These days, we recognise that cities play a starring role in the global climate crisis. Indeed, at least 70 % of global greenhouse gas (GHG) emissions come from cities themselves or activities that take place elsewhere but satisfy urban demand for goods and services. So, limiting warming to 1.5ºC above pre-industrial levels — a goal we must reach — hinges on urban sustainability solutions. Because the best urban levers to mitigate warming are variable, there is no one-size-fits-all approach to urban energy policy. And although outdoor urban lighting is probably not the most important source of GHG emissions, one urban energy solution is to retrofit traditional streetlighting technologies — especially high-pressure-sodium (HPS) vapour lamps — with light-emitting-diode (LED) lamps. These newer LED lights are not only way more energy-efficient, but also more durable, so these retrofits can save cities money and labour.

Amid widespread application of these retrofits, there is concern about the ecological impacts of LED lights, which emit whitish light, unlike HPS lamps, which glow orangish, given the known biological importance of light colour spectrum. For instance, LEDs may attract more phototactic insects (which orient to lights). It stands to reason that there may be a cascading effect on their predators, such as bats. In 2017, I conceptualised an experimental investigation of the impacts of the retrofit on insectivorous bats in Singapore (SG) — it would be the first such study anywhere in the tropics and to focus exclusively on a brightly lit city (the city-state of SG is the world’s most light polluted nation).

My student, Kenneth Ee Meng LEE, spent four months acoustically monitoring bats on paired experimental (LED-lit) and control (HPS-lit) streets, and another one of my students, Deon LUM, helped with data analyses. Our working hypothesis was that LED lights, by attracting more insects, would attract more bats known to hunt at streetlights — generally, urban-adapted, aerial insectivores, such as the pouched tomb bat (Saccolaimus saccolaimus) and lesser Asiatic yellow bat (Scotophilus kuhlii), which are common in SG. But the data told a different story — one of ecological meaninglessness, whereby there was no difference between control and experimental streets. And we suspect that is because these bats were mainly using both types of street to commute, not to forage.

This does not mean that LED-retrofits are ecologically benign in the tropics. Rather, it highlights the need for more studies on more taxa, including on other trophic guilds of bats (e.g., nectar- and fruit-eaters), because applying urban energy solutions without properly understanding the effects on wildlife may have detrimental consequences not only to biodiversity, but also to the ecosystem services it renders.

Check out the paper here.