Wildlife crossing structures (WCS) over or under highways have been proposed as a solution for road-related habitat fragmentation and wildlife collisions. To assure the efficacy of WCS, road related negative impacts that could cause animals to avoid WCS, such as noise and light, need to be considered. Human-sourced noise can affect habitat occupancy, and a suite of animal behaviors such as vigilance, communication, and predation efficiency, while artificial light, especially at night, can change animal’s perception of resources, foraging, mate selection, and navigation. Furthermore, the impact of noise and light varies among wildlife species, leading to differential responses within wildlife communities. To test whether traffic noise impacts species’ use of WCS, the authors quantified overnight (afternoon to early morning) road traffic noise, measured as dB(A), at 20 WCS positioned along four central California highways (I-5, I-80, 680 and 280), as well as historical WCS mammal use for 20 recorded days during the summer of 2012, 2015, and 2016. Using species richness inclusive of all taxa as the response variable, the only significant explanatory variable was annual average daily traffic (AADT) (p <0.001). However, when using disturbance-sensitive species richness as the response variable, the authoras found that sensitive species richness was negatively correlated with maximum noise (p < 0.05) and AADT (p < 0.01). Noise levels (dB(A)) and species richness at 8 WCS and adjacent habitats (> 800m from the highway) were monitored over a 20-day period, to further examine the impact of noise on WCS use. Species richness was greater (p<0.05) in adjacent habitats (mean 10.6) with low traffic noise levels than recorded at the WCS (mean 7.2). They also examined differences between disturbance-tolerant (e.g., raccoon, striped skunk) and sensitive (e.g., black bear, mountain lion, bobcat, coyote) species. Presence of tolerant species was greater at high noise locations, particularly at underpasses, while the number of sensitive species was lower in noisier areas. The authors also measured light intensity as total luminescence at eight WCS in the Sierra Nevada and San Francisco Bay Area. They used a novel approach employing a camera with a very wide-angle lens to capture low light levels, combined with software that estimates total illumination and light frequency. There was a suggestive, but not significant inverse relationship between total illumination and species richness, for these eight sites. Their results indicate that wildlife use of WCS could be disrupted by traffic, especially for disturbance-sensitive species. This has important ramifications for using WCS as primary approach to resolve road/highway disruption of wildlife movement and occupancy.

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