Shining a light on indirect interactions

Many of us are familiar with direct effects of light pollution but how do these effects cascade and have wider implications? Associate Editor, Owen Lewis discusses the recently published article by Bennie et al. Artificial light at night causes top‐down and bottom‐up trophic effects on invertebrate populations.

You’ve probably seen the images of a night-time earth taken from space: the cities of Europe and North America glowing brightly, the coastlines of the continents picked out with light, and remarkably little of the globe truly dark. Although artificial ‘light pollution’ is ubiquitous across much of the planet, its ecological consequences received relatively little study until an upsurge in attention during the past decade. Ecologists have now uncovered a wide range of direct ecological impacts on the behaviour of individuals, as well as emerging effects on populations, communities and even ecosystem functions. The new paper by Bennie et al. from the University of Exeter adds a new twist to the story, unravelling cascading effects of light pollution and identifying some of the mechanisms through which light pollution might have community-wide consequences.

While most studies on light pollution concentrate on the direct effects of illumination such as changes to behaviour or reproductive rates, Bennie et al.’s clever experimental design allowed them to check for indirect effects too, and to distinguish between those that are mediated by their predators versus those mediated via their resources. Top-down indirect effects occur when predators are responsible for changes in the abundance of the herbivores, while bottom-up indirect effects result from changes to their plant resources.

Graphical abstract
Graphical abstract courtesy of Bennie et al.

The researchers populated small cages (mesocosms) with communities comprising one (plants only), two (plants plus invertebrate herbivores) or three (plants, herbivores and their predators) trophic levels from a grassland food web. The mescosms were then exposed to ‘natural’ levels of illumination as a control, or to one of two light pollution treatments. These treatments involved amber light characteristic of low pressure sodium lamps, widely used for street lighting, and the white LED lights with which they are increasingly being replaced.

Focusing on the invertebrate herbivores (aphids and slugs), the researchers found that artificial light at night had both top-down and bottom-up effects, and that these differed between the different lighting technologies. Indirect effects were particularly pronounced for the white LED lights: numbers of slugs (generalist herbivores) were more than halved under this treatment, but only in mesocosms where a visually-hunting predatory carabid beetle was present. This reveals that the decrease in slug abundance in the white LED treatment is not a direct response, but results from predation, probably because broad-spectrum light helps the beetles find their prey at night.

In contrast, the amber light mimicking the sodium vapour lamps led to a bottom-up effect on the pea aphids, which are more specialised herbivores than the slugs. Aphid abundance decreased by 17% in the amber light treatment, linked to changes in the availability of their plant resources. There was no evidence that top-down effects of aphid predators differed among the lighting regimes.

While such effects are perhaps not unexpected, effects of light pollution that span trophic levels in this way have not previously been unravelled. It seems likely that they are widespread within ecological communities, particularly in places where artificial illumination is pronounced, including urban and suburban habitats, domestic gardens and roadsides.

The Exeter team’s findings, and other ecological work on ‘ALAN’ (Artificial Light at Night) have clear implications for public policy and the design of outdoor lighting systems. In general, minimising unnecessary night-time illumination is a sensible strategy. Where it is unavoidable, attention should be paid to the appropriate wavelength to minimise the sorts of cascading ecological effects that Bennie et al. document. Plants and their specialized herbivores seem most likely to be affected by longer wavelengths of light, while trophic interactions involving visually-hunting predators may be most strongly altered by broad-wavelength white light. In theory, the LED technologies now being phased in for outdoor illumination could allow greater flexibility in the wavelengths of light emitted, to avoid spectra that have marked direct and indirect effects on plants and animals.

Read the full article, Artificial light at night causes top‐down and bottom‐up trophic effects on invertebrate populations in Journal of Applied Ecology.

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