Importance of visibility when evaluating animal response to roads

Roads increase risk to animals via direct and indirect mechanisms yet, both positive and negative effects of animal space use in relation to roads have been reported. These contrasting reports may not actually represent animal ecology, but could be a product of the primary variable used to test the relationship between animals and roads. Animal-road associations are often evaluated using Euclidean distance. Euclidean, or straight-line, distance fails to account for the screening effects of vegetation and topography and may document spurious relationships. We evaluated the influence of Euclidean distance, visibility from road and forage quality on summer space use for male elk Cervus elaphus and female elk subherds in Custer State Park, South Dakota, USA. Models that included interactions with visibility from road metrics outperformed models that included only Euclidean distance to road as main effects. Elk response to roads varied by sex and road type, which functioned as an index for vehicle use. Male elk selected habitat away from roads with the greatest vehicle use, an effect that was greater if habitat was visible from those roads. Female elk tended to select habitat with high forage quality in areas visible from roads closed to vehicle use. Interestingly, both male and female elk selected habitat away from roads with secondary vehicle use and near to roads devoid of traffic, regardless of visibility. Our analysis highlights the importance of including both Euclidean distance and visibility from road metrics. Road effects research may be incomplete without consideration of visibility from roads, particularly for large mammals in landscapes with intense road use.