Light pollution: a landscape-scale issue requiring cross-realm consideration

Abstract

Terrestrial, marine and freshwater realms are inherently linked through ecological, biogeochemical and/or physical processes. An understanding of these connections is critical to optimise management strategies and ensure the ongoing resilience of ecosystems. Artificial light at night (ALAN) is a global stressor that can profoundly affect a wide range of organisms and habitats and impact multiple realms. Despite this, current management practices for light pollution rarely consider connectivity between realms. Here we discuss the ways in which ALAN can have cross-realm impacts and provide case studies for each example discussed. We identified three main ways in which ALAN can affect two or more realms: 1) impacts on species that have life cycles and/or stages in two or more realms, such as diadromous fish that cross realms during ontogenetic migrations and many terrestrial insects that have juvenile phases of the life cycle in aquatic realms; 2) impacts on species interactions that occur across realm boundaries, and 3) impacts on transition zones or ecosystems such as mangroves and estuaries. We then propose a framework for cross-realm management of light pollution and discuss current challenges and potential solutions to increase the uptake of a cross-realm approach for ALAN management. We argue that the strengthening and formalisation of professional networks that involve academics, lighting practitioners, environmental managers and regulators that work in multiple realms is essential to provide an integrated approach to light pollution. Networks that have a strong multi-realm and multi-disciplinary focus are important as they enable a holistic understanding of issues related to ALAN.

Keywords: ALAN; adaptive management; artificial light at night; ecological connectivity; light pollution; multi-disciplinary.

Figure 1

Schematic figure showing the potential cross-realm impacts of ALAN due to effects on different life stages in salmon. (A) Salmon spend their juvenile phase in rivers before migrating to sea to grow and mature. To complete their life cycle, they must return to the river to spawn. (B) ALAN at sea alters vertical movement of fish resulting in a mismatch between preferred light levels and optimal feeding zones. Additionally, ALAN results in increased predation of fish at sea and hence a decrease in adults returning to rivers. (C) ALAN along rivers disrupts synchronous emergence of juveniles resulting in increased predation which then reduces the recruitment of smolts out to sea. This reduction in adults returning to rivers and smolts migrating to sea results in trophic effects in both realms. (D) Illustrates one trophic effect in the terrestrial environment with reduced food resources for bears resulting in reduced nutrients into the terrestrial environment. Image created with BioRender.com.

Figure 2

(A) Schematic figure depicting the aquatic ecosystem with fishing bats under natural light (B) and how artificial light at night influences prey species. As artificial light is introduced, aquatic prey species migrate into the shadows, sediment or to greater depths, making them unavailable to bats. Additionally, some aquatic insects emerge as aerial adult forms that are attracted to light. Fishing bats avoid lit areas and cannot switch foraging strategies to take advantage of the new aerial prey that is attracted to lights. Image created with BioRender.com.

Figure 3

Proposed framework to explore the cross-realm impact of artificial light at night. 1. Defining light pollution – requires a shared understanding of what constitutes light pollution, and that its meaning and measurement is consistent across all stakeholders; 2. Accounting for cross-realm connections – requires knowledge of the ecological environment, the organisms, target species and how cross-realm impacts intersect; 3. Integrating effective cross-realm management – requires all stakeholders to be clear on objectives and outcomes; and 4. Effective scaling of management integrations – requires the scale of the management intervention to match the scale of impact. Image created with BioRender.com.