Potential of Small Culverts as Wildlife Passages on Forest Roads

Roads and traffic can cause animal mortality. Specifically, roads serve as barriers by impeding animal movement, resulting in demographic and genetic consequences. Drainage structures, such as culverts, can provide linkages between habitat patches. However, the potential of small culverts with diameters of <60 cm (e.g., wildlife passages that facilitate movement on forest roads) are relatively unknown. In this study, we used trail cameras to monitor the use of 14 small culverts, by mammals, along forest roads on Mt. Graham, home of the critically endangered Mt. Graham red squirrels ( Tamiasciurus hudsonicus grahamensis ), in southeastern Arizona, USA. From 2011 to 2013, we only recorded 20 completed road crossings through culverts. More than half of culvert uses were by striped skunks ( Mephitis mephitis ), followed by the rock squirrel ( Spermophilus variegatus ) and the bobcat ( Lynx rufus ). The Mt. Graham red squirrel was the only species that was common along the roads, but never crossed the roads. Culverts with higher usages were characterized by shorter culvert lengths and absence of accumulated soil inside the culverts. Our study shows that small-dimension drainage systems may provide alternative pathways for wildlife crossing roads, especially for slow moving and ground dwelling species. However, the potential of small culverts assisting wildlife crossings can only be maximized when culverts are accessible year-round.