Correspondence: Addressing and mitigating the ecological effects of light pollution requires ecological perspectives

Light pollution has received increasing attention over the last decade due to its potential environmental impacts. According to the International Commission on Illumination, light pollution can be defined as the ‘sum total of all adverse effects of artificial light’. ¹ Light pollution consists of three key areas: astronomical and atmospheric light pollution, ecological light pollution and impacts on humans. ²

Lighting Research and Technology has published studies investigating aspects of light pollution,^3–6 environmental lighting zones,^7,8 light pollution management ⁹ and impacts on humans, ¹⁰ but few studies can be found in the rapidly expanding research area of ecological light pollution. In this correspondence, we discuss light pollution from an ecological perspective and highlight the most urgent research challenges.

In January 2022, the findings were published from the conference Dark and Quiet Skies for Science and Society II, organised by the United Nations Office for Outer Space Affairs, the International Astronomical Union and the Spanish government. ¹¹ The report focuses on the feasibility of recommendations to mitigate interference from artificial light at night (ALAN), low earth orbit satellites and radio-wavelength emissions.

We concur that ALAN is an alarming environmental problem, degrading astronomical observations and potentially harming ecosystems. Night sky brightening threatens ground-based astronomical observatories operating at optical wavelength ranges. ¹² Strategies to decrease disturbances of astronomical observations, mainly focusing on reducing sky brightness and upward emitted light, are also ecologically motivated, as bright lights may affect nocturnally migrating species and avian orientation. ¹³ However, not all species are nocturnal migrators. Most species are potentially directly exposed to light emitted below the horizontal plane from the installations but also reflected and scattered light originating from infrastructure and light installations at levels far exceeding those of upward emitted light due to the inverse square law. The impact of light can occur locally, thereby affecting organisms in the vicinity of, for example, road or bridge lighting, but also when light emissions from cities or installations protrude into aquatic habitats or when light installations at high elevations spill light over into lower-situated nature reserves or photosensitive habitats. The viewing direction of many animals in their natural environment is towards the horizon or the ground ¹⁴ and rarely up into the sky. However, it is currently unknown how measurements of upward emitted radiation translate into light exposure for living organisms.

Very few ecological studies have investigated the impacts of light at night with an ecosystem approach or including several species and their interactions. A major challenge is to conserve biodiversity in ecosystems when studies on mitigating measures are largely based on a single species or a group of species. In ecology, studies have focused to a large extent on individual species of birds, bats, fish and turtles and on nocturnal insects as a group. ² Nevertheless, it is not currently possible to recommend any outdoor lighting solutions that would safeguard against all unwanted ecological impacts. This is because species differ in photosensitivity. ¹⁵ General strategies to reduce, for example, the amount of upward emitted light ¹⁶ or the blue-light content of outdoor lighting, and use more warm-white light sources, as proposed by many organisations (e.g., the Dark and Quiet Skies consortium ¹¹ and the International Union for Conservation of Nature ¹⁷ ) will not be effective in reducing impacts on all species. Unfortunately, these contradictions have not been thoroughly discussed by the responsible organisations with ecologists and lighting scientists. In short, we have identified a pressing need for research to answer the following questions:

1. Which species, ecosystems and ecological processes will benefit the most and the least from popular general strategies to mitigate light pollution?

2. How much will responses to ALAN differ between species (with various photosensitivities), as well as between ecosystems?

3. Do established applications (or adaptations) in light installations or conservation measures have the intended mitigating effects on species, biodiversity and ecosystems?

4. How can we measure ecological light pollution?

We acknowledge the need for sustained global efforts to curb light pollution but advise caution, since general strategies to mitigate light pollution cannot directly be translated into ecological benefits. To address these issues, ecological perspectives are crucial. We argue that recommendations must consider ecosystem- and species-specific photosensitivity and the long-term impacts on these populations’ survival. Long-term monitoring and assessment should be conducted of both terrestrial and aquatic ecosystems to cover species with various photosensitivities to accurately assess not only the most photosensitive species but also all species inhabiting these ecosystems. To fully understand the ecology, demography, migration, the various stages in species lifecycles and interactions with prey and predator and how these are affected by light at night, well-planned experimental studies are needed that incorporate carefully chosen ecological parameters and involve experienced ecologists and lighting scientists.