Moths confuse artificial light with the moon, by which they orient themselves. © Shutterstock
There are far more moths than butterflies, with nearly 800 moth species in the Netherlands. But their numbers are going down, and have fallen by nearly one quarter in the past 30 years.
For the first time, the researchers have now proven a strong negative link with light. The populations of species that are drawn to light have dropped much more markedly than those of species that are not attracted to light, or are active both by day and by night.
counts carried out using light traps. © Jurriën van Deijk
481 species
The researchers made use of data from Noctua, a database managed by the Dutch butterfly association Vlinderstichting. This includes counts carried out using light traps. For the years 1985-2015, the researchers plotted the number of moths caught against the time, producing trend lines for 481 species. Some species remained stable or even increased, but the overall trend is very clearly one of declining numbers. The researchers then used statistical analyses to look at the possible causes at play. It was as clear as day that artificial light is a factor in the decline.
The negative impact of artificial light had already been demonstrated for particular species. According to Wallis de Vries, this study shows for the first time what the effect is at population level. And it is quite remarkable. Artificial light is only used in a relatively small part of the country, and yet the effect found is an average for the whole of the Netherlands. It would seem, the researchers conclude, that the impact of artificial light reaches way beyond the immediate vicinity. One reason for this might be that well-lit roads form a barrier for moths, and another might be that the glow above cities has more far-reaching effects that we realized. Moths are the main source of food for bats and nocturnal birds. Their caterpillars are also important as herbivores, so they form an important link in the food web. The impact of artificial light on moths could therefore have a knock-on effect on other groups of species.