The Relationship Between Roads and Amphibians: Effects of Traffic Noise and Mitigation of Road Mortality

Abstract

Anuran abundance is negatively correlated with road traffic; this may be due in part to the interruption of mate attraction by traffic noise. The impact could be small if anurans can alter their vocalization characteristics to avoid masking of their calls by traffic noise. I compared vocalizations of four anuran species in locations with low traffic noise vs. sites with high traffic noise. I then broadcast traffic noise at low traffic sites, and compared the anuran vocalizations before vs. during the broadcast traffic noise. Finally, I compared vocalizations at high traffic sites to those produced at low traffic sites while broadcasting traffic noise. Some species produced different vocalization characteristics in the presence and absence of traffic noise. Broadcast traffic noise immediately altered vocalizations such that they became similar to individuals of the same species found in locations with high traffic noise. To test whether alterations compensate for an effect of traffic noise on mate attraction, I: (i) recorded a male calling at a quiet site; (ii) played traffic noise at the same male and recorded its altered call; and (iii) used these recordings to attract females to a trap at sites either with or without broadcast traffic noise. Results indicate that when necessary, males change their calls to compensate for potential effects of traffic noise on mate attraction. If my results apply to anurans in general, the previously documented negative effects of roads on populations are likely not caused by traffic noise; I hypothesize that road mortality is the main cause. Culvert-type ecopassages, along with fencing have been used to mitigate road mortality on herptiles. Effectiveness of the ecopassage itself, independent of fencing, is largely untested. I tested the hypothesis that pre-existing drainage culverts actually mitigate anuran road mortality. I found no evidence that culverts alone reduce road kill. In contrast, fencing with culverts was effective at reducing road mortality. Overall, my results suggest that mitigation of road effects on anurans should focus on ways of excluding the animals from the road surface.