Plants need light to grow, but many need a good dose of darkness as well. This is because some very effective pollinators wait until dark to visit plants’ flowers, meaning that a plant can work on reproduction night and day, growing more seeds for new plants. This has served plants well for millions of years, and a variety of very effective pollinators only come out at night, from moths to beetles to bats. Unfortunately, recent experiments in Switzerland indicate that humanity’s love of lighting may be casting a shadow over this otherwise efficient system.
The basic model to be tested was that artificial lighting is scaring nocturnal pollinators away from their favorite flowers. Setting up this experiment was tricky though, as artificial lighting in developed areas has left very few places in total darkness at night. This forced researchers from the University of Bern to head for the foot of the Alps to find some cabbage thistles (Cirsium oleraceum) that still enjoyed a decent amount of darkness each night, at which point they started shining lights on them. Half the thistle plants were left in natural conditions and monitored with night-vision goggles, while the others were illuminated by semi-portable LED lamps meant to imitate a streetlight, albeit one with a very long extension cord.
Staying out of the spotlight
Pollinators were counted and collected each night, and the various nocturnal critters clearly showed a preference for the dark. There were 62 percent fewer visits by pollinators, and 29 percent less variety among the pollinators that did risk exposure in the lights. Even though daytime pollinators visited both sets of thistles equally, the plants that missed their nighttime visitors showed a significant decrease in the number of seeds they produced. The decrease in seeds actually outweighed the decline in pollinator visits, suggesting that nighttime pollinators may do a better job of moving pollen on a per-visit basis. In other words, adding more bees and butterflies during the day wouldn’t easily replace the lack of moths and beetles at night.
One hope is that the plants still left in darkness are getting extra pollinator traffic from all the visitors that are scared off by human-made lighting. However, the pockets of darkness left in some areas are so isolated in many places that these more successful plants probably won’t make up for the losses experienced by their well-lit counterparts. This isn’t the only concern that’s been raised about artificial lighting, indicating that we may have to reconsider how badly we really need all our outdoor night-lights.
Source: Artificial Light Deters Nocturnal Pollinators, Study Suggests by Scott Neuman, The Two-Way