Impact of light pollution on moth morphology–A 137-year study in Germany

Increasing artificial illumination during night has multifaceted effects on species. Moths are shown to be distracted and attracted by artificial light sources, leading to increased mortality through predation or exhaustion. Increased mortality can be expected to increase selection pressure on morphology, particularly those being functional in light detection and flight ability. We were thus interested if intraspecific traits differ between areas and times with differing light pollution values. We chose the moth Agrotis exclamationis, a common species in the Berlin-Brandenburg region, Germany, a region that offers very different levels of light pollution across space and time. We examined body length, eye size and forewing length, traits likely targeted through selection due to light pollution. We examined moths collected over the past 137 years. We predicted decreasing forewing length, body and eye size, in response to increasing light pollution and expected to see trait changes from the past to today, and from rural to urban areas, representing temporal and spatial gradients of increasing light pollution. In order to determine current levels of light pollution, we used radiance values of the years 2012 to 2019. These values were the base to extrapolate previous radiance values for all sample sites and years. We observed no trait differences along the spatial gradient, but trait and sex dependant changes along the temporal gradient. We could not confirm a direct causal link between changes in body size and female eye size. However, we revealed indirect effects of light pollution, and assume habitat fragmentation and host-plants to be the main drivers for these effects. A trend towards smaller-eyed females in ‘medium’ and ‘high’ light-polluted areas over time could be a first indication that morphological trait changes to light pollution are taking place.