Not all ‘bats’ are created equal- The impacts of light pollution on bat foraging.

Understanding exactly how bats are impacted by light is really important as it can put into context the difference your efforts are making! Artificial light can affect several bat behaviours and as a result, impact their survival. This week I’ll be discussing how light alters the foraging behaviour of different bat species and their prey.    

All our UK bats eat insects. Most eat very small bugs such as mosquitoes and midges, but they are also known to munch on larger insects including moths and beetles [1]. Bats forage where prey is most abundant, such as pastures, ponds and woodland [1]. But what happens when insects gather at a light?

As mentioned last week, Ultraviolet (UV) and blue-white light emitting lamps attract many species of insect [2]. At first, this sounds great for bats, and for some it is! However, as different bats feed in different ways some species are much more light-sensitive than others. This means only light-tolerant bats will take advantage of the insect clusters at a light source. These are usually fast-flying bats which are adapted to forage in large, open spaces [2]. For example, the common pipistrelle (Pipistrellus pipistrellus), soprano pipistrelle (Pipistrellus pygmaeus) and noctule (Nyctalus noctula) have all been recorded foraging at streetlights [2].

Photo credit: Forschungsverbund Berlin e.V (FVB)

As larger moths are more attracted to short-wavelength (UV/blue) light than smaller moths, light-tolerant bats can gain more energy much quicker, meaning they can forage for less time and reserve energy [3]. This means they have a competitive advantage over light-sensitive species. It has been suggested that some bat species have already evolved to forage at a light source. For example, in southern Europe, the Kuhl’s pipistrelle (Pipistrellus kuhlii) is thought to have adapted the morphology of its skull to eat larger moths [4]! However, despite the obvious benefits for light-tolerant bats, they too can be negatively impacted. Bats are sensitive to UV lights as it can damage their eyes, meaning those which spend lots of time feeding at a light may have reduced eyesight over time [4]. This is concerning considering the long lifespan many bats have, particularly for Nyctalus species which travel long distances and need to be able to navigate. Additionally, feeding at a light source can increase the mortality rate for bats as vehicle-collision and predation risk increases, especially for juveniles which are slower [2].

So, what about the light-sensitive bats?

As may have guessed, they will avoid the light, even if there’s an abundance of insects at it. Their sensitivity is thought to be a result of their slower flight and later emergence times for reducing predation [4]. This means many species, such as brown long-eared bats (Plecotus auritus) and Natterer’s bats (Myotis nattereri), will lose foraging areas to artificial light [2]. A ‘vacuum effect’ can also occur, which is where insects are attracted away from dark areas towards a light [3]. This can be really serious for light-sensitive bats as they will struggle to find food in their dark foraging spots. This means they will have to fly further (using more energy) to find food or must rely on suboptimal prey [1]. Reduced foraging opportunities mean light-sensitive bats may be out-competed by light-tolerant bats and they may suffer from long-term effects such as reduced juvenile growth rates [1]. The morphology of light-sensitive species has been associated with a higher extinction risk, meaning conserving them and reducing our impact on them is even more of a priority!

Photo credit: Eleonimages/Shuttercock

As well as affecting bats’ foraging behaviour, artificial light largely impacts their prey. In fact, it is considered to be a main driver in the insect biodiversity crisis [3]. Attracting insects to a light source increases their mortality rate in various ways. First of all, lights can be confused for the moon, which they use for orientation, meaning they will fly around the lamp until they eventually die of exhaustion or collision with the hot bulb [3]. Diverting insects away from important activities such as foraging or mating can also cause survival problems [4]. Insects’ risk of predation also largely increases because of artificial light. For example, they can be ‘dazzled’ by the light meaning they are immobilised and rest on the ground or vegetation allowing for predators to easily find them [3]. As mentioned earlier, as they cluster at light, insects are easy prey for light-tolerant bats, which leads to disproportionate amounts being killed. This can seriously impact insect populations and reduce food availability for light-sensitive bats, disadvantaging them even more [2]!

Additionally, as certain species of insects are attracted more than others, the high mortality at lights will alter the composition of insect communities [2]. For example, predatory insects are more attracted to High Pressure Sodium lamps (HPS), meaning if large quantities of them die there will be an increase in their prey populations, which will have knock-on effects for the whole ecosystem [1]. Finally, the ‘vacuum effect’ can result in local extinction of insects which are drawn away from their natural ecosystem [3]. As well as being massively disruptive to insect communities, many species which rely on them will be affected. This of course, includes bats, particularly light-sensitive species, which can be largely impacted even by local reductions in prey [1].  

As you can clearly see, light pollution impacts bat foraging in lots of different ways! As well as creating competition between species, artificial light affects bats through altering their food availability. It’s so important we ensure we’re all limiting our contribution to the issue. In terms of minimising changes in foraging behaviour, we can reduce our outside lighting, making sure it is only on when needed and switching to non-UV emitting, ‘warm’ lamps. Contacting your local council about switching to more bat-friendly streetlights can also make a massive difference!

Next week I’ll be continuing to explain the effects of light pollution on bat behaviours, focusing on roosting!

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#pf7
  3. URL: https://link.springer.com/chapter/10.1007/978-3-319-25220-9_7
  4. URL: https://royalsocietypublishing.org/doi/10.1098/rstb.2014.0124