Illuminating complexity: How artificial light at night influences insect richness and abundance across a vehicle pollution gradient

Controlled studies assessing the effects of artificial light at night (ALAN) on insects have revealed strong effects of the color, strength, and duration of nighttime light on insects and their host plants. These studies suggest that lighting within urban areas is likely to have important but complex effects on biodiversity. Here, we assessed ALAN as a driver of insect community richness and abundance across Sacramento, CA, USA. Originally, the sites used for this study were chosen because focal trees (Quercus lobata) were in areas that represented a gradient of vehicle pollution. We found in previous studies that these sites also have variable light regimes driven by street lights. This variation in light is not substantially correlated with vehicle pollution, which presents an opportunity to determine their relative effects on insects. Our results indicate that the relationship between ALAN and family-level insect richness and overall abundance depends on the level of vehicle pollution. At sites where vehicle pollution is low, there is a negative effect of ALAN on family-level insect richness and abundance. However, the negative effects of ALAN become less negative as vehicle pollution increases, likely because there is a negative effect of vehicle pollution on certain insects. We found a small but significant positive interaction between ALAN and vehicle pollution, suggesting that ALAN effects hinge on vehicle pollution levels insect communities face. Our study highlights the importance of considering other anthropogenic factors, such as vehicle pollution, when studying the effects of ALAN in urban and rural environments.