Role of Fencing in Promoting Wildlife Underpass Use and Highway Permeability
Ungulate-proof fencing has been used successfully to mitigate the incidence of wildlife-vehicle collisions on highways throughout North America. And while fencing is often regarded as an integral component of effective wildlife passage structures, limited information or guidelines exist for the application of fencing in conjunction with wildlife passages. Fencing itself may limit wildlife permeability across highways and exacerbate the barrier effect of highways on wildlife populations. An 8-km section of highway reconstructed from a two- to four-lane divided highway in central Arizona was opened to traffic six months before ungulate-proof fencing was erected linking four wildlife underpasses (UP) and three bridges. To assess the role of strategically placed fencing along 49% of the section, we compared before and after fencing Rocky Mountain elk (Cervus elaphus nelsoni)-vehicle collision incidence, wildlife use of UP, and elk highway permeability. From 2002–2006, we documented 110 elk-vehicle collisions. The incidence of collisions increased over three fold after highway reconstruction was completed but before fencing was erected. After fencing, the incidence of elk collisions declined 87%. We employed video camera surveillance systems at two UP to compare wildlife use for nine months before and 11 months after fencing was erected. Before fencing, we recorded 500 elk and deer (Odocoileus spp.) at the UP, of which only 12% successfully passed through the UP; 81% of animals continued to cross the highway at grade. After fencing, of 595 elk and deer recorded, 56% crossed successfully and no animals crossed the highway at grade. The probability of an approaching animal crossing through an UP increased from 0.09 to 0.56 with fencing, and the combined odds of a crossing through the UP after fencing was 13.6:1 compared to before fencing. We used Global Positioning System (GPS) telemetry to assess highway permeability and crossing patterns. We instrumented 22 elk (16 female, 6 male) with GPS receiver collars April 2004–October 2005, during which time our collars accrued 87,745 GPS fixes. The elk highway passage rate, our measure of permeability, after the highway was opened to traffic but before fencing was erected (0.54 crossings/approach) was 32% lower than the level determined from a previous study for the section during reconstruction (0.79 crossings/approach). Once fencing was erected, the passage rate increased 52% to 0.82 crossings/approach. The proportion of elk crossings that occurred along fenced highway stretches declined 50% while the proportion of crossings along unfenced highway increased 40%. Fencing plays an important role in reducing the incidence of wildlife-vehicle collisions and increasing the effectiveness of wildlife passage structures. Furthermore, fencing in combination with a relatively high density of passages (1 struc¬ture/1.1 km) promoted elk highway permeability by funneling animals toward the UP where resistance to crossing was lower than that associated with crossings at grade.