Estimating the expected crossing frequency of road mitigation structures in South Texas based on proximity and vegetation

T. Miles Hopkins, University of Texas Rio Grande Valley
Thomas Yamashita, University of Texas Rio Grande Valley
Anna Rivera Roy, University of Texas Rio Grande Valley
Kevin Ryer, University of Texas Rio Grande Valley
John Young Jr., University of Texas Rio Grande Valley
Richard Kline, University of Texas Rio Grande Valley
Topic Area
Terrestrial Wildlife and Ecosystem Interactions with Transportation

The endangered ocelot (Leopardus pardalis) is estimated to have fewer than 80 individuals remaining in the United States, with the entire population constrained to South Texas. Within this population, 40% of known ocelot deaths are due to vehicle collisions. To prevent additional road mortalities on state highway 100 in Cameron County, the Texas Department of Transportation (TxDOT) constructed 11.5 km of wildlife exclusion fencing and eighteen wildlife guards. To maintain landscape connectivity, five wildlife crossing structures were modified or constructed within the exclusion fencing. Camera trap arrays at each road mitigation structure monitor actual wildlife use through wildlife crossing structures and at wildlife guards. Though important, this baseline data collection does not address wildlife crossing structure performance. Structure performance may be estimated by determining the influence of wildlife crossing structure location and design. The influence of wildlife crossing structure location can be obtained using a robust study of wildlife in the surrounding area. The objective of this study was to determine the effect of structure location on performance using expected crossing frequencies estimated from control sites. Two control camera arrays were deployed to determine expected crossing frequencies. The first was based on proximity to wildlife crossing structures, where sites were not placed farther than 150 meters from the roadway or more than 500 meters from the structure. The second array was placed in a stratified random design based on similar vegetation surrounding wildlife crossing structures. Vegetation was an important factor to incorporate as ocelots are specialists, preferring dense thornscrub, a rare and decreasing vegetation type that may not be preferred by other species. The expected crossing frequencies from both control arrays were compared to determine if expected crossing frequencies based on vegetation may be used instead of those based on proximity. To determine performance, the expected crossing frequencies calculated from each array were compared to the actual rate of use at mitigation structures. Estimating the performance of these structures with control sites can account for the influence of location, allowing for the comparison of wildlife crossing structure design.

Abstract Keywords
Road Mitigation Structures Expected Crossing Frequency Control-Impact Study