Mitigating the environmental and ecological impacts of evolving city-scale streetlighting installations

Artificial light at night (ALAN), from streetlights and other sources, is ubiquitous across modern towns and cities and has wide-ranging impacts upon the natural environment. The extent, spectra and timing of light influence the physiology, behaviour and fitness of individuals of many species, shape the structure of ecological communities and the functioning of ecosystems. To date, however, it has been challenging to characterize this lighting at sufficiently fine spatial resolutions across city-wide extents. Here, we apply a Monte Carlo radiative transfer model to simulate in three dimensions the light environment resulting from emission from streetlights using the city of Exeter, UK, as an exemplar. We show that this technique can model the evolving lighting landscape of modern cities at scales, and through observables, suitable for both ecological studies and lighting professionals. We estimate measures of melatonin suppression, induced photosynthesis and phytochrome photostationary state from our models, probing how the transition towards light-emitting diode street lighting impacts physiological processes in plants and animals throughout the city. Our simulations illustrate that although the area lit by ALAN is decreasing overall at metre scales, which is lit is becoming more hostile towards many organisms.