Low Levels of Artificial Light at Night Strengthen Top-Down Control in Insect Food Web

Artificial light has transformed the nighttime environment of large areas of the earth, with 88% of Europe and almost 50% of the United States experiencing light-polluted night skies 1Falchi F. Cinzano P. Duriscoe D. Kyba C.C.M. Elvidge C.D. Baugh K. Portnov B.A. Rybnikova N.A. Furgoni R. The new world atlas of artificial night sky brightness.Sci. Adv. 2016; 2: e1600377Crossref PubMed Scopus (682) Google Scholar. The consequences for ecosystems range from exposure to high light intensities in the vicinity of direct light sources to the very widespread but lower lighting levels further away 2Bennie J. Davies T.W. Cruse D. Inger R. Gaston K.J. Cascading effects of artificial light at night: resource-mediated control of herbivores in a grassland ecosystem.Philos. Trans. R. Soc. Lond. B Biol. Sci. 2015; 370: 20140131Crossref PubMed Scopus (103) Google Scholar. While it is known that species exhibit a range of physiological and behavioral responses to artificial nighttime lighting e.g., 3Gaston K.J. Bennie J. Davies T.W. Hopkins J. The ecological impacts of nighttime light pollution: a mechanistic appraisal.Biol. Rev. Camb. Philos. Soc. 2013; 88: 912-927https://doi.org/10.1111/brv.12036Crossref PubMed Scopus (591) Google Scholar, 4Miller M.W. Apparent effects of light pollution on singing behavior of American robins.Condor. 2006; 108: 130-139Crossref Scopus (147) Google Scholar, 5Stone E.L. Jones G. Harris S. Street lighting disturbs commuting bats.Curr. Biol. 2009; 19: 1123-1127Abstract Full Text Full Text PDF PubMed Scopus (278) Google Scholar, there is a need to gain a mechanistic understanding of whole ecological community impacts 6Gaston K.J. Davies T.W. Bennie J. Hopkins J. Reducing the ecological consequences of night-time light pollution: options and developments.J. Appl. Ecol. 2012; 49: 1256-1266Crossref PubMed Scopus (314) Google Scholar, 7Knop E. Zoller L. Ryser R. Gerpe C. Hörler M. Fontaine C. Artificial light at night as a new threat to pollination.Nature. 2017; 548: 206-209Crossref PubMed Scopus (225) Google Scholar, especially to different light intensities. Using a mesocosm field experiment with insect communities, we determined the impact of intensities of artificial light ranging from 0.1 to 100 lux on different trophic levels and interactions between species. Strikingly, we found the strongest impact at low levels of artificial lighting (0.1 to 5 lux), which led to a 1.8 times overall reduction in aphid densities. Mechanistically, artificial light at night increased the efficiency of parasitoid wasps in attacking aphids, with twice the parasitism rate under low light levels compared to unlit controls. However, at higher light levels, parasitoid wasps spent longer away from the aphid host plants, diminishing this increased efficiency. Therefore, aphids reached higher densities under increased light intensity as compared to low levels of lighting, where they were limited by higher parasitoid efficiency. Our study highlights the importance of different intensities of artificial light in driving the strength of species interactions and ecosystem functions.