Pinot et al. 2016).

Our study, which experimentally investigated the effectiveness of wire netting fence to stop small terrestrial vertebrates (five species of small mammals and amphibians) from entering into road infrastructures, clearly demonstrates the limitations of such structures.

Without an overhang at the top of the wire netting fence, individuals of all tested species, adults and juveniles, were able to pass the structure. Clearly, wire netting fences without overhang should be avoided in future constructions. Furthermore, even the addition of an overhang only marginally increased the effectiveness of the wire netting fence in blocking the tested mammalian species. Individuals of all small mammal species tested were still able to cross the fence, including the common vole despite some difficulties, even with a long, 15 cm overhang. For example, hamsters were sufficiently large to reach the far end of the overhang, so that they could pull themselves up and cross the fence. However, some adult individuals were unable to cross the fence, which was likely explained by their body condition (i.e. these were the largest and heaviest adult hamsters). Since the hamsters tested were captive individuals from a breeding center, they were presumably fatter and less agile than wild hamsters. Wood mice were able to reach the far end of the overhang by climbing along the mesh, upside down. However, changes in the design of the overhang structure, like the use of a solid structure (e.g. a metal plate), without gripping possibility, might prevent such small/agile species from crossing. Nevertheless, structures similar to the ones used in our study should be avoided, at least for small mammals.

By contrast, for amphibians, the tested wire netting fence might prove effective when combined with a 10 cm overhang. Adult individuals of European green toads and marsh frogs were unable to pass such a structure during our tests. However, since juvenile frogs were able to pass through the mesh of wire netting fences, even at a relatively small mesh size, their use should be avoided at the proximity of ponds. They should also be avoided when more “agile” amphibians, such as achieved jumping (i.e. Agile frog, Rana dalmatina) or climbing species (i.e. European tree frog, Hyla arborea or newts) are present. These species were not tested in our study but have been shown to easily cross a 40 cm concrete fence (Conan et al. 2021).

Given our current test results, we suggest to avoid the use of wire netting fence along motorways. In eastern France, 70.4% of overhangs of wire netting fences along motorways inspected by Jumeau (2017) had a length of less than 9 cm, which is lower than the 10 cm overhangs that proofed effective for the tested amphibians in our study. The author explained this situation by a lack of information/communication on behalf of the work crews installing these fences along the roads. If during construction the fences were buried a few centimetres too deep, while fence height above ground was maintained, bending the top of the fence resulted in a too short overhang. In addition, the author reported that 78.2% of inspected fences, even recently built fences, showed signs of deterioration, such as broken mesh, too high vegetation (allowing animals to climb over the fence; Arntzen et al. 1995), as well as deteriorated or absent overhang. These results are especially troubling, since such a state will also reduce the effectiveness of fences for their second role, namely to guide animals to wildlife passages (Clevenger et al. 2001; Beebee 2013; Testud and Miaud 2018).

For several years, studies have highlighted the ineffectiveness of wire netting fences in excluding animals from road infrastructures, especially for amphibians (Schmidt et al. 2008; Testud 2020). Nevertheless, these fences are still being used along newly built roads, even when they are located in the dispersal corridors of endangered species (e.g. green toad and European hamster in Alsace, France). Therefore, we recommend that these fences should be replaced by viable alternatives. Opaque fences, for example, may be more effective in guiding small animals to the wildlife passages, and experimental tests to confirm this are urgently needed. It is, however, important to note that effective fences can impact the movement of individuals on both sides of a road and consequently lead to a decrease in gene flow if individuals are unable to reach wildlife passages (e.g. newt; Matos et al. 2018). In this context, testing the effectiveness of structures to guide animals to wildlife passages is needed in controlled and field conditions while an increase in the number of wildlife passages might also be necessary.

Wire netting fence between 30 and 60 cm is a commonly used mitigation device to prevent small vertebrate species from entering/crossing roads and reduce roadkill. This study showed that its effectiveness is very limited. Accordingly, we suggest that this device should be avoided and replaced by more effective and durable fences.

All manipulations were carried out after obtaining the legal authorizations for capture and transport (2019-DREAL-EBP-0031) and a certificate permitting the detention of wildlife species in captivity (DDPP67-SPAE-FSC-2019-04). The experimental protocol was approved by the ethics committee (CREMEAS and Ministry) under the agreement number (#18546-2019011810282677 v7).

We thank all the students who participated in this study and anonymous reviewers for their valuable comments. We thank Manfred Enstipp for the English editing. Special thanks to Frederic Voegel and Laurence Feltmann. This study was funded by the French Minister of Ecology (DREAL Grand-Est), the Région Grand-Est, and the Collectivité européenne d’Alsace (CeA). They had no role in the study design, writing, collection, analysis and interpretation of data. They agree to the publication of this study.