Continued Declines and an Aged Salamander Breeding Population 16-Years After Installation of the First Amphibian Underpass System in a National Park in Canada

Abstract

Pond-breeding amphibians are highly vulnerable to the effects of roads, prompting the installation of road underpasses and exclusion fencing in many places. Although such systems are often successful at reducing amphibian road mortality, post-installation monitoring is rarely conducted for more than a few years. Consequently, the long-term use and impacts of such infrastructure on amphibian populations is unclear. The first amphibian road mitigation system in a National Park in Canada was installed in 2008 in Waterton Lakes National Park to protect long-toed salamanders (Ambystoma macrodactylum krausei) moving to and from Linnet Lake. However, despite this intervention, surveys of the system shortly after installation pointed to declines of the Linnet Lake population relative to baseline (pre-tunnel) census size. In this study, I used a pitfall trap array and visual surveys to reassess salamander use of this system and associated breeding site 16-years post-installation. Despite observing zero road mortality, I observed just 55 salamanders along the fence and tunnel system and in the terrestrial environment around the lake - a dramatic decrease relative to >500 individuals observed during the last survey and the thousands of individuals present pre-mitigation. Early spring surveys and quantification of migratory orientation around the lake rule out shifts in breeding phenology or changes in migratory routes as explanations for the observed decline in individuals observed. Instead, a high fraction of cross-survey recaptures (i.e. ~35% individuals >13 years old) and lack of successful recruitment (i.e. no young-of-year) suggest that this breeding population has continued to decline and is now at risk of being lost entirely. In addition to the road, salamanders at this site face threats from fish predators and climate and landscape change. My results thus suggest that mitigating road mortality alone may be insufficient for preventing the loss of associated amphibian breeding populations.