Trapped in the Light

Night is the peak activity period for many animal species. In the Western Andes of Ecuador, the Chocó golden scarab flies between forest patches during the night, but urban lighting interferes with their paths and jeopardises populations already struggling to persist in fragmented native forests.

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Urban development has created a network of illuminated infrastructure that allows our society to function day and night without interruption. It is no surprise that with so much artificial light, we increasingly have to move farther away from towns and cities to see a sky full of stars.

Light pollution poses a challenge for nocturnal species that have adapted to living in the dimness of night (1, 2) — see documentaries about the impacts of artificial light on wildlife and insects, and a related scientific talk. This problem might be one of the causes of the global decline in insects (3, 4), in turn negatively affecting their role in maintaining agricultural systems through pest control, pollination, and soil quality (5). These concepts are featured by the documentaries The Insect Apocalypse and The Great Death of Insects.

_{Chocó golden scarab (Chrysina argenteola) walking on forest litter in La Maná (Cotopaxi, Ecuador). Growing to up to 4 cm in length, this species inhabits the tropical rainforest of the Chocó region in the Western Andes (10), where it is frequently attracted to artificial lights at night. The striking color of this ‘jewel scarab’ is an optical illusion. The exoskeleton is covered with overlapping layers of chitin that polarise light and reflect hues of blue, gold, green, silver, or reddish tones, depending on the species (16). The metallic sheen appears to deter bird predation (17) and might serve as camouflage as well as aid in individual recognition (11). The eyes of insects are ‘compound’ — composed of 100s to 1000s of tubular eyelets (‘ommatidia’), each with its own cornea and lens (18), and all collectively contributing to insect vision. In nocturnal species like the golden scarab, the photoreceptor cells (at the base of each ommatidium) respond more slowly to light compared to diurnal species, allowing the former to collect more nocturnal light per unit of time before forming an image (19). However, just as staring at the sun blinds us, eyes adapted for night vision become overwhelmed by excessive artificial light, disrupting the behavior of these species. Below the scarab image are two photographs contrasting the day and night landscapes of the same location in Pedro Vicente Maldonado (Pichincha, Ecuador) within the species’ distribution range. Photos courtesy of Martín Bustamante (animal) and Luis Camacho (city).}

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When flying, nocturnal insects orient their backs toward the sky, using the light of the moon and stars as a reference (6) (explained here and here). However, when they encounter artificial lights, they can no longer distinguish up from down, and so they can become disoriented, flying erratically, like a moth circling a streetlight.

It is estimated that a third of the insects attracted to artificial light die from collisions, burn injuries, exhaustion, and/or predation (7). In the tropics, finding countless dead insects at the base of urban lights is a common scene. Equally important is that artificial light also hinders migration, foraging, and the search for mates in many nocturnal species (1, 8, 9).

Nocturnal jewels

Camacho and collaborators evaluated the effect of artificial lighting at night on the Chocó golden scarab (Chrysina argenteola) (10). This species inhabits the tropical rainforests of the Western Andes from Ecuador to Colombia, and is a member of the group known as ‘jewel scarabs‘ due to their metallic body coloration (11). Because of its nocturnal habits and the larvae’s dependence on wood for food (12), the golden scarab has been increasingly affected by the loss of native forest in combination with light pollution from rural and urban expansion.

In the last two centuries approximately two-thirds of the native forest, which serve as habitat for this scarab, have been lost in Ecuador. However, the conservation status of the species is poorly understood, and little is known of its natural history while its populations are difficult to monitor using established entomological survey methods.

As an alternative to studying the golden scarab directly, Camacho relied on local ecological knowledge (13). In 2014, he did interviews with 395 locals, gathering 362 reports of golden scarab sightings, including their latitude and longitude. Most sightings consisted of individuals in flight and spanned a region encompassing 45 towns. Ninety-six per cent of sightings occurred in cultivated or urban areas, with 78% involving individuals flying around artificial lights. Comacho and his team cross-referenced the geographical coordinates of each sighting with satellite images (also compiled in 2014) to assess forest cover and the intensity of artificial light at each scarab sighting.

Camacho’s team found that in areas with more forest cover, sightings of the beetles were more likely in places with higher artificial light. However, in regions with less forest, sightings were more likely in dimly lit areas. The probability of encountering the beetles was highest in forests that were both abundant and more fragmented by illuminated roads and pathways (10).

