Streetlights disrupt both flying and crawling invertebrates—but not in the same ways

A new study by Lockett and colleagues explores how the proximity, intensity and colour of streetlighting impacts arthropods occupying different spatial niches, by simultaneously sampling flying and ground-dwelling invertebrates.

It is well known that artificial light at night (ALAN) can attract and kill insects and other invertebrates, but do creepy-crawlies and flying invertebrates respond the same way? Does the colour and intensity of lighting matter? And do effects persist in urban landscapes exposed to decades of artificial light?

To answer these questions our study simultaneously sampled flying and ground-dwelling invertebrates directly under 30 suburban streetlights, and in the darker spaces in between. Invertebrates were sampled along grass verges using intercept traps 1.5 metres above the ground and pitfall traps set into the ground nearby.

Samples were taken day and night over 72 hours across three locations in suburban Melbourne, Australia (10 streetlights per location), and repeated at identical locations a year later.We counted the number and type of invertebrates caught in each trap, then compared the effects of streetlight proximity (under streetlight versus between streetlights), intensity (brightness) and spectrum (colour) on the abundance and composition of the flying and ground-dwelling groups.

We measured the intensity and spectrum of light using common measures that are weighted to reflect human visual sensitivity (illuminance (Lux) and correlated colour temperature (CCT) respectively). Our streetlights emitted a range of lighting intensities (illuminance: 6.0 to 26.8 Lux) and spectra (CCT: 3373 to 4859 °K). However, because invertebrate visual systems are highly varied and have spectral sensitivities different to those of humans, we also took unweighted measures of intensity and spectrum, based on photon counts (photon flux). We modelled invertebrate abundance and community composition using both the human-weighted and unweighted measures, and got largely consistent results.

Locket fig 1 — Simultaneous day and night sampling of airborne and ground-dwelling invertebrates showed that the two groups respond differently to the proximity, intensity and colour of streetlights

Even in our long-illuminated, heavily urbanised study sites, we found that ALAN continues to strongly alter the abundance and (to a lesser extent) composition of invertebrate communities. However, the proximity, intensity and colour of streetlighting affected airborne invertebrates very differently to those on the ground.

ALAN clearly disrupts the spatial distribution of flying invertebrates at night, with twice as many caught under streetlights as between streetlights. This effect was strongest under the whitest streetlights (i.e. those with a high proportion of short wavelength ‘blue’ light and a low proportion of long wavelength ‘red’ light).

The types of airborne invertebrates found under streetlights were also different to those between streetlights—most notably, non-biting midges, ground beetles, flying ants and moths were all much more prevalent under streetlights. This community composition effect continued (weakly) into the daytime, but abundance effects were only seen at night.

In contrast, the abundance and composition of ground-dwelling invertebrates were similar under and between streetlights, both during the day and at night, and were largely unaffected by lighting colour. However, brighter streetlights reduced the abundance of ground-dwelling invertebrates at all traps, including those between streetlights, and changed the mix of ground-dwelling invertebrates present.

Locket fig 3 — Flying invertebrates were more abundant under streetlights than between, however this effect was only present at night (a) and was strongest in ‘whiter’ lights with a low proportion of red light and high proportion of blue light (b). No similar effect of proximity was seen in ground-dwelling invertebrates (c), however abundance at all traps was lower when streetlights were brighter (d).

These findings are consistent with the growing body of evidence linking ALAN to changes in invertebrate community abundance, composition and spatial distribution, and highlight the particular effects of more intense lighting, and short-wavelength ‘white’ lights. Our study demonstrates that these effects are present even in long-illuminated, highly modified suburban streetscapes. Public lighting managers should consider ALAN impacts on invertebrate communities not only when introducing lighting to naïve environments, but also when changing lighting in highly urbanised areas.

Further, while the proximity and colour of streetlights disrupted flying invertebrates, lighting brightness was the key characteristic affecting those on the ground. Because of these differing responses, lighting proposals and environmental monitoring of invertebrate communities must take into account effects on both ground-dwelling and airborne assemblages, and must consider the placement, intensity and spectrum of lights employed.

Read the full paper Urban street lighting differentially affects community attributes of airborne and ground-dwelling invertebrate assemblages in Journal of Applied Ecology.