Nocturnal city lighting elicits a macroscale response from an insect outbreak population

Abstract

Anthropogenic environmental change affects organisms by exposing them to enhanced sensory stimuli that can elicit novel behavioural responses. A pervasive feature of the built environment is artificial nocturnal lighting, and brightly lit urban areas can influence organism abundance, distribution and community structure within proximate landscapes. In some cases, the attractive or disorienting effect of artificial light at night can draw animals into highly unfavourable habitats, acting as a macroscale attractive ecological sink. Despite their significance for animal ecology, identifying cases of these phenomena and determining their effective scales and the number of organisms impacted remains challenging. Using an integrated set of remote-sensing observations, we quantify the effect of a large-scale attractive sink on nocturnal flights of an outbreak insect population in Las Vegas, USA. At the peak of the outbreak, over 45 million grasshoppers took flight across the region, with the greatest numbers concentrating over high-intensity city lighting. Patterns of dusk ascent from vegetated habitat toward urban areas suggest a daily pull toward a time-varying nocturnal attractive sink. The strength of this attractor varies with grasshopper density. These observations provide the first macroscale characterization of the effects of nocturnal urban lighting on the behaviour of regional insect populations and demonstrate the link between insect perception of the built environment and resulting changes in spatial and movement ecology. As human-induced environmental change continues to affect insect populations, understanding the impacts of nocturnal light on insect behaviour and fitness will be vital to developing robust large-scale management and conservation strategies.

Keywords: Orthoptera; aeroecology; artificial lights at night; global change; grasshopper; radar entomology.

Figure 1.

Nocturnal urban lighting as a macroscale sensory stimulus for an outbreak population of grasshoppers. (a) The pallid-winged grasshopper (Trimerotropis pallidipennis). (b) An aerial view of the Las Vegas strip, illustrating an extreme case of large-scale artificial light at night. (c) Mass attraction of grasshoppers to the high-intensity light column of the Luxor Hotel and Casino on the Las Vegas strip. (d) An example of the widespread nocturnal grasshopper aggregations that were characteristic of this outbreak event. (e) A snapshot of the regional airspace taken by the Las Vegas weather surveillance radar (red circle) at the peak of the outbreak event on 27 July, 2019 07:07 UTC (00:07 local time), detecting approximately 45 million grasshoppers aloft. Downtown Las Vegas is denoted by a white circle. (f) Landscape-scale variability in vegetation index at the time of the grasshopper outbreak. (g) Landscape-scale variability in nocturnal radiance at the time of the grasshopper outbreak. (h–l) Dusk ascent and within-night movement dynamics of grasshopper aggregations across southern Nevada on the night of 25 July, 2019. (h–j) The abundance and distribution of grasshoppers aloft with respect to downtown Las Vegas (white circle) measured by the Las Vegas weather surveillance radar (red circle) at 03:35, 05:35 and 07:38 UTC. Colour maps in (h–j) are the same as (e) and weather signals are denoted by greyscale. (k) Time series of within-night variability in grasshopper abundance across the full radar surveillance domain (black). (l) Time series of within-night variability in Pearson correlation coefficient between the spatial distribution of grasshoppers and vegetation index (green) and nocturnal light intensity (blue) throughout the night. The yellow dotted lines indicate the times of local sunset and sunrise and filled circles correspond with the times shown in (h–j).

Figure 2.

Variability in abundance of grasshoppers across the 75-day analysis period and resulting impacts on landscape associations. (a) Across-night variability in mean grasshopper abundance over the full radar surveillance domain (black) from 1 June to 14 August. ((a), inset) The electromagnetic scattering model of a pallid-winged grasshopper. (b) Nightly mean Pearson correlation coefficient between the spatial distribution of grasshoppers and vegetation index as a function of grasshopper abundance for the analysis period. (c) Nightly mean Pearson correlation coefficient between the spatial distribution of grasshoppers and nocturnal light intensity as a function of grasshopper abundance for the analysis period. Nightly means are calculated from 1 h before sunset to 1 h after sunrise.