  It is now well established that amphibians are suffering widespread decline
and extinctions. Among other causes, urbanization is responsible for habitat
reduction, habitat fragmentation and massive road kills. In this context, it is
urgent to develop and assess appropriate conservation measures. Using yearly
censuses of migrating adults of two anuran species at one location in
Switzerland, we examined the impact of a road mitigation measure - permanent
under-road tunnels with guiding trenches - along a road separating wintering
forests from breeding wetlands. We observe that the adult migrating populations
do not exhibit any long-term trend but undergo a transient increase a few years
after the installation of the road mitigation measure. Using additional
datasets like climatic data and censuses obtained in a control area, we show
that the observed pattern of migrating populations cannot be explained by any
other data at our disposal. We then checked as a working hypothesis whether the
installation of under-road tunnels could explain the observed transient or not.
To this end, we use a simple population model and show that the road mitigation
measure together with competition for resources can successfully explain the
experimental observations. We conclude by discussing the requirements for
further assessment of this hypothesis as well as consequences for conservation
planners.
