At the intersection of soundscapes and roads: Quantifying anthrophony's influence on wildlife crossing structure use

Anthropogenic noise (anthrophony) can have significant negative effects on wildlife, causing both physiological (i.e., increased stress hormone production) and behavioral (i.e., altered anti-predator behaviors, space use, or diel activity) changes in individuals. Roads are a major source of anthrophony, often contributing the most to the anthrophony in rural areas. Most efforts to reduce road effects on wildlife have focused on decreasing road-associated mortality through the construction of wildlife crossing structures (WCSs) with little consideration for the anthrophony associated with these structures. Given the impacts of anthrophony on wildlife behavior, the effectiveness of WCSs could be altered without consideration of noise pollution. Therefore, understanding how anthrophony is structured in space and time and how it impacts WCS use is an important aspect of assessing the effectiveness of WCSs. We developed a framework for assessing anthrophony at WCS using an array of autonomous recording units to monitor overall acoustic conditions. We then examined how wildlife crossing rates were associated with anthrophony using camera traps. We monitored five underpass-style WCSs built in the Lower Rio Grande Valley of South Texas, USA, using camera traps and acoustic recording units. We measured sound pressure level (SPL [dB]) and relative level of anthrophony (using the normalized difference soundscape index [NDSI]) at six positions around each WCS: two at elevation (road grade) with the road surface (west and east), two at the WCS entrances, and two in the middle of the WCSs. We then used SPL and NDSI to predict the probability of a successful crossing by Virginia opossum (Didelphis virginiana), a common, disturbance-tolerant mammal. While the relative amount of anthrophony did not differ, smaller WCSs and those with less traffic were up to 40 dB quieter than larger WCSs and those with more traffic. Opossums spent more time at WCSs when it was quieter on average and were more likely to successfully cross through a WCS when there was less vehicle noise. Our study highlights the importance of considering soundscapes in assessing WCS effectiveness and represents a framework that can be used for further exploration of the impacts of anthrophony on WCS use.