Thursday, 1 November 2012

Artificial lighting has very real effects on invertebrates

Researchers estimate that artificial lighting is increasing at a rate of 6% each year around the world. This could dramatically reduce the amount of "naturally unlit habitat" available to wildlife, and have significant impacts on organisms that have evolved to deal with specific characteristics and regimes of light. By altering how much light is present in a habitat, where it occurs, how bright it is, what wavelengths it contains, and when it illuminates the environment, we might inadvertently impact a variety of wildlife traits, including behaviors (such as foraging, navigation, and communication), physiology (including cyclical release of hormones and other important chemicals), and interactions with other species. In fact, researchers have already gathered evidence indicating that light pollution does effect these characteristics in at least some species. Until recently, however, scientists had no information on whether artificial lighting might alter higher-level processes, such as the formation of animal communities.

(Image courtesy of Redlands Labour.)

Last summer, a team of researchers from the University of Exeter's Environment and Sustainability Institute investigated these potential higher-level effects of light pollution using an elegantly simple study design: Over a period of three days and three nights during the annual peak of insect abundance, they set out traps to collect invertebrates utilizing habitat either under street lamps or in unlit areas between two neighboring streetlamps. Traps were emptied twice a day: 30 minutes before sunrise, and again 30 minutes before sunset. This allowed the researchers to determine whether the street lamps were having a permanent impact on invertebrate communities, or whether differences were only found at night, when the lamps were switched on and were actively attracting insects from the neighboring habitat.

The scientists performed several different analyses on their data. First, they looked for differences in invertebrate abundance (measured as the total number of individuals caught per trap over the entire sampling period) between the lit and unlit patches. Second, they performed a similar analysis that took into account invertebrate community composition--in other words, not just how many organisms were caught, but what species those animals belonged to. Third, they quantified the amount of vegetation present around each trap in order to determine whether the lights were directly impacting invertebrate communities, or whether the lights were having an indirect effect by altering vegetation under each street lamp. Finally, in order to get a clearer idea of how the effects of light might impact ecosystem structure and function, they classified all trapped invertebrates more broadly, both taxonomically speaking ("harvestmen," "mites," "amphipods," etc.) and trophically speaking ("predators," "scavengers," parasites," etc.), and again tested for differences between the two types of patch.

(Example of a pitfall trap similar to those used in the current study. Image courtesy of INHS.)

The results of their work, published this autumn in the academic journal Biology Letters, were pretty emphatic. Their final analysis included data from 1,194 ground-dwelling invertebrates from 60 different taxa. On average, traps directly under the street lamps caught significantly more species (+ 5) than traps between the lamps; likewise, composition of invertebrate communities was significantly different in lit and unlit areas. These patterns were found both during the day and at night, indicating that street lamps not only impact vertebrate communities, but also that they do so permanently, and not just when they are turned on at night.

Five taxonomic groups (harvestmen, ants, ground beetles, woodlice, and amphipods) were more abundant in lit patches. For the first three of these groups, this relationship was independent of time, suggesting--again--that the lights have a permanent effect on invertebrate communities. Unfortunately, woodlice and amphipods were only captured at night, and so there is no way to determine whether they, too, are consistently more common in illuminated patches. Results of the trophic analysis also revealed long-term effects of streetlamps: Predators and scavengers were both more common under the lights, both during the day and at night.

(A harvestman, one of several taxonomic groups more common under street lamps. Image courtesy of FCPS.)

In addition to revealing the powerful effects of anthropogenic light pollution on invertebrate communities, this study is also the first to show that ground-dwelling invertebrates--and not just their aerial relatives--are attracted to human lighting. The next obvious question is whether the effects of these community shifts can be felt by organisms other than the invertebrates themselves. In order to explore this possibility, it may be particularly interesting to research the effects of light on trophic interactions, since invertebrates are an essential dietary component of many species (and, of course, are also consumers, themselves). Should this light-mediated community restructuring have wider ecosystem impacts, it will be important to identify these and determine whether they might alter essential ecosystem services provided to humans.

As researchers continue to explore the relationships between light pollution and wildlife, they will need to keep up to speed with changes in artificial lighting technology. Variations in bulb and lamp design can impact the wavelengths of light emitted, the brightness of the resulting glow, and the total area of the environment that is illuminated by light pollution. Each of these factors can contribute to the impacts of human light sources on wildlife and species assemblages.

Davies, T.W., Bennie, J., and Gaston, K.J. 2012. Street lighting changes the composition of invertebrate communities. Biology Letters 8:764-767.

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