Bats on the road — a systematic review of the impacts of roads and traffic on bats
Written By Daniel F. Ramalho & Ludmilla M S. Aquiar
University of Brasilia
25th February 2022
Bats are represented by over 1,400 species with a large repertoire of feeding habits, such as insectivorous, frugivorous, hematophagous (feeding on blood), and nectar-feeding species. Because of that diversity, bats play an important ecological role by providing ecosystem services such as pollination, seed dispersal, and insect control. Despite their importance, bats face different threats.
One of the main threats most animals suffer is the conversion of natural habitat for human activities, such as agriculture, urbanization, and road construction. The first studies considering the impacts of roads on bats were published in the early 1990s, when researchers began to observe bat roadkill and bat activity near road lamps. We conducted a systematic review to explore the global impacts of roads and traffic on bats and we found 43 studies that quantified impacts on 127 species. While studies were found from around the world, most were conducted in Europe, with a few in Oceania, North America, and South America.
Effects of roads on bats
Bat mortality due to roadkill
Roadkill is one of the main anthropogenic causes of mortality for several organisms in the wild, often exceeding rates of mortality from hunting or habitat loss. Even though mammals represent a high number of road-killed individuals, few studies have evaluated bat roadkill and which factors influence those collisions. Because of their small body size, bat mortality on roads is almost certainly underestimated, since roadkilled individuals may be quickly squashed or removed by scavengers.
Low-flying species (which fly at traffic height), male individuals (which may have larger home ranges), and young bats (which fly at lower heights and are more naive) tend to be the most frequently observed in roadkill counts. The risk of collision appears to be higher in deforested areas or areas with shorter trees because bats tend to fly closer to the ground. The risk of bat-vehicle collision is also increased by the construction of roads across existing migration or commuting routes and features that improve habitat quality near roads, such as wetlands that many bats preferentially forage above.
Some studies we reviewed suggested that highway mortality may affect approximately 5% of a colony, indicating that collisions with vehicles are an important threat to bats and should not be overlooked, especially in areas with a dense road network.
Photo: Daniel Ramalho
Bat activity near roads
The presence of roads heavily affects the surrounding area. Habitat alteration, the presence of artificial lighting, lights from vehicles, and traffic noise are important elements that should be considered in those areas. For bats, it has been observed that presence of roads may reduce foraging efficiency and foraging areas, as well as act as barriers to roost switching, which can restrict access to more suitable environments.
Distance from the road appears to be an important factor influencing bat activity, especially because environmental alterations and sound pollution derived from the presence of roads decreases with distance from such structures. In open habitats, however, activity seems to increase in some areas near roads, possibly because of the higher feeding opportunities in those areas, which can increase due to the presence of artificial lights and trees planted along roadsides.
When comparing roads with different traffic volume, most studies reported less bat activity near highways with intense traffic, probably because bats tend to avoid foraging in loud environments as noise could interfere with acoustic cues from prey or predators. Also, when areas near roads have shorter trees or no trees at all, the effects of vehicle noise are more evident, and bats are more likely to avoid noisy roadside environments.
Streetlights are beneficial to some species of bats which are attracted to the insects that swarm around the artificial light, while other species of bats avoid artificially-lit areas.
Mitigation measures and future strategies
To date, most of the mitigation measures were implemented in order to avoid collision, while other impacts, such as traffic noise and light pollution on roads, are relatively overlooked. Crossing structures are the main strategy used to avoid roadkill, however, their effectiveness is still unclear. Previous studies suggest that they tend to be more successful in areas with pre-existing commuting routes. Because commuting bats do not always use crossing structures, small habitat alterations, such as placing trees or fences along the road, could guide bats to safe places to cross or encourage them to fly higher, avoiding collision.
The majority of studies were conducted in US and Europe, and the impacts of roads and effectiveness of mitigation strategies in other parts of the world is more uncertain. Studies are urgently required in other areas, especially in tropical countries, which harbour high richness of bat species and are facing a rapid increase in road construction. Also, studies that experimentally test for causes of reduced activity by some species along roads are necessary in order to develop effective mitigation strategies to protect and conserve bats.
To achieve global conservation of bats it is essential that we understand how bats are affected in all parts of the world; this is especially needed to protect migrating species and those with small population sizes. Studies that consider all direct and indirect road impacts on different bat species, including evolutionarily distant groups, are essential to ensure conservation of the species and preserve the ecosystem services they provide.
Author information:
Daniel F. Ramalho, Ph.D. in Ecology – University of Brasília – daniel.f.ramalho@gmail.com
Ludmilla M. S. Aguiar, Ph.D. in Ecology – University of Brasília – ludmillaaguiar@unb.br
Source:
Ramalho, D.F. & Aguiar, L.M.S. 2020. Bats on the road — a review of the impacts of roads and highways on bats. Acta Chiropterologica, 22(2), 417-433. DOI: 10.3161/15081109ACC2020.22.2.015
Editor:
Rodney van der Ree
Cite this summary:
Ramalho, D.F. & Aguiar, L.M.S. (2022). Bats on the road — a systematic review of the impacts of roads and traffic on bats Edited by van der Ree, R. TransportEcology.info, Accessed at: https://transportecology.info/research/review-of-road-impacts-on-bats [Date accessed].
