Energy-saving street lights might be bad news for bugs

Wildlife managers and conservationists are often faced with dilemmas in which they are forced to choose the less bad of two bad choices. For example, some might argue that limited trophy hunting is preferable to habitat loss, if it encourages landowners to maintain ecosystems in their natural state (or something close to it). Here’s another dilemma: is it better to swap older street lights for LED lamps, even if the new energy-saving bulbs are detrimental for declining wildlife? These are not easy decisions to make, but in order to make them it helps to know exactly what the stakes are.

Each year artificial lighting dumps some 1.9 billion tonnes of carbon dioxide into the earth’s atmosphere. As a result of energy saving policies, many local and national governments are working on plans to replace sodium and mercury vapor street lights with LEDs. The idea is to reduce global energy consumption. But the problem, according to University of Bristol researchers Andrew Wakefield, Emma L. Stone, Gareth Jones, and Stephen Harris, is that “these lights are being installed en masse without adequate research to establish their effects on human health and wildlife.” They argue that the change especially from the ubiquitous sodium lights that throw-off a narrow range of spectra to broad-spectrum white LEDs could disrupt or alter a fair number of species interactions. One important interaction that’s affected by artificial light is between bats and moths.

The negative effects of light pollution aside, around one third of flying insects attracted to artificial lights die. In many cases, that’s because artificial lights serve as something of a buffet table for hungry bats. Lots of UK moth species are in decline, and that is at least in part thanks to artificial lighting. Previous research showed that moths were less able to evade bats under mercury vapor lights compared with sodium lights, and that’s despite the fact that many moths are able to detect the bats’ echolocation calls. It could be because mercury vapor lights contain broader spectra than sodium lights, or it could be because they contain UV radiation along with visible light. Nobody has tested LED lights yet, which are broad-spectrum but do not contain UV.

The researchers set about testing groups of wild moths at a series of sites around Bristol, UK that were free from artificial lights. They erected a commonly available street light containing an array of 24 LEDs atop a 4-meter tall tripod at boundaries between woodlands and open fields—common hangouts for the winged bugs. Once the moths showed up, Wakefield and his team broadcasted pre-recorded bat calls at them and recorded their responses on video.

By the end of 16 nights of data collection, the researchers had good data for 94 individual moths. They found that moths were 36% less likely to evade a perceived bat (using a “powerdive” maneuver) when the LED light was on than when it was off, though they still executed their powerdives more often when the LED light was on and a bat signal was broadcast than when the researchers did not play the bat call. So the good news is that LED lights don’t completely obliterate the moths’ ability to survive a bat attack, but the bad news is that they do severely disrupt it.

It isn’t clear why this is the case, but it doesn’t appear to be associated with UV radiation, which is absent from LED lights. The researchers are also confident that the majority of moths tested are indeed capable of hearing bat calls. More than 90% of moths attracted to artificial lights in southwestern UK are from the families Geometridae, Noctuidae or Notodontidae, all of which have ultrasonic hearing abilities.

One possibility is that moths have evolved the ability to respond contingently to potential threats according to the time of day. Most daytime sounds for nocturnal moths are harmless, and reacting unnecessarily to those sounds would be a waste of energy. Instead, they focus their anti-predator vigilance on the darker nighttime hours. But if some forms of artificial lighting – especially broad-spectrum lights – succeed in fooling moths into believing that they’re flying about in the daytime, then that might explain their relative inability to avoid the threat of predation in those circumstances.

Insects perform essential services, especially in terms of pollination and pest control, and artificial lighting system that’s detrimental to insects could pose severe consequences for the ecosystems in which they live. Especially for ecosystems, as in the UK, where insects such as moths are in decline. But this isn’t to say that LED lights should necessarily be avoided. LEDs could still prove to be more insect friendly, all things considered, than other lighting technologies like metal halide, mercury vapor, or high-pressure sodium bulbs. The challenge now is to carry out that research. “We need a better understanding of how different taxa respond to various artificial lights as negative impacts are likely to have cascading ecosystem effects,” the researchers conclude. – Jason G. Goldman | 07 August 2015

Wakefield A, Stone EL, Jones G, Harris S. 2015 Light-emitting diode street lights reduce last-ditch evasive manoeuvres by moths to bat echolocation calls. Royal Society Open Science, 2: 150291. DOI: 10.1098/rsos.150291.

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