Artificial light at night erases positive interactions across trophic levels

Abstract

Artificial light at night (ALAN) is one of the most recently recognized sources of anthropogenic disturbance, with potentially severe effects on biological systems that are still to be fully explored. Among marine ecosystems, high‐shore habitats are those more likely to be impacted by ALAN, due to a more intense exposition to outdoor nocturnal lightings (mostly from lamps along coastal streets and promenades, or within harbours, ports and marinas). By performing in situ nocturnal manipulations of a direct source of white LED light and presence of herbivores in a Mediterranean high‐shore habitat, we assessed the interactive effects of light pollution and grazing on two key functional components of the epilithic microbial community (the cyanobacteria, as the main photoautotrophic component, and the other bacteria, mainly dominated by heterotrophs) developing on rocky shores. Results showed an unexpected increase in the diversity of epilithic bacterial biofilm at unlit sites in the presence of grazers, that was more evident on the other (mainly heterotrophic) bacterial component, when giving weight to more abundant families. This effect was likely related to the mechanical removal of dead cells through the grazing activity of consumers. ALAN significantly modified this scenario, by reducing the density of grazers and thus erasing their effects on bacteria, and by increasing the diversity of more abundant cyanobacterial families. Overall, direct and indirect effects on ALAN resulted in a significant increase in the diversity of the photoautotrophic component and a decrease in the heterotrophic one, likely affecting key ecosystem functions acting on rocky shore habitats. ALAN may represent a threat for natural systems through the annihilation of positive interactions across trophic levels, potentially impairing the relationship between biodiversity and functioning of ecosystems and interacting with other global and local stressors currently impinging on coastal areas. A free Plain Language Summary can be found within the Supporting Information of this article.

Biomass responses of widely and less‐widely naturalized alien plants to artificial light at night

“…irradiance; 1.8% per year between 2012 and 2016) (Kyba et al., 2017). As artificial light at night could potentially affect many ecological processes, from individuals via communities to ecosystems (Bennie et al., 2015; Gaston, 2018; Hölker et al., 2010; Irwin, 2018; Maggi et al., 2019), it has emerged as an important topic in ecological research (Davies & Smyth, 2018).…”

Artificial Light at Night: A New Challenge in Microphytobenthos Research

“…Intensity, spectrum, spatial and temporal variability of ALAN are key features of a recently recognized source of anthropogenic disturbance, that is impinging on coastal organisms at a global scale. The role of light pollution on physiological and ecological aspects of MPB is still in its infancy, but some recent studies anticipate a potential for either direct or indirect influences on the spatial and temporal variability of biomass and diversity of MPB assemblages, mediated by trophic relationships (Maggi and Benedetti-Cecchi, 2018;Maggi et al, 2019Maggi et al, , 2020, as well as for interactive effects with additional stressors (Pu et al, 2019). Here we emphasize the need for including ALAN among the new challenges in microphytobenthos research, through the development of common monitoring approaches and the unambiguous assessment of its potential effects on MPB by means of manipulative experiment.…”

“…Finally, the ecological effect of ALAN on MPB cannot be fully assessed without considering top-down and bottom-up effects related to their consumers (Leroux and Loreau, 2015;Lynam et al, 2017). Rates of grazing activity might be influenced by ALAN, with positive or negative effects cascading down to MPB, depending on the suppression or enhancement of consumers' metabolism, respectively (Maggi and Benedetti-Cecchi, 2018;Maggi et al, 2020). Potential changes in behavior of herbivores might even affect the spatial variability in the MPB biomass, as observed under increased water temperature (Como et al, 2014).…”

“…Knowledge on the effect of ALAN on coastal organisms and habitats is still fragmented, but published research has already highlighted a variety of impacts, including effects on settlement processes (both in invertebrates and bacteria; Davies et al, 2015;Maggi and Benedetti-Cecchi, 2018), changes in behavior (e.g., orientation of turtles, vertical migration of zooplankton and fish, anti-predator and locomotor activities, trophic pressure; e.g., Witherington and Bjorndal, 1991;Underwood et al, 2017;Ludvigsen et al, 2018;Duarte et al, 2019;Maggi et al, 2019) and composition of assemblages (Garratt et al, 2019;Maggi et al, 2020).…”

Exposure to Artificial Light at Night and the Consequences for Flora, Fauna, and Ecosystems

“…N. lapillus are widely distributed across the North Atlantic (including illuminated coastal areas) and feed on barnacles and mussels; they were more likely to, respond to, and handle prey under, a white LED light (∼21 lx) compared to controls, irrespective of the presence of a snail predator (the common shore crab Carcinus maenas ) ( Underwood et al, 2017 ). Alterations of trophic interactions were also reported to occur under ALAN in studies performed along the Italian coast, where the population of grazing snails Melarhaphe neritoides has positive effects on the diversity of epilithic heterotrophic bacteria under a natural LD cycle ( Maggi et al, 2020 ). ALAN (white LED, 27 lx) modified this by reducing the density of grazers (thus erasing the positive effects on heterotrophic bacteria) and increasing autotrophic Cyanobacteria.…”