Abstract
Cities around the world are transitioning to more efficient lighting schemes, especially retrofitting traditional, high-pressure sodium (HPS) streetlights with light-emitting diode (LED) lights. Although these initiatives aim to address the problems of urban sustainability and save money, the ecological impacts of these retrofits remain poorly understood, especially in brightly lit cities and in the tropics, where urbanisation is most rapid. We performed an experimental study of the retrofit in Singapore–focusing on insectivorous bats, whose activity we monitored acoustically along paired control (HPS-lit) and treatment (LED-lit) streets. We recorded seven species along these streets, but only obtained enough recordings to measure the effect of light type for three of them–all of which can reasonably be described as urban adapters. The strongest predictor of bat activity (an index of habitat use) was rainfall–it has a positive effect. Light type did not influence bat activity or species composition of the bat assemblage along these streets, though it did interact with the effects of rainfall and traffic noise for one bat species. Ultimately, the retrofit may be ecologically meaningless to urban-adapted, tropical insectivores that already experience high levels of light pollution as they do in Singapore. However, while our findings may appear reassuring to those concerned with such retrofits in other tropical and/or brightly-lit cities, they also highlight the contextual nature of ecological impacts. We point out that they should not be prematurely generalised to other locales and systems. In particular, they do not imply no impact on species that are less urban-adapted, and there is a clear need for further studies, for example, on responses of other foraging guilds and of bats (and insects) throughout the tropics.
Highlights
Urbanisation and artificial light at night (ALAN) are inexorably linked– the widespread use of remotely-sensed nighttime light as a metric of urbanisation [1]
In contrast to S. kuhlii and S. saccolaimus, which we detected on almost every sampling event and whose activities were fairly evenly distributed among events, that of M. muricola was distributed unevenly, with 664 passes recorded during two sampling events
The retrofit does not seem to affect insectivorous bats, and we find no evidence to support our hypothesis that it benefits them
Summary
Urbanisation and artificial light at night (ALAN) are inexorably linked– the widespread use of remotely-sensed nighttime light as a metric of urbanisation [1]. Lighting up the night has many consequences, including biological and ecological ones, which are well-documented (albeit with some gaps). Certain impacts, such as the fatal attraction of moths to lamps or the disorientation of sea-turtle hatchlings on beaches fringed by lit buildings, are more well-known than others [5]. Impacts are documented in a vast literature and diversity of organisms ranging from fungi [e.g., 6] to plants [7] to marine invertebrates [e.g., 8] to large mammals [9]. These population- or community-level impacts should alter ecosystem structure and function [10]
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