Road Mortality Relative to Wheel-Track Position During Spring Migration of Common Toads in Norway
Road mortality is a widely recognized, but rarely quantified threat to the viability of amphibian populations. This paper summarizes road mortality during the spring migration for a northern Common Toad (Bufo bufo) population in Norway for 10 of the last 22 years, in addition to assessing where on the road (which wheel-track) toads were most exposed to mortality risk for 2 observation years. Percentage of observed dead toads for all observation years ranged from a mean of 56.9% in 2001 to 83.2% in 2006. The highest annual number of migrating toads was 1,082 in 2006, of which 764 were killed. When adjusting for effective counting days with positive toad activity, average number of toads per day decreased from 40.5 in the years 2001–2007 to 24.2 in 2020–2023, an average decrease of 40%. As a global mean for all 10 counting years, 69.4% of toads were killed on the road. When assigning dead toads to the closest wheel-track, the highest mortality in numbers was found at the start of the road, at wheel-track 1, for both years. Fractional risk of mortality was highest at the fourth wheel-track in 2007, with 47% of toads entering this track being killed, whereas the first wheel-track had the highest fractional death in 2023, with 43% dead animals. Among live migrating animals in 2023, there were 3 females and 56 males, indicating a highly male-dominated migrating population of toads.ABSTRACT
Study area. (a) Map of Norway showing the position of our study area; (b) Illustrates the location of the study site indicated by a blue circle; (c) Illustrates the sampling area where observations started each dusk at the site marked with (1). Following the walking path in the northern direction (thin line in red), each living toad was counted, and each dead toad was marked with spray. At the site marked with (2), the observer crossed the road and repeated the protocol, now counting the “newcomers” to the road. The observations took about one hour at a slow pace; (d) Illustrates the wheel-tracks and the migration pattern (orange arrow).
Boxplots summarizing the total daily number of observed toads (gray) and daily total of dead toads (red) for each sampling year. Boxplots show the 25–75% quantiles (boxes), medians across sampling days (black/red horizontal bold line), 95% limits (bars) and outliers (open circles). Overall average percent of dead toads per year from the total are plotted as a line (orange).
Line plots displaying total number of toads observed per year (light gray circles) and total number of recorded dead toads per year (red squares). To adjust for differences in number of counting days per year (e.g., a long migration season with few migrating animals for many days until the peak vs. a shorter concentrated migration period), the 6 days per year with most activity were plotted as separate lines, illustrating total adjusted number of toads (dark gray diamonds) and adjusted number of dead toads (red triangles).
Results from (a) 2007 and (b) 2023. Bars show number of toads that were entering each wheel-track (divided by 4) where numbers on each bar are observed numbers of live toads entering each wheel-track; red boxes show the actual number of dead toads for each individual wheel-track (divided by 4); black line displays percentage of dead toads per wheel-track (dead toads/entering toads*100); orange line shows accumulated deaths from all wheel-tracks, summing to 84% in 2007 and 77% in 2023.
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