The effect of a major road on bat activity and diversity


1. It is well known that roads can have a significant impact, usually negative, on species and ecosystems. However, despite their protected status in many countries, little research has been done into the effects of roads on bats. With a view to making more informed management recommendations, we address the simple question: are bat activity and diversity (as measured with ultrasonic detectors) correlated with distance from a major road?

2. Broadband acoustic surveys were conducted on 20 walked transects perpendicular to the M6, a major road in Cumbria (UK), with bat activity recorded at eight spot checks per transect at different distances from the road. Climatic and habitat variables were also recorded, and the relationships between bat activity and these variables were investigated using generalised estimated equations (GEE) and ordinal logistic regression.

3. Total bat activity, the number of species and the activity of Pipistrellus pipistrellus (the most abundant species) were all positively correlated with distance from the road. Total activity increased more than threefold between 0 and 1600 m from the road. These effects were found to be consistent over 2 years.

4. Synthesis and applications. This study is one of the first to show that roads have a major negative impact on bat foraging activity and diversity and is broadly applicable to insectivorous bat communities worldwide. Mitigation requires that roads are made more permeable to bats through the use of effective crossings, such as underpasses and overpasses, and that habitat is improved within 1 km of major roads. Because the effectiveness of current mitigation measures is unknown, well-designed monitoring of mitigation is essential.

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Roads destroy and degrade habitat and dissect the natural landscape (Forman et al. 2003); yet as recently as 1998, Forman and Alexander described ‘road ecology’ as ‘the sleeping giant’, drawing attention to the potentially devastating, but largely unstudied, effects of roads on the natural world. Road ecology is now increasingly well studied (e.g. Frair et al. 2008; McGregor, Bender & Fahrig 2008;Halfwerk et al. 2011; Summers, Cunnington & Fahrig 2011), but there are relatively few studies of bats.

The density and diversity of a range of vertebrates, including birds (e.g. Canaday 1996; Summers, Cunnington & Fahrig 2011), are negatively correlated with road density and positively correlated with distance from the road (early work is reviewed by Trombulak & Frissell 2000; Coffin 2007). Roads may have a positive effect on some species. For example, densities of white-footed mice Peromyscus leucopus increase in proximity to roads because of the creation of favourable habitats along road verges and a reduction in predators (Rytwinski & Fahrig 2007), but the same species is reluctant to cross roads (McGregor, Bender & Fahrig 2008). However, it is likely that these positive effects are limited to relatively few species. Fahrig & Rytwinski (2009) reviewed 79 studies that between them investigated 131 species and found that negative effects were far more prevalent than positive effects.

Possible effects of roads on wildlife include mortality from vehicle collisions, habitat destruction, habitat fragmentation, barrier effects, edge effects and habitat degradation or disturbance from light, noise and chemical pollution. The magnitude of road effects is likely to vary over time and multiple effects will usually be cumulative in the long term (Balkenhol & Waits 2009). There may also be far reaching effects, such as the cascading consequences that can occur in ecological communities when the abundances of key species are altered (Francis, Ortega & Cruz 2009).

Most of the literature on road effects has focused on terrestrial mammals, amphibians and birds, with little research into the effects on bats. There are 17 resident species of bat in the UK and all are protected by both UK (The Wildlife and Countryside Act 1981; The Countryside and Rights Of Way Act 2000) and EU legislation [The Conservation (Natural Habitats &c.) Regulations 1994; The Conservation (Natural Habitats &c.) (Amendment) Regulations 2007]. All bat species are an important consideration in national and local recovery plans, and a licence must be obtained if it is necessary to disturb any species of bat in the UK. Furthermore, developers must demonstrate that they will put in place mitigation measures to minimise the impact and compensate for any loss to bat foraging or roosting habitat (Mitchell-Jones 2004). Similar laws apply in many other countries.

It is likely that bats are particularly vulnerable to road developments and will be slow to recover from disturbance because of their life history strategy of low fecundity, their longevity and their use of large areas of the landscape (Altringham 2008). Roads may affect bats in three principle ways: (i) kill by collision with vehicles, (ii) damage or degrade roosts and foraging areas and (iii) sever critical flight routes used for commuting and migration. Several recent studies show that bats of many species are killed by collision with vehicles (Lesinski 2007; Gaisler, Rehak & Bartonicka 2009; Russell et al. 2009; Lesinski, Sikora & Olszewski 2010). Mortality in many of these studies is probably severely underestimated because of the difficulty of finding corpses and their removal by scavengers (Slater 2002). Kerth & Melber (2009) found that a major road in Germany restricted habitat accessibility in female Bechstein’s bats Myotis bechsteinii resulting in smaller foraging areas and reduced reproductive success. Noise pollution from traffic reduced foraging efficiency of M. myotis, a species that forages by passive listening (Schaub, Ostwald & Siemers 2008; Siemers & Schaub 2011), and Stone, Jones & Harris (2009) have shown that street lighting is a major deterrent to foraging and commuting lesser horseshoe bats Rhinolophus hipposideros. Other slow flying and/or gleaning species are also likely to be deterred by lights, but some faster flying species forage beneath street lamps that attract insect prey (e.g. Blake et al. 1994). Lights may also alter local bat population dynamics by attracting insects out of woodland, leading to diffuse exploitative competition (Arlettaz, Godat & Meyer 2000). Zurcher, Sparks & Bennett (2010) found evidence for road avoidance behaviour: bats approaching a road bisecting a commuting route were found to reverse their course more frequently in the presence of traffic.

These studies, and inference from studies of bats described later, suggest that roads are likely to have significant negative impacts on bats, leading to a reduction in population sizes. Bat populations have declined dramatically in the last century in the UK (Harris et al. 1995) and in many other countries, leading to increasingly strong legal protection. To satisfy legal requirements, costly mitigation measures are employed on road developments throughout Europe to reduce their impact on bats. However, there is little satisfactory evidence to support their effectiveness (e.g. Altringham 2008), and we have little knowledge of just how much roads do affect bats. This study is a step towards a more evidence-based approach to the bat–road issue. We ask the simple question: are bat activity and diversity (as measured with ultrasonic detectors) correlated with distance from a major road? We show that roads do affect bat activity, suggest what mechanisms underlie the effect and discuss appropriate mitigation and monitoring strategies.


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Megabats and Microbats: The effect of a major road on bat activity and diversity
The effect of a major road on bat activity and diversity
The effect of a major road on bat activity and diversity
Megabats and Microbats
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