Abstract
Artificial light at night (ALAN) is a widespread alteration of the natural environment that can affect the functioning of ecosystems. ALAN can change the movement patterns of freshwater animals that move into the adjacent riparian and terrestrial ecosystems, but the implications for local riparian consumers that rely on these subsidies are still unexplored. We conducted a two-year field experiment to quantify changes of freshwater-terrestrial linkages by installing streetlights in a previously light-naive riparian area adjacent to an agricultural drainage ditch. We compared the abundance and community composition of emerging aquatic insects, flying insects, and ground-dwelling arthropods with an unlit control site. Comparisons were made within and between years using generalized least squares and a BACI design (Before-After Control-Impact). Aquatic insect emergence, the proportion of flying insects that were aquatic in origin, and the total abundance of flying insects all increased in the ALAN-illuminated area. The abundance of several night-active ground-dwelling predators (Pachygnatha clercki, Trochosa sp., Opiliones) increased under ALAN and their activity was extended into the day. Conversely, the abundance of nocturnal ground beetles (Carabidae) decreased under ALAN. The changes in composition of riparian predator and scavenger communities suggest that the increase in aquatic-to-terrestrial subsidy flux may cascade through the riparian food web. The work is among the first studies to experimentally manipulate ALAN using a large-scale field experiment, and provides evidence that ALAN can affect processes that link adjacent ecosystems. Given the large number of streetlights that are installed along shorelines of freshwater bodies throughout the globe, the effects could be widespread and represent an underestimated source of impairment for both aquatic and riparian systems.
Highlights
The continuing global increase of artificial light at night (ALAN) and its effects on organisms and ecosystems have received considerable attention in recent years (Hölker et al, 2010)
The model of comparison BACIacute found no significant difference in emergence among phases and sites, i.e. before and after Artificial light at night (ALAN) treatment in 2012 (Table 1, Figure 2)
We found a higher flux of aquatic insects into the illuminated riparian area through an increase in emergence directly under lamps
Summary
The continuing global increase of artificial light at night (ALAN) and its effects on organisms and ecosystems have received considerable attention in recent years (Hölker et al, 2010). Aquatic-terrestrial subsidy fluxes can be reciprocal, benefitting consumers in both habitats (Polis et al, 1997; Nakano and Murakami, 2001; Richardson et al, 2010) while contributing to food-web and ecosystem stability (Takimoto et al, 2002). The few studies to investigate this possibility have suggested that riparian consumers may respond to the increased availability of prey when aquatic insects are attracted to light sources (Perkin et al, 2011; Meyer and Sullivan, 2013). A difficulty with previous studies has been the inability to disentangle the effects of ALAN from the many associated anthropogenic stressors such as urbanisation, sealing (paving) of the ground, increased noise, and chemical pollution (Perkin et al, 2011)
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