Neglected juveniles; a call for integrating all amphibian life stages in assessments of mitigation success (and how to do it)

Abstract

Road networks are central drivers of biodiversity loss and their impacts are rapidly expanding. Pond-breeding amphibians are highly vulnerable to road impacts because they typically travel from terrestrial to aquatic habitats to breed and leave the aquatic habitat for the terrestrial as newly emerged juveniles. If amphibians must cross roads during migrations, mass mortalities and local extirpation can result. While mortality at any life stage is concerning, juveniles should be of special interest when assessing road mortality and mitigation (e.g., tunnels to facilitate safe seasonal migrations) because their fate can have a disproportionate impact on population dynamics. We highlight a pervasive lack of information about juveniles which contributes to an inability to demonstrate the effect of road mitigation actions at the population level. This limits our capacity to implement conservation strategies and improve outcomes for vulnerable amphibian populations. We examine this knowledge gap using published mathematical models of amphibian populations and studies on efforts to mitigate road impacts. We further discuss the successful use of volunteers (i.e., citizen science) and identify how these efforts might be more broadly applied to address the dearth of data on juveniles and to mitigate the levels of mortality caused by roads. There are discrepancies between theoretical population models and conservation practice and we evaluate the potential causes and implications for the effectiveness of conservation projects in improving population persistence. Road mortality is a common and increasingly important challenge for amphibian conservation. Understanding juvenile demographics, movement ecology and response to mitigation structures is critical for expanding beyond short-term road mortality prevention to long-term mitigation (population persistence).