Lighting the way- The impacts of light pollution on bat commuting behaviour.

This week is my last slightly sciencey blog focusing on the different bat behaviours impacted by light pollution! I will have covered three major areas, but it’s important to know that light’s effect extends to other aspects of bats’ lives. Although understanding the impacts on foraging, roosting and commuting will give you a good grasp of the issue and allow you to make informed choices about your lighting.

How does artificial light affect bat commuting?

Bats commute between their roosts and foraging sites using a network of routes [1]. These are often linear and consist of a variety of features including gardens, woodland edges, waterways, rows of street trees and hedgerows [1, 2]. Commuting routes are important as it helps bats to navigate their way around and acts as a form of protection against bad weather and predators [1]. Bats often use the most efficient route to maximise their foraging time and minimise energy losses [1]. However, if light spills onto a commuting route the network can become fragmented, causing a variety of responses and knock-on effects [1, 3].

Photo credit: Emma Stone

One response recorded is avoidance, as many bats will turn away from a light source and may fly back to their roost [4]. This can mean the availability of foraging sites are reduced, limiting how much food and energy they can gather. Returning to their roost without feeding can also negatively impact bats, as they waste unnecessary energy flying to and from [3].

Another response adopted by many bats is using an alternative route, which can have several negative consequences [1, 3]. Firstly, these routes may be longer, meaning they spend more time flying and less time foraging, resulting in fewer insects eaten [1]. Arriving at the foraging site later may also mean bats miss the peak prey abundance, having to rely on suboptimal or less food [3]. The less energy gained along with higher energy costs from flying further and increased stress levels can reduce the fitness of bats [4]. This can then lead to reduced reproductive success, meaning lighting of commuting routes during their reproductive period can be particularly serious due to their high energy requirements [3, 4].   Alternative routes can also provide less vegetation cover, which can expose bats to predators and bad weather, increasing both their predation risk and energy costs [1, 3]. If bats perceive the energy costs too high, they may be cut off from their foraging site and left without an alternative [3]. This can be really serious for bats’ survival, as they may be forced to abandon their roost, causing further damage to the overall functioning of the ecosystem [1].

Photo credit: Graham Horn

Certain bat species rely on these routes more than others. For example, slow flying bats, such as horseshoes (Rhinolophus) and Myotis species, are more vulnerable to predators in open spaces, meaning commuting routes are an important source of protection [2, 5]. Even low levels of light can impact them, with activity levels of the rare lesser horseshoe (Rhinolophus hipposideros) reducing at only 3.6 lux (lower than side street lighting) [5, 6].  Faster flying bats including Pipistrellus, Nyctalus and Eptesicus species have been found to either be unaffected or benefit from lighting on routes [7]. As mentioned in my previous blog on foraging, slow flying bats are more light-sensitive meaning the combined effects of reduced foraging and fragmented commuting routes can have more serious implications.

If you live close to an area which you suspect may be a suitable commuting route, it’s important to try to avoid lighting falling onto or even around it, as our more vulnerable bats can be very sensitive to low brightness levels! An effective way to improve bat commuting routes is by contacting your local council to suggest changing to more bat-friendly lights near dark corridors or even creating a corridor! This could include low level lighting or motion-sensor lights to limit the time they are switched on for.

Next week, I’ll be moving away from the impacts of lighting and suggesting to you a variety of ways you can help to make a big difference!

You can access information on this issue and find out how to help by following Blinded by the Night’s Facebook and Instagram pages. If you have any questions about the campaign or would like to get involved, please contact me via email at Beth2.Gerrard@live.uwe.ac.uk.

How to get involved:

Facebook: Blinded by the Night @blindedbythenightbristol

Instagram: @blindedbythenight_bats

References:

  1. URL: https://cdn.bats.org.uk/pdf/Bats_and_Lighting_-_Overview_of_evidence_and_mitigation_-_2014_UPDATE.pdf
  2. URL: https://www.researchgate.net/publication/272889669_Impacts_of_artificial_lighting_on_bats_A_review_of_challenges_and_solutions
  3. URL: https://www.sciencedirect.com/science/article/pii/S0960982209011932
  4. URL: https://arnhemspeil.nl/docs/2008-07-10-vzz-experimental-evidence-of-light-disturbance-along-the-commuting-routes-of-pond-bats.pdf
  5. URL: https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2486.2012.02705.x
  6. URL: https://cdn.bats.org.uk/pdf/Resources/ilp-guidance-note-8-bats-and-artificial-lighting-compressed.pdf?mtime=20181113114229&focal=none
  7. URL: https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.14462