Bright Side of the Night – 2.3.2

Is Artificial Light at Night a Stress Factor for Biodiversity?

Artificial Light – The Great Disruptor

Light pollution plays a role in the global loss of biodiversity. It is disruptive at all levels of biodiversity – in terms of genes, species, communities, populations, and ecosystems. It has the potential to further disrupt ecosystems that are already suffering from multiple weakening factors.

When Systems Are Out of Joint

In habitats illuminated by street lights, invertebrate predators and scavengers have been observed to occur in larger numbers than they do in comparative locations without artificial light. In particular, scientists found that amber light and, to a lesser extent, white light suppress the flowering of the greater birdsfoot trefoil (Lotus pedunculatus). For instance, when pea aphids (Acyrthosiphon pisum) switched from sucking the plant juices from the shoots to the flowers in August, the number of animals in the amber light boxes in the field trial decreased significantly – presumably because fewer inflorescences were present. Another example of this phenomenon can be seen in some mountain valleys in Switzerland, where the lesser horseshoe bat has disappeared completely after street lighting was installed. Concurrently, the less light-shy common pipistrelle has spread into these areas. The suspicion is that the lesser horseshoe bat disappeared due to competitive pressure. Both species are about the same size and eat the same spectrum of food.

Insects as Ecosystem Service Providers

In the adult stage, many insects feed on nectar and pollen from plants. Passing from flower to flower, pollen are transferred. If the pollen lands on the carpel of a seed plant of the same species, fertilization can take place. For the plant however, the focus goes beyond reproduction to the rearrangement of genetic material. All this makes pollination one of the most significant interactions between plants and animals, and thus, one of the crucial ecosystem services of nocturnal insects.

Nocturnal Insects are Key Pollinators

Nocturnal insects thus make essential contributions to the maintenance of biodiversity, pest regulation and soil quality. It was found in a study[1] that flowers cultivated under artificial lighting received 62 percent fewer visits from nocturnal pollinators than those growing at dark reference sites. The loss in terms of pollination by nocturnal insects was not compensated by daytime pollinators, as the case of the cabbage thistle (Cirsium oleraceum) showed. The reduced number of nocturnal pollinators resulted in a 13 percent decrease in the number of fruits per plant. Artificial light thus clearly influenced the sexual propagation of the plants. In the absence of pollination, the long-term composition of vegetation changes, sometimes even leading to a loss of species.

From Vertical Migration to Irritation

Under cover of darkness, microbial fauna swim to the surface of the water to feed on algae. In the daytime, zooplankton can be found in lower layers of water. Light pollution—even the relatively weak brightness caused by urban light pollution—suppresses this nightly vertical migration; the zooplankton no longer ascends. This disrupts the aquatic food chain and leads to more frequent algal bloom, and thus, to deterioration in the quality of the water.

[1] Knop et al. (2017)

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*Eyed hawk-moth. Image by Ian Lindsay – Pixabay*

*Pipistrellus pipistrellus bat. Image by Jakob Fahr – iNaturalist*

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Artificial light at night: Potential sources, biodiversity impacts and responses are complex.