Viability analysis and the importance of fine‐scale variables influencing roadkills of a toad population

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Abstract Amphibians are among the most threatened vertebrate groups globally, with roadkills representing a major factor contributing to population decline and isolation. We investigated fine‐scale temporal variables that could influence rates of roadkills and conducted population viability analyses to assess the impact of roadkills on long‐term population persistence. We focused on the natterjack toad ( Epidalea calamita ) as a case study. In 2021, during the breeding season (late January to late May), we conducted 16 sampling sessions in which we recorded toad carcasses along a regional road in Madrid, Spain (central Iberian Peninsula), and censused the breeding population within a 700‐m buffer—the species' seasonal maximum dispersal distance—from the road during each session. We also performed searcher efficiency and carcass persistence experiments to correct our mortality estimates. Following a hypothesis‐driven approach, we modeled 2 sets of predictors to identify the variables influencing roadkill rates: one incorporating all carcasses, with temporal variables averaged between consecutive surveys, and one based on fresh carcasses (those observed within a 24‐hour window of mortality) and temporal variables within the same 24‐hour window. In the first set of models, the presence of embankments was associated with reduced mortality rates and mortality declined as the season progressed, suggesting a reduction in toad displacement after breeding. In the second set of models, higher nearby relative abundance equated to increased mortality rate and the number of ponds (which may have been unoccupied) had no significant effect. These findings suggest that population size is a better predictor of roadkill rates than indirect proxies such as the number of nearby ponds. Although road mortality was high (12% of the population), population viability analyses suggested that natterjack toad persistence is not threatened in the long‐term, even under hypothetical scenarios involving 1.5 and 2 times the current mortality rate. However, this outcome could change following the anticipated negative impacts of climate change on ephemeral pond specialists. Therefore, we recommend implementing safe crossing structures—such as fencing to funnel individuals toward road culverts—particularly where roads bisect habitats with high population densities.

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