New Hope Creek Bridge as a Wildlife Underpass

Abstract

Roads pose many threats to wildlife. One such threat, wildlife-vehicle collisions, is a danger to humans as well as wildlife. Bridges built to facilitate movement of wildlife under roads may reduce threats that exist when large mammals attempt to cross roadways. My study is the first phase of a two-stage investigation of whether a bridge designed to function as a wildlife underpass influences wildlife use of the U.S. Highway 15/501 bridge over New Hope Creek (NHC) near Durham, North Carolina. This underpass is important as a wildlife passage, particularly for white-tailed deer (Odocoileus virginianus), because the forests associated with NHC create a corridor between two natural areas: Duke Forest to the north and B. Everett Jordan Lake to the south. Phase One involves monitoring wildlife use of the current bridge structure using video cameras. In 2007, a longer bridge will be constructed. Phase Two will be the replication of my study upon completion of the new bridge. Wildlife use of the current and future underpass will be compared to determine whether underpass dimensions influence wildlife use of the underpass. Wildlife use of the NHC underpass was recorded continuously from December 2003 through May 2005. During the study period, 126 crossings were observed in the sample of video data by the following species: white-tailed deer, woodchuck (Marmota monax), chipmunk (Tamias striatus), raccoon (Procyon lotor), red or gray fox (Vulpes vulpes or Urocyon cinereoargenteus), gray squirrel (Sciurus carolinensis), domestic dog or coyote (Canis spp.), domestic cat (Felis catus), muskrat (Ondatra zibethicus), hispid cotton rat (Sigmodon hispidis), and unidentifiable small and medium-sized mammals. Based on the sampling technique, 42.2% of wildlife crossings were observed. Thus, an estimated 299 wildlife crossings occurred throughout the study period. Seventy-five deer were observed in the sample of video data using the underpass, while 17 deer approached and retreated. Using the sampling technique, 40.5% of deer crossings and 92.1% of deer approaches were observed. Thus, an estimated 185 deer crossings and 18 approaches occurred during the study period. One-hundred forty-six people were observed near the underpass in the sample of video data. Based on the sample, 80.8% of human activity near the underpass was detected. Thus, an estimated 181 people were observed near the underpass during the study period. Five potential road crossings, including three by small mammals and two by deer, were observed in the sample of video footage. As an index of road mortality near the NHC underpass, weekly surveys of vehicle-killed animals were conducted while driving north and south on the 1.8 km section of Highway 15/501 containing the underpass from December 2003 through June 2005. The surveys revealed that five individuals were killed by vehicle collisions, including raccoon, opossum (Didelphus virginiana), woodchuck, wild turkey (Meleagris gallapavo), and one unidentifiable mammal. North Carolina Department of Transportation records of wildlife collisions occurring from January 1, 1990 through October 30, 2004 revealed that deer-vehicle collisions on the section of Highway 15/501 containing the NHC underpass were infrequent, with 16 deer-related vehicle collisions occurring. Because several years will separate the current study from the future study of the expanded NHC underpass, a deer abundance index was developed using counts of deer observed during a driving route along roads west and north of the NHC underpass. From March 23, 2004 through May 31, 2005, 205 deer were observed during 53 driving counts. These data suggest that the Highway 15/501 underpass provides landscape connectivity between habitats on opposite sides of the highway and likely increases motorist safety by providing deer and other wildlife with an alternate route for reaching habitat on the far side of the highway without crossing onto the road.