_{Combined effect of Andean forest condition and artificial lighting at night on the Chocó golden scarab (Chrysina argenteola) using data obtained from local interviewees living in the provinces of Pichincha and Santo Domingo de los Tsáchilas (Ecuador) (10). Trend lines on left show that as tree cover increases, scarabs are more frequently observed in areas where the forest is more fragmented (number of fragments: 16–35 [high], 6–15 , 1–5 [low]). The central graph illustrates that in areas with high tree cover (4–16 km² of forest within a 3-km radius of the sighting point), the probability of spotting the scarab increases as locations go from less to more illuminated. This increase is weaker in areas with intermediate forest cover (1–4 km²). In contrast, in areas with low forest cover (0–1 km²), it is more likely to observe these insects in places with less artificial light. The scarab sightings (362 in total) were gathered from interviews with 395 residents of the study area. The photo on the right (20-sec exposure) shows the erratic flight trajectories of hundreds of insects drawn to a light post above a football field in Pedro Vicente Maldonado (Pichincha). Photo by Luis Camacho.}

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Dazzling aerial landscapes

Over 80% of humanity (99% in Europe and the USA) lives under skies polluted by artificial light (14) — see animation of the world atlas of artificial night-sky brightness. Its effects on wildlife interact with other impacts through climate change, noise pollution, and habitat loss (9).

Such is the case for the Chocó golden scarab. Understanding their response to light pollution requires considering the state of the forest. In healthy forests with abundant resources for many individuals, reproduction can offset the mortality caused by light pollution. However, in areas where the forest is more degraded, the losses are not replenished, putting the beetle at high risk of local extinction. In other words, Camacho’s team’s findings (10) do not support the hypothesis that insects are more frequently trapped by artificial lights near mature forests. Rather, these arthropods are more abundant in such areas, making it more likely to observe them disoriented there compared to degraded forests or urban areas, where their populations have already waned due to low habitat quality.

The aerial zone immediately above the ground is part of the living space for species that fly. The study of this zone has given birth to the discipline of aeroecology (15) — a paradigm for cross-disciplinary research through biology, chemistry, ecology, genetics and physics.

If we perceive the land-air interface as a true ecosystem, artificial light (as much as airplane routes or the physical presence of buildings) fragments the aerial habitat of flying species at night. It is as though the animals were flying over a ‘minefield’ of urban lights that intercept them (1).

This fragmentation of the air mirrors how the construction of roads or the clearing of forests fragments the habitat of non-flying animals. Within that rationale, there is an urgent need to assess how environmental policies aimed at preventing fragmentation of terrestrial habitats can also be applied to aerial habitats and to what extent innovative actions are required when light pollution is the dominant mechanism causing habitat fragmentation (8).

Salvador Herrando-Pérez and Luis F. Camacho

References

1. Gaston KJ et al. 2021. Pervasiveness of biological impacts of artificial light at night. Integrative and Comparative Biology 61: 1098-1110
2. Kehoe R, Sanders, D & van Veen, FJF. 2022. Towards a mechanistic understanding of the effects of artificial light at night on insect populations and communities. Current Opinion in Insect Science 53: 100950
3. Owens ACS et al. 2020. Light pollution is a driver of insect declines. Biological Conservation 241: 108259
4. Kalinkat G et al. 2021. Assessing long-term effects of artificial light at night on insects: what is missing and how to get there. Insect Conservation and Diversity 14: 260-270
5. Grubisic M et al. 2018. Insect declines and agroecosystems: does light pollution matter? Annals of Applied Biology 173: 180-189
6. Fabian ST et al. 2024. Why flying insects gather at artificial light. Nature Communications 15: 689
7. Eisenbeis G & Hänel, A, 2009. in Ecology of Cities and Towns: A Comparative Approach, MJ McDonnell, AK Hahs & JH Breuste, Eds. (Cambridge University Press), pp. 243-263
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10. Camacho LF, Barragán, G & Espinosa, S. 2021. Local ecological knowledge reveals combined landscape effects of light pollution, habitat loss, and fragmentation on insect populations. Biological Conservation 262: 109311
11. Thomas DB, Seago, A & Robacker, DC. 2007. Reflections on golden scarabs. American Entomologist 53: 224-230
12. Hawks DC. 2002. Jewel scarabs. Museum Notes (University of Nebraska State Museum) 112: 1-4
13. Joa B, Winkel, G & Primmer, E. 2018. The unknown known – a review of local ecological knowledge in relation to forest biodiversity conservation. Land Use Policy 79: 520-530
14. Falchi F et al. 2016. The new world atlas of artificial night sky brightness. Science Advances 2: e1600377
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16. Fernández del Río L, Arwin, H & Järrendahl, K. 2016. Polarizing properties and structure of the cuticle of scarab beetles from the Chrysina genus. Physical Review E 94: 012409
17. Kjernsmo K et al. 2020. Iridescence as camouflage. Current Biology 30: 551-555
18. Makarova AA et al. 2022. Scaling of the sense organs of insects. 1. Introduction. Compound eyes. Entomological Review 102: 161-181
19. Warrant EJ. 2017. The remarkable visual capacities of nocturnal insects: vision at the limits with small eyes and tiny brains. Philosophical Transactions of the Royal Society B 372: 20160063