Habitat Connectivity: Placement of Crossing Structures

The extensive and growing road network in the United States has substantial ecological, economic, and social impacts. In the case of the endangered Florida Key deer (Odocoileus virginianus clavium), nearly 50 percent of the total mortality is attributed to deer-vehicle collisions. Over half of the deer-vehicle collisions occur on U. S. Highway 1, the only highway linking the Keys to the mainland. Since the early 1990’s, various agencies and stakeholders have been trying to address deer-vehicle collisions in the Florida Keys. Initially, underpasses in combination with fencing were chosen to address deer-vehicle collisions. An apparently simple solution, however, was complicated due to access management issues and environmental regulations related to urban development. The Florida Department of Transportation (FDOT) was instrumental in resolving many of these issues, and provided resources and expertise which served as a catalyst in this process. The FDOT’s U. S. Highway 1 improvement project, testing of a bridge grating system, and a habitat conservation plan illustrate successful conservation strategies in the Florida Keys. In the continental United States, roads and roadsides cover approximately 1 percent of the surface area, and impact 22 percent of it ecologically (Forman 2000). For species that readily cross roads, wildlife-vehicle collisions can have serious costs in several forms. For example, each year in the United States, deer-vehicle (Odocoileus virginianus) collisions cost $1.1 billion in property damage or losses, and cause an estimated 29,000 human injuries and 211 human fatalities (Conover et al. 1995). Continued urban sprawl and suburban development are likely to increase costs associated with deer-vehicle collisions. Florida Key deer (O. v. clavium) occupy 20-25 islands in the lower Florida Keys and are the smallest sub-species of white-tailed deer in the United States (Hardin et al. 1984, Lopez 2001, fig. 1). Approximately 75 percent of the overall population is found on Big Pine and No Name keys (Lopez et al. 2003a). Since 1960, urban development and habitat fragmentation have threatened the Key deer (Lopez 2001, Lopez et al. 2003c). In addition to a loss of habitat, an increase in urban development is of particular concern because highway mortality accounts for the majority of the total deer mortality. Over half of the deer-vehicle collisions occur on U. S. Highway 1 (US 1), the only highway linking the Keys to the mainland (fig. 1, Lopez et al. 2003c). Since the late 1980’s, U. S. Fish and Wildlife Service (USFWS), Florida Department of Transportation (FDOT), and local residents have been trying to address deer-vehicle collisions on Big Pine Key (Lopez et al. 2003c). In 1993, FDOT began efforts to reduce Key deer mortality along the US 1 corridor on Big Pine Key. This proactive effort resulted in the formation of the Key Deer Ad-Hoc Committee in 1993. Based on recommendations from the committee, the Key Deer/Motorist Conflict Concept Study was initiated in 1995 to evaluate viable solutions in reducing Key deer mortality along US 1 (Calvo 1996, Calvo and Silvy 1996). Key Deer Dilemma Recommendations from the Key Deer/Motorist Conflict Concept Study proposed the construction of barriers (fences) with two wildlife crossings (underpasses) to prevent Key deer access onto US 1 (fig. 2, Calvo 1996, Calvo and Silvy 1996). Underpasses in combination with fencing have been successfully used to reduce wildlife(Foster and Humphrey 1995, Clevenger and Waltho 2000) and deer(Bellis and Graves 1971, Falk et al. 1978, Reed et al. 1975, Ford 1980, Reed et al. 1974, Reed et al. 1975) vehicle collisions in many parts of the country. Furthermore, FDOT has successfully reduced road mortality for other federally-listed species (e.g., Florida panther (Puma concolor coryi) and Florida black bear (Ursus americanus floridanus) with the use of wildlife crossings. At this point, an apparently simple solution was complicated due to access management issues and environmental regulations related to urban development. First, access management is a critical factor in the success of fence/underpass-type wildlife crossings. For example, along the US 1 corridor there are a number of access points (e.g., side roads, driveways) making continuous fencing impossible (fig. 2). Previous studies (Reed et al. 1974, Reed et al. 1975, Sebesta 2000) proposed modified cattle guards or “deer guards” (defined as cattle guards adapted for deer) as a method to